Ramda

__

Added in v0.6.0

A special placeholder value used to specify "gaps" within curried functions, allowing partial application of any combination of arguments, regardless of their positions.

If g is a curried ternary function and _ is R.__, the following are equivalent:

  • g(1, 2, 3)
  • g(_, 2, 3)(1)
  • g(_, _, 3)(1)(2)
  • g(_, _, 3)(1, 2)
  • g(_, 2, _)(1, 3)
  • g(_, 2)(1)(3)
  • g(_, 2)(1, 3)
  • g(_, 2)(_, 3)(1)
const greet = R.replace('{name}', R.__, 'Hello, {name}!');
greet('Alice'); //=> 'Hello, Alice!'

add

Parameters
  • a
  • b
Returns Number

Added in v0.1.0

Adds two values.

See also subtract. 
R.add(2, 3);       //=>  5
R.add(7)(10);      //=> 17

addIndex

Parameters
  • fn

    A list iteration function that does not pass index or list to its callback
Returns function An altered list iteration function that passes (item, index, list) to its callback

Added in v0.15.0

Creates a new list iteration function from an existing one by adding two new parameters to its callback function: the current index, and the entire list.

This would turn, for instance, R.map function into one that more closely resembles Array.prototype.map. Note that this will only work for functions in which the iteration callback function is the first parameter, and where the list is the last parameter. (This latter might be unimportant if the list parameter is not used.)

const mapIndexed = R.addIndex(R.map);
mapIndexed((val, idx) => idx + '-' + val, ['f', 'o', 'o', 'b', 'a', 'r']);
//=> ['0-f', '1-o', '2-o', '3-b', '4-a', '5-r']

adjust

Parameters
  • idx

    The index.
  • fn

    The function to apply.
  • list

    An array-like object whose value at the supplied index will be replaced.
Returns Array A copy of the supplied array-like object with the element at index `idx` replaced with the value returned by applying `fn` to the existing element.

Added in v0.14.0

Applies a function to the value at the given index of an array, returning a new copy of the array with the element at the given index replaced with the result of the function application.

See also update. 
R.adjust(1, R.toUpper, ['a', 'b', 'c', 'd']);      //=> ['a', 'B', 'c', 'd']
R.adjust(-1, R.toUpper, ['a', 'b', 'c', 'd']);     //=> ['a', 'b', 'c', 'D']

all

Parameters
  • fn

    The predicate function.
  • list

    The array to consider.
Returns Boolean `true` if the predicate is satisfied by every element, `false` otherwise.

Added in v0.1.0

Returns true if all elements of the list match the predicate, false if there are any that don't.

Dispatches to the all method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also any, none, transduce. 
const equals3 = R.equals(3);
R.all(equals3)([3, 3, 3, 3]); //=> true
R.all(equals3)([3, 3, 1, 3]); //=> false

allPass

Parameters
  • predicates

    An array of predicates to check
Returns function The combined predicate

Added in v0.9.0

Takes a list of predicates and returns a predicate that returns true for a given list of arguments if every one of the provided predicates is satisfied by those arguments.

The function returned is a curried function whose arity matches that of the highest-arity predicate.

See also anyPass. 
const isQueen = R.propEq('rank', 'Q');
const isSpade = R.propEq('suit', '♠︎');
const isQueenOfSpades = R.allPass([isQueen, isSpade]);

isQueenOfSpades({rank: 'Q', suit: '♣︎'}); //=> false
isQueenOfSpades({rank: 'Q', suit: '♠︎'}); //=> true

always

Parameters
  • val

    The value to wrap in a function
Returns function A Function :: * -> val.

Added in v0.1.0

Returns a function that always returns the given value. Note that for non-primitives the value returned is a reference to the original value.

This function is known as const, constant, or K (for K combinator) in other languages and libraries.

const t = R.always('Tee');
t(); //=> 'Tee'

and

Parameters
  • a
  • b
Returns Any

Added in v0.1.0

Returns the first argument if it is falsy, otherwise the second argument. Acts as the boolean and statement if both inputs are Booleans.

See also both, or. 
R.and(true, true); //=> true
R.and(true, false); //=> false
R.and(false, true); //=> false
R.and(false, false); //=> false

andThen

Parameters
  • onSuccess

    The function to apply. Can return a value or a promise of a value.
  • p
Returns Promise The result of calling `p.then(onSuccess)`

Added in v0.27.1

Returns the result of applying the onSuccess function to the value inside a successfully resolved promise. This is useful for working with promises inside function compositions.

See also otherwise. 
const makeQuery = email => ({ query: { email }});
const fetchMember = request =>
  Promise.resolve({ firstName: 'Bob', lastName: 'Loblaw', id: 42 });

//getMemberName :: String -> Promise ({ firstName, lastName })
const getMemberName = R.pipe(
  makeQuery,
  fetchMember,
  R.andThen(R.pick(['firstName', 'lastName']))
);

getMemberName('bob@gmail.com').then(console.log);

any

Parameters
  • fn

    The predicate function.
  • list

    The array to consider.
Returns Boolean `true` if the predicate is satisfied by at least one element, `false` otherwise.

Added in v0.1.0

Returns true if at least one of the elements of the list match the predicate, false otherwise.

Dispatches to the any method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also all, none, transduce. 
const lessThan0 = R.flip(R.lt)(0);
const lessThan2 = R.flip(R.lt)(2);
R.any(lessThan0)([1, 2]); //=> false
R.any(lessThan2)([1, 2]); //=> true

anyPass

Parameters
  • predicates

    An array of predicates to check
Returns function The combined predicate

Added in v0.9.0

Takes a list of predicates and returns a predicate that returns true for a given list of arguments if at least one of the provided predicates is satisfied by those arguments.

The function returned is a curried function whose arity matches that of the highest-arity predicate.

See also allPass. 
const isClub = R.propEq('suit', '♣');
const isSpade = R.propEq('suit', '♠');
const isBlackCard = R.anyPass([isClub, isSpade]);

isBlackCard({rank: '10', suit: '♣'}); //=> true
isBlackCard({rank: 'Q', suit: '♠'}); //=> true
isBlackCard({rank: 'Q', suit: '♦'}); //=> false

ap

Parameters
  • applyF
  • applyX
Returns *

Added in v0.3.0

ap applies a list of functions to a list of values.

Dispatches to the ap method of the second argument, if present. Also treats curried functions as applicatives.

R.ap([R.multiply(2), R.add(3)], [1,2,3]); //=> [2, 4, 6, 4, 5, 6]
R.ap([R.concat('tasty '), R.toUpper], ['pizza', 'salad']); //=> ["tasty pizza", "tasty salad", "PIZZA", "SALAD"]

// R.ap can also be used as S combinator
// when only two functions are passed
R.ap(R.concat, R.toUpper)('Ramda') //=> 'RamdaRAMDA'

aperture

Parameters
  • n

    The size of the tuples to create
  • list

    The list to split into n-length tuples
Returns Array The resulting list of `n`-length tuples

Added in v0.12.0

Returns a new list, composed of n-tuples of consecutive elements. If n is greater than the length of the list, an empty list is returned.

Acts as a transducer if a transformer is given in list position.

See also transduce. 
R.aperture(2, [1, 2, 3, 4, 5]); //=> [[1, 2], [2, 3], [3, 4], [4, 5]]
R.aperture(3, [1, 2, 3, 4, 5]); //=> [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
R.aperture(7, [1, 2, 3, 4, 5]); //=> []

append

Parameters
  • el

    The element to add to the end of the new list.
  • list

    The list of elements to add a new item to. list.
Returns Array A new list containing the elements of the old list followed by `el`.

Added in v0.1.0

Returns a new list containing the contents of the given list, followed by the given element.

See also prepend. 
R.append('tests', ['write', 'more']); //=> ['write', 'more', 'tests']
R.append('tests', []); //=> ['tests']
R.append(['tests'], ['write', 'more']); //=> ['write', 'more', ['tests']]

apply

Parameters
  • fn

    The function which will be called with args
  • args

    The arguments to call fn with
Returns * result The result, equivalent to `fn(...args)`

Added in v0.7.0

Applies function fn to the argument list args. This is useful for creating a fixed-arity function from a variadic function. fn should be a bound function if context is significant.

See also call, unapply. 
const nums = [1, 2, 3, -99, 42, 6, 7];
R.apply(Math.max, nums); //=> 42

applySpec

Parameters
  • spec

    an object recursively mapping properties to functions for producing the values for these properties.
Returns function A function that returns an object of the same structure as `spec', with each property set to the value returned by calling its associated function with the supplied arguments.

Added in v0.20.0

Given a spec object recursively mapping properties to functions, creates a function producing an object of the same structure, by mapping each property to the result of calling its associated function with the supplied arguments.

See also converge, juxt. 
const getMetrics = R.applySpec({
  sum: R.add,
  nested: { mul: R.multiply }
});
getMetrics(2, 4); // => { sum: 6, nested: { mul: 8 } }

applyTo

Parameters
  • x

    The value
  • f

    The function to apply
Returns * The result of applying `f` to `x`

Added in v0.25.0

Takes a value and applies a function to it.

This function is also known as the thrush combinator.

const t42 = R.applyTo(42);
t42(R.identity); //=> 42
t42(R.add(1)); //=> 43

ascend

Parameters
  • fn

    A function of arity one that returns a value that can be compared
  • a

    The first item to be compared.
  • b

    The second item to be compared.
Returns Number `-1` if fn(a) < fn(b), `1` if fn(b) < fn(a), otherwise `0`

Added in v0.23.0

Makes an ascending comparator function out of a function that returns a value that can be compared with < and >.

See also descend. 
const byAge = R.ascend(R.prop('age'));
const people = [
  { name: 'Emma', age: 70 },
  { name: 'Peter', age: 78 },
  { name: 'Mikhail', age: 62 },
];
const peopleByYoungestFirst = R.sort(byAge, people);
  //=> [{ name: 'Mikhail', age: 62 },{ name: 'Emma', age: 70 }, { name: 'Peter', age: 78 }]

assoc

Parameters
  • prop

    The property name to set
  • val

    The new value
  • obj

    The object to clone
Returns Object A new object equivalent to the original except for the changed property.

Added in v0.8.0

Makes a shallow clone of an object, setting or overriding the specified property with the given value. Note that this copies and flattens prototype properties onto the new object as well. All non-primitive properties are copied by reference.

See also dissoc, pick. 
R.assoc('c', 3, {a: 1, b: 2}); //=> {a: 1, b: 2, c: 3}

assocPath

Parameters
  • path

    the path to set
  • val

    The new value
  • obj

    The object to clone
Returns Object A new object equivalent to the original except along the specified path.

Added in v0.8.0

Makes a shallow clone of an object, setting or overriding the nodes required to create the given path, and placing the specific value at the tail end of that path. Note that this copies and flattens prototype properties onto the new object as well. All non-primitive properties are copied by reference.

See also dissocPath. 
R.assocPath(['a', 'b', 'c'], 42, {a: {b: {c: 0}}}); //=> {a: {b: {c: 42}}}

// Any missing or non-object keys in path will be overridden
R.assocPath(['a', 'b', 'c'], 42, {a: 5}); //=> {a: {b: {c: 42}}}

binary

Parameters
  • fn

    The function to wrap.
Returns function A new function wrapping `fn`. The new function is guaranteed to be of arity 2.

Added in v0.2.0

Wraps a function of any arity (including nullary) in a function that accepts exactly 2 parameters. Any extraneous parameters will not be passed to the supplied function.

See also nAry, unary. 
const takesThreeArgs = function(a, b, c) {
  return [a, b, c];
};
takesThreeArgs.length; //=> 3
takesThreeArgs(1, 2, 3); //=> [1, 2, 3]

const takesTwoArgs = R.binary(takesThreeArgs);
takesTwoArgs.length; //=> 2
// Only 2 arguments are passed to the wrapped function
takesTwoArgs(1, 2, 3); //=> [1, 2, undefined]

bind

Parameters
  • fn

    The function to bind to context
  • thisObj

    The context to bind fn to
Returns function A function that will execute in the context of `thisObj`.

Added in v0.6.0

Creates a function that is bound to a context. Note: R.bind does not provide the additional argument-binding capabilities of Function.prototype.bind.

See also partial. 
const log = R.bind(console.log, console);
R.pipe(R.assoc('a', 2), R.tap(log), R.assoc('a', 3))({a: 1}); //=> {a: 3}
// logs {a: 2}

both

Parameters
  • f

    A predicate
  • g

    Another predicate
Returns function a function that applies its arguments to `f` and `g` and `&&`s their outputs together.

Added in v0.12.0

A function which calls the two provided functions and returns the && of the results. It returns the result of the first function if it is false-y and the result of the second function otherwise. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a false-y value.

In addition to functions, R.both also accepts any fantasy-land compatible applicative functor.

See also either, and. 
const gt10 = R.gt(R.__, 10)
const lt20 = R.lt(R.__, 20)
const f = R.both(gt10, lt20);
f(15); //=> true
f(30); //=> false

R.both(Maybe.Just(false), Maybe.Just(55)); // => Maybe.Just(false)
R.both([false, false, 'a'], [11]); //=> [false, false, 11]

call

Parameters
  • fn

    The function to apply to the remaining arguments.
  • args

    Any number of positional arguments.
Returns *

Added in v0.9.0

Returns the result of calling its first argument with the remaining arguments. This is occasionally useful as a converging function for R.converge: the first branch can produce a function while the remaining branches produce values to be passed to that function as its arguments.

See also apply. 
R.call(R.add, 1, 2); //=> 3

const indentN = R.pipe(
  R.repeat(' '),
  R.join(''),
  R.replace(/^(?!$)/gm)
);

const format = R.converge(
  R.call,
  [
    R.pipe(R.prop('indent'), indentN),
    R.prop('value')
  ]
);

format({indent: 2, value: 'foo\nbar\nbaz\n'}); //=> '  foo\n  bar\n  baz\n'

chain

Parameters
  • fn

    The function to map with
  • list

    The list to map over
Returns Array The result of flat-mapping `list` with `fn`

Added in v0.3.0

chain maps a function over a list and concatenates the results. chain is also known as flatMap in some libraries.

Dispatches to the chain method of the second argument, if present, according to the FantasyLand Chain spec.

If second argument is a function, chain(f, g)(x) is equivalent to f(g(x), x).

Acts as a transducer if a transformer is given in list position.

const duplicate = n => [n, n];
R.chain(duplicate, [1, 2, 3]); //=> [1, 1, 2, 2, 3, 3]

R.chain(R.append, R.head)([1, 2, 3]); //=> [1, 2, 3, 1]

clamp

Parameters
  • minimum

    The lower limit of the clamp (inclusive)
  • maximum

    The upper limit of the clamp (inclusive)
  • value

    Value to be clamped
Returns Number Returns `minimum` when `val < minimum`, `maximum` when `val > maximum`, returns `val` otherwise

Added in v0.20.0

Restricts a number to be within a range.

Also works for other ordered types such as Strings and Dates.

R.clamp(1, 10, -5) // => 1
R.clamp(1, 10, 15) // => 10
R.clamp(1, 10, 4)  // => 4

clone

Parameters
  • value

    The object or array to clone
Returns * A deeply cloned copy of `val`

Added in v0.1.0

Creates a deep copy of the source that can be used in place of the source object without retaining any references to it. The source object may contain (nested) Arrays and Objects, Numbers, Strings, Booleans and Dates. Functions are assigned by reference rather than copied.

Dispatches to a clone method if present.

Note that if the source object has multiple nodes that share a reference, the returned object will have the same structure, but the references will be pointed to the location within the cloned value.

const objects = [{}, {}, {}];
const objectsClone = R.clone(objects);
objects === objectsClone; //=> false
objects[0] === objectsClone[0]; //=> false

collectBy

Parameters
  • fn

    Function :: a -> Idx
  • list

    The array to group
Returns Array An array of arrays where each sub-array contains items for which the String-returning function has returned the same value.

Added in v0.28.0

Splits a list into sub-lists, based on the result of calling a key-returning function on each element, and grouping the results according to values returned.

See also groupBy, partition. 
R.collectBy(R.prop('type'), [
  {type: 'breakfast', item: '☕️'},
  {type: 'lunch', item: '🌯'},
  {type: 'dinner', item: '🍝'},
  {type: 'breakfast', item: '🥐'},
  {type: 'lunch', item: '🍕'}
]);

// [ [ {type: 'breakfast', item: '☕️'},
//     {type: 'breakfast', item: '🥐'} ],
//   [ {type: 'lunch', item: '🌯'},
//     {type: 'lunch', item: '🍕'} ],
//   [ {type: 'dinner', item: '🍝'} ] ]

comparator

Parameters
  • pred

    A predicate function of arity two which will return true if the first argument is less than the second, false otherwise
Returns function A Function :: a -> b -> Int that returns `-1` if a < b, `1` if b < a, otherwise `0`

Added in v0.1.0

Makes a comparator function out of a function that reports whether the first element is less than the second.

const byAge = R.comparator((a, b) => a.age < b.age);
const people = [
  { name: 'Emma', age: 70 },
  { name: 'Peter', age: 78 },
  { name: 'Mikhail', age: 62 },
];
const peopleByIncreasingAge = R.sort(byAge, people);
  //=> [{ name: 'Mikhail', age: 62 },{ name: 'Emma', age: 70 }, { name: 'Peter', age: 78 }]

complement

Parameters
  • f
Returns function

Added in v0.12.0

Takes a function f and returns a function g such that if called with the same arguments when f returns a "truthy" value, g returns false and when f returns a "falsy" value g returns true.

R.complement may be applied to any functor

See also not. 
const isNotNil = R.complement(R.isNil);
R.isNil(null); //=> true
isNotNil(null); //=> false
R.isNil(7); //=> false
isNotNil(7); //=> true

compose

Parameters
  • ...functions

    The functions to compose
Returns function

Added in v0.1.0

Performs right-to-left function composition. The last argument may have any arity; the remaining arguments must be unary.

Note: The result of compose is not automatically curried.

See also pipe. 
const classyGreeting = (firstName, lastName) => "The name's " + lastName + ", " + firstName + " " + lastName
const yellGreeting = R.compose(R.toUpper, classyGreeting);
yellGreeting('James', 'Bond'); //=> "THE NAME'S BOND, JAMES BOND"

R.compose(Math.abs, R.add(1), R.multiply(2))(-4) //=> 7

composeWith

Parameters
  • transformer

    The transforming function
  • functions

    The functions to compose
Returns function

Added in v0.26.0

Performs right-to-left function composition using transforming function. The last function may have any arity; the remaining functions must be unary.

Note: The result of composeWith is not automatically curried. Transforming function is not used on the last argument.

See also compose, pipeWith. 
const composeWhileNotNil = R.composeWith((f, res) => R.isNil(res) ? res : f(res));

composeWhileNotNil([R.inc, R.prop('age')])({age: 1}) //=> 2
composeWhileNotNil([R.inc, R.prop('age')])({}) //=> undefined

concat

Parameters
  • firstList

    The first list
  • secondList

    The second list
Returns Array A list consisting of the elements of `firstList` followed by the elements of `secondList`.

Added in v0.1.0

Returns the result of concatenating the given lists or strings.

Note: R.concat expects both arguments to be of the same type, unlike the native Array.prototype.concat method. It will throw an error if you concat an Array with a non-Array value.

Dispatches to the concat method of the first argument, if present. Can also concatenate two members of a fantasy-land compatible semigroup.

R.concat('ABC', 'DEF'); // 'ABCDEF'
R.concat([4, 5, 6], [1, 2, 3]); //=> [4, 5, 6, 1, 2, 3]
R.concat([], []); //=> []

cond

Parameters
  • pairs

    A list of [predicate, transformer]
Returns function

Added in v0.6.0

Returns a function, fn, which encapsulates if/else, if/else, ... logic. R.cond takes a list of [predicate, transformer] pairs. All of the arguments to fn are applied to each of the predicates in turn until one returns a "truthy" value, at which point fn returns the result of applying its arguments to the corresponding transformer. If none of the predicates matches, fn returns undefined.

Please note: This is not a direct substitute for a switch statement. Remember that both elements of every pair passed to cond are functions, and cond returns a function.

See also ifElse, unless, when. 
const fn = R.cond([
  [R.equals(0),   R.always('water freezes at 0°C')],
  [R.equals(100), R.always('water boils at 100°C')],
  [R.T,           temp => 'nothing special happens at ' + temp + '°C']
]);
fn(0); //=> 'water freezes at 0°C'
fn(50); //=> 'nothing special happens at 50°C'
fn(100); //=> 'water boils at 100°C'

construct

Parameters
  • fn

    The constructor function to wrap.
Returns function A wrapped, curried constructor function.

Added in v0.1.0

Wraps a constructor function inside a curried function that can be called with the same arguments and returns the same type.

See also invoker. 
// Constructor function
function Animal(kind) {
  this.kind = kind;
};
Animal.prototype.sighting = function() {
  return "It's a " + this.kind + "!";
}

const AnimalConstructor = R.construct(Animal)

// Notice we no longer need the 'new' keyword:
AnimalConstructor('Pig'); //=> {"kind": "Pig", "sighting": function (){...}};

const animalTypes = ["Lion", "Tiger", "Bear"];
const animalSighting = R.invoker(0, 'sighting');
const sightNewAnimal = R.compose(animalSighting, AnimalConstructor);
R.map(sightNewAnimal, animalTypes); //=> ["It's a Lion!", "It's a Tiger!", "It's a Bear!"]

constructN

Parameters
  • n

    The arity of the constructor function.
  • Fn

    The constructor function to wrap.
Returns function A wrapped, curried constructor function.

Added in v0.4.0

Wraps a constructor function inside a curried function that can be called with the same arguments and returns the same type. The arity of the function returned is specified to allow using variadic constructor functions.

// Variadic Constructor function
function Salad() {
  this.ingredients = arguments;
}

Salad.prototype.recipe = function() {
  const instructions = R.map(ingredient => 'Add a dollop of ' + ingredient, this.ingredients);
  return R.join('\n', instructions);
};

const ThreeLayerSalad = R.constructN(3, Salad);

// Notice we no longer need the 'new' keyword, and the constructor is curried for 3 arguments.
const salad = ThreeLayerSalad('Mayonnaise')('Potato Chips')('Ketchup');

console.log(salad.recipe());
// Add a dollop of Mayonnaise
// Add a dollop of Potato Chips
// Add a dollop of Ketchup

converge

Parameters
  • after

    A function. after will be invoked with the return values of fn1 and fn2 as its arguments.
  • functions

    A list of functions.
Returns function A new function.

Added in v0.4.2

Accepts a converging function and a list of branching functions and returns a new function. The arity of the new function is the same as the arity of the longest branching function. When invoked, this new function is applied to some arguments, and each branching function is applied to those same arguments. The results of each branching function are passed as arguments to the converging function to produce the return value.

See also useWith. 
const average = R.converge(R.divide, [R.sum, R.length])
average([1, 2, 3, 4, 5, 6, 7]) //=> 4

const strangeConcat = R.converge(R.concat, [R.toUpper, R.toLower])
strangeConcat("Yodel") //=> "YODELyodel"

count

Parameters
  • predicate

    to match items against
Returns Array list of items to count in

Added in v0.28.0

Returns the number of items in a given list matching the predicate f

const even = x => x % 2 == 0;

R.count(even, [1, 2, 3, 4, 5]); // => 2
R.map(R.count(even), [[1, 1, 1], [2, 3, 4, 5], [6]]); // => [0, 2, 1]

countBy

Parameters
  • fn

    The function used to map values to keys.
  • list

    The list to count elements from.
Returns Object An object mapping keys to number of occurrences in the list.

Added in v0.1.0

Counts the elements of a list according to how many match each value of a key generated by the supplied function. Returns an object mapping the keys produced by fn to the number of occurrences in the list. Note that all keys are coerced to strings because of how JavaScript objects work.

Acts as a transducer if a transformer is given in list position.

const numbers = [1.0, 1.1, 1.2, 2.0, 3.0, 2.2];
R.countBy(Math.floor)(numbers);    //=> {'1': 3, '2': 2, '3': 1}

const letters = ['a', 'b', 'A', 'a', 'B', 'c'];
R.countBy(R.toLower)(letters);   //=> {'a': 3, 'b': 2, 'c': 1}

curry

Parameters
  • fn

    The function to curry.
Returns function A new, curried function.

Added in v0.1.0

Returns a curried equivalent of the provided function. The curried function has two unusual capabilities. First, its arguments needn't be provided one at a time. If f is a ternary function and g is R.curry(f), the following are equivalent:

  • g(1)(2)(3)
  • g(1)(2, 3)
  • g(1, 2)(3)
  • g(1, 2, 3)

Secondly, the special placeholder value R.__ may be used to specify "gaps", allowing partial application of any combination of arguments, regardless of their positions. If g is as above and _ is R.__, the following are equivalent:

  • g(1, 2, 3)
  • g(_, 2, 3)(1)
  • g(_, _, 3)(1)(2)
  • g(_, _, 3)(1, 2)
  • g(_, 2)(1)(3)
  • g(_, 2)(1, 3)
  • g(_, 2)(_, 3)(1)
See also curryN, partial. 
const addFourNumbers = (a, b, c, d) => a + b + c + d;

const curriedAddFourNumbers = R.curry(addFourNumbers);
const f = curriedAddFourNumbers(1, 2);
const g = f(3);
g(4); //=> 10

curryN

Parameters
  • length

    The arity for the returned function.
  • fn

    The function to curry.
Returns function A new, curried function.

Added in v0.5.0

Returns a curried equivalent of the provided function, with the specified arity. The curried function has two unusual capabilities. First, its arguments needn't be provided one at a time. If g is R.curryN(3, f), the following are equivalent:

  • g(1)(2)(3)
  • g(1)(2, 3)
  • g(1, 2)(3)
  • g(1, 2, 3)

Secondly, the special placeholder value R.__ may be used to specify "gaps", allowing partial application of any combination of arguments, regardless of their positions. If g is as above and _ is R.__, the following are equivalent:

  • g(1, 2, 3)
  • g(_, 2, 3)(1)
  • g(_, _, 3)(1)(2)
  • g(_, _, 3)(1, 2)
  • g(_, 2)(1)(3)
  • g(_, 2)(1, 3)
  • g(_, 2)(_, 3)(1)
See also curry. 
const sumArgs = (...args) => R.sum(args);

const curriedAddFourNumbers = R.curryN(4, sumArgs);
const f = curriedAddFourNumbers(1, 2);
const g = f(3);
g(4); //=> 10

dec

Parameters
  • n
Returns Number n - 1

Added in v0.9.0

Decrements its argument.

See also inc. 
R.dec(42); //=> 41

defaultTo

Parameters
  • default

    The default value.
  • val

    val will be returned instead of default unless val is null, undefined or NaN.
Returns * The second value if it is not `null`, `undefined` or `NaN`, otherwise the default value

Added in v0.10.0

Returns the second argument if it is not null, undefined or NaN; otherwise the first argument is returned.

const defaultTo42 = R.defaultTo(42);

defaultTo42(null);  //=> 42
defaultTo42(undefined);  //=> 42
defaultTo42(false);  //=> false
defaultTo42('Ramda');  //=> 'Ramda'
// parseInt('string') results in NaN
defaultTo42(parseInt('string')); //=> 42

descend

Parameters
  • fn

    A function of arity one that returns a value that can be compared
  • a

    The first item to be compared.
  • b

    The second item to be compared.
Returns Number `-1` if fn(a) > fn(b), `1` if fn(b) > fn(a), otherwise `0`

Added in v0.23.0

Makes a descending comparator function out of a function that returns a value that can be compared with < and >.

See also ascend. 
const byAge = R.descend(R.prop('age'));
const people = [
  { name: 'Emma', age: 70 },
  { name: 'Peter', age: 78 },
  { name: 'Mikhail', age: 62 },
];
const peopleByOldestFirst = R.sort(byAge, people);
  //=> [{ name: 'Peter', age: 78 }, { name: 'Emma', age: 70 }, { name: 'Mikhail', age: 62 }]

difference

Parameters
  • list1

    The first list.
  • list2

    The second list.
Returns Array The elements in `list1` that are not in `list2`.

Added in v0.1.0

Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. Objects and Arrays are compared in terms of value equality, not reference equality.

See also differenceWith, symmetricDifference, symmetricDifferenceWith, without. 
R.difference([1,2,3,4], [7,6,5,4,3]); //=> [1,2]
R.difference([7,6,5,4,3], [1,2,3,4]); //=> [7,6,5]
R.difference([{a: 1}, {b: 2}], [{a: 1}, {c: 3}]) //=> [{b: 2}]

differenceWith

Parameters
  • pred

    A predicate used to test whether two items are equal.
  • list1

    The first list.
  • list2

    The second list.
Returns Array The elements in `list1` that are not in `list2`.

Added in v0.1.0

Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.

See also difference, symmetricDifference, symmetricDifferenceWith. 
const cmp = (x, y) => x.a === y.a;
const l1 = [{a: 1}, {a: 2}, {a: 3}];
const l2 = [{a: 3}, {a: 4}];
R.differenceWith(cmp, l1, l2); //=> [{a: 1}, {a: 2}]

dissoc

Parameters
  • prop

    The name of the property to dissociate
  • obj

    The object to clone
Returns Object A new object equivalent to the original but without the specified property

Added in v0.10.0

Returns a new object that does not contain a prop property.

See also assoc, omit. 
R.dissoc('b', {a: 1, b: 2, c: 3}); //=> {a: 1, c: 3}

dissocPath

Parameters
  • path

    The path to the value to omit
  • obj

    The object to clone
Returns Object A new object without the property at path

Added in v0.11.0

Makes a shallow clone of an object, omitting the property at the given path. Note that this copies and flattens prototype properties onto the new object as well. All non-primitive properties are copied by reference.

See also assocPath. 
R.dissocPath(['a', 'b', 'c'], {a: {b: {c: 42}}}); //=> {a: {b: {}}}

divide

Parameters
  • a

    The first value.
  • b

    The second value.
Returns Number The result of `a / b`.

Added in v0.1.0

Divides two numbers. Equivalent to a / b.

See also multiply. 
R.divide(71, 100); //=> 0.71

const half = R.divide(R.__, 2);
half(42); //=> 21

const reciprocal = R.divide(1);
reciprocal(4);   //=> 0.25

drop

Parameters
  • n
  • list
Returns * A copy of list without the first `n` elements

Added in v0.1.0

Returns all but the first n elements of the given list, string, or transducer/transformer (or object with a drop method).

Dispatches to the drop method of the second argument, if present.

See also take, transduce, dropLast, dropWhile. 
R.drop(1, ['foo', 'bar', 'baz']); //=> ['bar', 'baz']
R.drop(2, ['foo', 'bar', 'baz']); //=> ['baz']
R.drop(3, ['foo', 'bar', 'baz']); //=> []
R.drop(4, ['foo', 'bar', 'baz']); //=> []
R.drop(3, 'ramda');               //=> 'da'

dropLast

Parameters
  • n

    The number of elements of list to skip.
  • list

    The list of elements to consider.
Returns Array A copy of the list with only the first `list.length - n` elements

Added in v0.16.0

Returns a list containing all but the last n elements of the given list.

Acts as a transducer if a transformer is given in list position.

See also takeLast, drop, dropWhile, dropLastWhile. 
R.dropLast(1, ['foo', 'bar', 'baz']); //=> ['foo', 'bar']
R.dropLast(2, ['foo', 'bar', 'baz']); //=> ['foo']
R.dropLast(3, ['foo', 'bar', 'baz']); //=> []
R.dropLast(4, ['foo', 'bar', 'baz']); //=> []
R.dropLast(3, 'ramda');               //=> 'ra'

dropLastWhile

Parameters
  • predicate

    The function to be called on each element
  • xs

    The collection to iterate over.
Returns Array A new array without any trailing elements that return `falsy` values from the `predicate`.

Added in v0.16.0

Returns a new list excluding all the tailing elements of a given list which satisfy the supplied predicate function. It passes each value from the right to the supplied predicate function, skipping elements until the predicate function returns a falsy value. The predicate function is applied to one argument: (value).

Acts as a transducer if a transformer is given in list position.

See also takeLastWhile, addIndex, drop, dropWhile. 
const lteThree = x => x <= 3;

R.dropLastWhile(lteThree, [1, 2, 3, 4, 3, 2, 1]); //=> [1, 2, 3, 4]

R.dropLastWhile(x => x !== 'd' , 'Ramda'); //=> 'Ramd'

dropRepeats

Parameters
  • list

    The array to consider.
Returns Array `list` without repeating elements.

Added in v0.14.0

Returns a new list without any consecutively repeating elements. R.equals is used to determine equality.

Acts as a transducer if a transformer is given in list position.

See also transduce. 
R.dropRepeats([1, 1, 1, 2, 3, 4, 4, 2, 2]); //=> [1, 2, 3, 4, 2]

dropRepeatsWith

Parameters
  • pred

    A predicate used to test whether two items are equal.
  • list

    The array to consider.
Returns Array `list` without repeating elements.

Added in v0.14.0

Returns a new list without any consecutively repeating elements. Equality is determined by applying the supplied predicate to each pair of consecutive elements. The first element in a series of equal elements will be preserved.

Acts as a transducer if a transformer is given in list position.

See also transduce. 
const l = [1, -1, 1, 3, 4, -4, -4, -5, 5, 3, 3];
R.dropRepeatsWith(R.eqBy(Math.abs), l); //=> [1, 3, 4, -5, 3]

dropWhile

Parameters
  • fn

    The function called per iteration.
  • xs

    The collection to iterate over.
Returns Array A new array.

Added in v0.9.0

Returns a new list excluding the leading elements of a given list which satisfy the supplied predicate function. It passes each value to the supplied predicate function, skipping elements while the predicate function returns true. The predicate function is applied to one argument: (value).

Dispatches to the dropWhile method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also takeWhile, transduce, addIndex. 
const lteTwo = x => x <= 2;

R.dropWhile(lteTwo, [1, 2, 3, 4, 3, 2, 1]); //=> [3, 4, 3, 2, 1]

R.dropWhile(x => x !== 'd' , 'Ramda'); //=> 'da'

either

Parameters
  • f

    a predicate
  • g

    another predicate
Returns function a function that applies its arguments to `f` and `g` and `||`s their outputs together.

Added in v0.12.0

A function wrapping calls to the two functions in an || operation, returning the result of the first function if it is truth-y and the result of the second function otherwise. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a truth-y value.

In addition to functions, R.either also accepts any fantasy-land compatible applicative functor.

See also both, or. 
const gt10 = x => x > 10;
const even = x => x % 2 === 0;
const f = R.either(gt10, even);
f(101); //=> true
f(8); //=> true

R.either(Maybe.Just(false), Maybe.Just(55)); // => Maybe.Just(55)
R.either([false, false, 'a'], [11]) // => [11, 11, "a"]

empty

Parameters
  • x
Returns *

Added in v0.3.0

Returns the empty value of its argument's type. Ramda defines the empty value of Array ([]), Object ({}), String (''), TypedArray (Uint8Array [], Float32Array [], etc), and Arguments. Other types are supported if they define <Type>.empty, <Type>.prototype.empty or implement the FantasyLand Monoid spec.

Dispatches to the empty method of the first argument, if present.

R.empty(Just(42));               //=> Nothing()
R.empty([1, 2, 3]);              //=> []
R.empty('unicorns');             //=> ''
R.empty({x: 1, y: 2});           //=> {}
R.empty(Uint8Array.from('123')); //=> Uint8Array []

endsWith

Parameters
  • suffix
  • list
Returns Boolean

Added in v0.24.0

Checks if a list ends with the provided sublist.

Similarly, checks if a string ends with the provided substring.

See also startsWith. 
R.endsWith('c', 'abc')                //=> true
R.endsWith('b', 'abc')                //=> false
R.endsWith(['c'], ['a', 'b', 'c'])    //=> true
R.endsWith(['b'], ['a', 'b', 'c'])    //=> false

eqBy

Parameters
  • f
  • x
  • y
Returns Boolean

Added in v0.18.0

Takes a function and two values in its domain and returns true if the values map to the same value in the codomain; false otherwise.

R.eqBy(Math.abs, 5, -5); //=> true

eqProps

Parameters
  • prop

    The name of the property to compare
  • obj1
  • obj2
Returns Boolean

Added in v0.1.0

Reports whether two objects have the same value, in R.equals terms, for the specified property. Useful as a curried predicate.

const o1 = { a: 1, b: 2, c: 3, d: 4 };
const o2 = { a: 10, b: 20, c: 3, d: 40 };
R.eqProps('a', o1, o2); //=> false
R.eqProps('c', o1, o2); //=> true

equals

Parameters
  • a
  • b
Returns Boolean

Added in v0.15.0

Returns true if its arguments are equivalent, false otherwise. Handles cyclical data structures.

Dispatches symmetrically to the equals methods of both arguments, if present.

R.equals(1, 1); //=> true
R.equals(1, '1'); //=> false
R.equals([1, 2, 3], [1, 2, 3]); //=> true

const a = {}; a.v = a;
const b = {}; b.v = b;
R.equals(a, b); //=> true

evolve

Parameters
  • transformations

    The object specifying transformation functions to apply to the object.
  • object

    The object to be transformed.
Returns Object The transformed object.

Added in v0.9.0

Creates a new object by recursively evolving a shallow copy of object, according to the transformation functions. All non-primitive properties are copied by reference.

A transformation function will not be invoked if its corresponding key does not exist in the evolved object.

const tomato = {firstName: '  Tomato ', data: {elapsed: 100, remaining: 1400}, id:123};
const transformations = {
  firstName: R.trim,
  lastName: R.trim, // Will not get invoked.
  data: {elapsed: R.add(1), remaining: R.add(-1)}
};
R.evolve(transformations, tomato); //=> {firstName: 'Tomato', data: {elapsed: 101, remaining: 1399}, id:123}

F

Parameters
Returns Boolean

Added in v0.9.0

A function that always returns false. Any passed in parameters are ignored.

See also T. 
R.F(); //=> false

filter

Parameters
  • pred
  • filterable
Returns Array Filterable

Added in v0.1.0

Takes a predicate and a Filterable, and returns a new filterable of the same type containing the members of the given filterable which satisfy the given predicate. Filterable objects include plain objects or any object that has a filter method such as Array.

Dispatches to the filter method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also reject, transduce, addIndex. 
const isEven = n => n % 2 === 0;

R.filter(isEven, [1, 2, 3, 4]); //=> [2, 4]

R.filter(isEven, {a: 1, b: 2, c: 3, d: 4}); //=> {b: 2, d: 4}

find

Parameters
  • fn

    The predicate function used to determine if the element is the desired one.
  • list

    The array to consider.
Returns Object The element found, or `undefined`.

Added in v0.1.0

Returns the first element of the list which matches the predicate, or undefined if no element matches.

Dispatches to the find method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also transduce. 
const xs = [{a: 1}, {a: 2}, {a: 3}];
R.find(R.propEq('a', 2))(xs); //=> {a: 2}
R.find(R.propEq('a', 4))(xs); //=> undefined

findIndex

Parameters
  • fn

    The predicate function used to determine if the element is the desired one.
  • list

    The array to consider.
Returns Number The index of the element found, or `-1`.

Added in v0.1.1

Returns the index of the first element of the list which matches the predicate, or -1 if no element matches.

Acts as a transducer if a transformer is given in list position.

See also transduce, indexOf. 
const xs = [{a: 1}, {a: 2}, {a: 3}];
R.findIndex(R.propEq('a', 2))(xs); //=> 1
R.findIndex(R.propEq('a', 4))(xs); //=> -1

findLast

Parameters
  • fn

    The predicate function used to determine if the element is the desired one.
  • list

    The array to consider.
Returns Object The element found, or `undefined`.

Added in v0.1.1

Returns the last element of the list which matches the predicate, or undefined if no element matches.

Acts as a transducer if a transformer is given in list position.

See also transduce. 
const xs = [{a: 1, b: 0}, {a:1, b: 1}];
R.findLast(R.propEq('a', 1))(xs); //=> {a: 1, b: 1}
R.findLast(R.propEq('a', 4))(xs); //=> undefined

findLastIndex

Parameters
  • fn

    The predicate function used to determine if the element is the desired one.
  • list

    The array to consider.
Returns Number The index of the element found, or `-1`.

Added in v0.1.1

Returns the index of the last element of the list which matches the predicate, or -1 if no element matches.

Acts as a transducer if a transformer is given in list position.

See also transduce, lastIndexOf. 
const xs = [{a: 1, b: 0}, {a:1, b: 1}];
R.findLastIndex(R.propEq('a', 1))(xs); //=> 1
R.findLastIndex(R.propEq('a', 4))(xs); //=> -1

flatten

Parameters
  • list

    The array to consider.
Returns Array The flattened list.

Added in v0.1.0

Returns a new list by pulling every item out of it (and all its sub-arrays) and putting them in a new array, depth-first.

See also unnest. 
R.flatten([1, 2, [3, 4], 5, [6, [7, 8, [9, [10, 11], 12]]]]);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

flip

Parameters
  • fn

    The function to invoke with its first two parameters reversed.
Returns * The result of invoking `fn` with its first two parameters' order reversed.

Added in v0.1.0

Returns a new function much like the supplied one, except that the first two arguments' order is reversed.

const mergeThree = (a, b, c) => [].concat(a, b, c);

mergeThree(1, 2, 3); //=> [1, 2, 3]

R.flip(mergeThree)(1, 2, 3); //=> [2, 1, 3]

forEach

Parameters
  • fn

    The function to invoke. Receives one argument, value.
  • list

    The list to iterate over.
Returns Array The original list.

Added in v0.1.1

Iterate over an input list, calling a provided function fn for each element in the list.

fn receives one argument: (value).

Note: R.forEach does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.forEach method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach#Description

Also note that, unlike Array.prototype.forEach, Ramda's forEach returns the original array. In some libraries this function is named each.

Dispatches to the forEach method of the second argument, if present.

See also addIndex. 
const printXPlusFive = x => console.log(x + 5);
R.forEach(printXPlusFive, [1, 2, 3]); //=> [1, 2, 3]
// logs 6
// logs 7
// logs 8

forEachObjIndexed

Parameters
  • fn

    The function to invoke. Receives three argument, value, key, obj.
  • obj

    The object to iterate over.
Returns Object The original object.

Added in v0.23.0

Iterate over an input object, calling a provided function fn for each key and value in the object.

fn receives three argument: (value, key, obj).

const printKeyConcatValue = (value, key) => console.log(key + ':' + value);
R.forEachObjIndexed(printKeyConcatValue, {x: 1, y: 2}); //=> {x: 1, y: 2}
// logs x:1
// logs y:2

fromPairs

Parameters
  • pairs

    An array of two-element arrays that will be the keys and values of the output object.
Returns Object The object made by pairing up `keys` and `values`.

Added in v0.3.0

Creates a new object from a list key-value pairs. If a key appears in multiple pairs, the rightmost pair is included in the object.

See also toPairs, pair. 
R.fromPairs([['a', 1], ['b', 2], ['c', 3]]); //=> {a: 1, b: 2, c: 3}

groupBy

Parameters
  • fn

    Function :: a -> Idx
  • list

    The array to group
Returns Object An object with the output of `fn` for keys, mapped to arrays of elements that produced that key when passed to `fn`.

Added in v0.1.0

Splits a list into sub-lists stored in an object, based on the result of calling a key-returning function on each element, and grouping the results according to values returned.

Dispatches to the groupBy method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also reduceBy, transduce, indexBy. 
const byGrade = R.groupBy(function(student) {
  const score = student.score;
  return score < 65 ? 'F' :
         score < 70 ? 'D' :
         score < 80 ? 'C' :
         score < 90 ? 'B' : 'A';
});
const students = [{name: 'Abby', score: 84},
                {name: 'Eddy', score: 58},
                // ...
                {name: 'Jack', score: 69}];
byGrade(students);
// {
//   'A': [{name: 'Dianne', score: 99}],
//   'B': [{name: 'Abby', score: 84}]
//   // ...,
//   'F': [{name: 'Eddy', score: 58}]
// }

groupWith

Parameters
  • fn

    Function for determining whether two given (adjacent) elements should be in the same group
  • list

    The array to group. Also accepts a string, which will be treated as a list of characters.
Returns List A list that contains sublists of elements, whose concatenations are equal to the original list.

Added in v0.21.0

Takes a list and returns a list of lists where each sublist's elements are all satisfied pairwise comparison according to the provided function. Only adjacent elements are passed to the comparison function.

R.groupWith(R.equals, [0, 1, 1, 2, 3, 5, 8, 13, 21])
//=> [[0], [1, 1], [2], [3], [5], [8], [13], [21]]

R.groupWith((a, b) => a + 1 === b, [0, 1, 1, 2, 3, 5, 8, 13, 21])
//=> [[0, 1], [1, 2, 3], [5], [8], [13], [21]]

R.groupWith((a, b) => a % 2 === b % 2, [0, 1, 1, 2, 3, 5, 8, 13, 21])
//=> [[0], [1, 1], [2], [3, 5], [8], [13, 21]]

const isVowel = R.test(/^[aeiou]$/i);
R.groupWith(R.eqBy(isVowel), 'aestiou')
//=> ['ae', 'st', 'iou']

gt

Parameters
  • a
  • b
Returns Boolean

Added in v0.1.0

Returns true if the first argument is greater than the second; false otherwise.

See also lt. 
R.gt(2, 1); //=> true
R.gt(2, 2); //=> false
R.gt(2, 3); //=> false
R.gt('a', 'z'); //=> false
R.gt('z', 'a'); //=> true

gte

Parameters
  • a
  • b
Returns Boolean

Added in v0.1.0

Returns true if the first argument is greater than or equal to the second; false otherwise.

See also lte. 
R.gte(2, 1); //=> true
R.gte(2, 2); //=> true
R.gte(2, 3); //=> false
R.gte('a', 'z'); //=> false
R.gte('z', 'a'); //=> true

has

Parameters
  • prop

    The name of the property to check for.
  • obj

    The object to query.
Returns Boolean Whether the property exists.

Added in v0.7.0

Returns whether or not an object has an own property with the specified name

const hasName = R.has('name');
hasName({name: 'alice'});   //=> true
hasName({name: 'bob'});     //=> true
hasName({});                //=> false

const point = {x: 0, y: 0};
const pointHas = R.has(R.__, point);
pointHas('x');  //=> true
pointHas('y');  //=> true
pointHas('z');  //=> false

hasIn

Parameters
  • prop

    The name of the property to check for.
  • obj

    The object to query.
Returns Boolean Whether the property exists.

Added in v0.7.0

Returns whether or not an object or its prototype chain has a property with the specified name

function Rectangle(width, height) {
  this.width = width;
  this.height = height;
}
Rectangle.prototype.area = function() {
  return this.width * this.height;
};

const square = new Rectangle(2, 2);
R.hasIn('width', square);  //=> true
R.hasIn('area', square);  //=> true

hasPath

Parameters
  • path

    The path to use.
  • obj

    The object to check the path in.
Returns Boolean Whether the path exists.

Added in v0.26.0

Returns whether or not a path exists in an object. Only the object's own properties are checked.

See also has. 
R.hasPath(['a', 'b'], {a: {b: 2}});         // => true
R.hasPath(['a', 'b'], {a: {b: undefined}}); // => true
R.hasPath(['a', 'b'], {a: {c: 2}});         // => false
R.hasPath(['a', 'b'], {});                  // => false

head

Parameters
  • list
Returns *

Added in v0.1.0

Returns the first element of the given list or string. In some libraries this function is named first.

See also tail, init, last. 
R.head(['fi', 'fo', 'fum']); //=> 'fi'
R.head([]); //=> undefined

R.head('abc'); //=> 'a'
R.head(''); //=> ''

identical

Parameters
  • a
  • b
Returns Boolean

Added in v0.15.0

Returns true if its arguments are identical, false otherwise. Values are identical if they reference the same memory. NaN is identical to NaN; 0 and -0 are not identical.

Note this is merely a curried version of ES6 Object.is.

const o = {};
R.identical(o, o); //=> true
R.identical(1, 1); //=> true
R.identical(1, '1'); //=> false
R.identical([], []); //=> false
R.identical(0, -0); //=> false
R.identical(NaN, NaN); //=> true

identity

Parameters
  • x

    The value to return.
Returns * The input value, `x`.

Added in v0.1.0

A function that does nothing but return the parameter supplied to it. Good as a default or placeholder function.

R.identity(1); //=> 1

const obj = {};
R.identity(obj) === obj; //=> true

ifElse

Parameters
  • condition

    A predicate function
  • onTrue

    A function to invoke when the condition evaluates to a truthy value.
  • onFalse

    A function to invoke when the condition evaluates to a falsy value.
Returns function A new function that will process either the `onTrue` or the `onFalse` function depending upon the result of the `condition` predicate.

Added in v0.8.0

Creates a function that will process either the onTrue or the onFalse function depending upon the result of the condition predicate.

See also unless, when, cond. 
const incCount = R.ifElse(
  R.has('count'),
  R.over(R.lensProp('count'), R.inc),
  R.assoc('count', 1)
);
incCount({ count: 1 }); //=> { count: 2 }
incCount({});           //=> { count: 1 }

inc

Parameters
  • n
Returns Number n + 1

Added in v0.9.0

Increments its argument.

See also dec. 
R.inc(42); //=> 43

includes

Parameters
  • a

    The item to compare against.
  • list

    The array to consider.
Returns Boolean `true` if an equivalent item is in the list, `false` otherwise.

Added in v0.26.0

Returns true if the specified value is equal, in R.equals terms, to at least one element of the given list; false otherwise. Also works with strings.

See also any. 
R.includes(3, [1, 2, 3]); //=> true
R.includes(4, [1, 2, 3]); //=> false
R.includes({ name: 'Fred' }, [{ name: 'Fred' }]); //=> true
R.includes([42], [[42]]); //=> true
R.includes('ba', 'banana'); //=>true

indexBy

Parameters
  • fn

    Function :: a -> Idx
  • array

    The array of objects to index
Returns Object An object indexing each array element by the given property.

Added in v0.19.0

Given a function that generates a key, turns a list of objects into an object indexing the objects by the given key. Note that if multiple objects generate the same value for the indexing key only the last value will be included in the generated object.

Acts as a transducer if a transformer is given in list position.

See also groupBy. 
const list = [{id: 'xyz', title: 'A'}, {id: 'abc', title: 'B'}];
R.indexBy(R.prop('id'), list);
//=> {abc: {id: 'abc', title: 'B'}, xyz: {id: 'xyz', title: 'A'}}

indexOf

Parameters
  • target

    The item to find.
  • xs

    The array to search in.
Returns Number the index of the target, or -1 if the target is not found.

Added in v0.1.0

Returns the position of the first occurrence of an item in an array, or -1 if the item is not included in the array. R.equals is used to determine equality.

See also lastIndexOf, findIndex. 
R.indexOf(3, [1,2,3,4]); //=> 2
R.indexOf(10, [1,2,3,4]); //=> -1

init

Parameters
  • list
Returns *

Added in v0.9.0

Returns all but the last element of the given list or string.

See also last, head, tail. 
R.init([1, 2, 3]);  //=> [1, 2]
R.init([1, 2]);     //=> [1]
R.init([1]);        //=> []
R.init([]);         //=> []

R.init('abc');  //=> 'ab'
R.init('ab');   //=> 'a'
R.init('a');    //=> ''
R.init('');     //=> ''

innerJoin

Parameters
  • pred
  • xs
  • ys
Returns Array

Added in v0.24.0

Takes a predicate pred, a list xs, and a list ys, and returns a list xs' comprising each of the elements of xs which is equal to one or more elements of ys according to pred.

pred must be a binary function expecting an element from each list.

xs, ys, and xs' are treated as sets, semantically, so ordering should not be significant, but since xs' is ordered the implementation guarantees that its values are in the same order as they appear in xs. Duplicates are not removed, so xs' may contain duplicates if xs contains duplicates.

See also intersection. 
R.innerJoin(
  (record, id) => record.id === id,
  [{id: 824, name: 'Richie Furay'},
   {id: 956, name: 'Dewey Martin'},
   {id: 313, name: 'Bruce Palmer'},
   {id: 456, name: 'Stephen Stills'},
   {id: 177, name: 'Neil Young'}],
  [177, 456, 999]
);
//=> [{id: 456, name: 'Stephen Stills'}, {id: 177, name: 'Neil Young'}]

insert

Parameters
  • index

    The position to insert the element
  • elt

    The element to insert into the Array
  • list

    The list to insert into
Returns Array A new Array with `elt` inserted at `index`.

Added in v0.2.2

Inserts the supplied element into the list, at the specified index. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.

R.insert(2, 'x', [1,2,3,4]); //=> [1,2,'x',3,4]

insertAll

Parameters
  • index

    The position to insert the sub-list
  • elts

    The sub-list to insert into the Array
  • list

    The list to insert the sub-list into
Returns Array A new Array with `elts` inserted starting at `index`.

Added in v0.9.0

Inserts the sub-list into the list, at the specified index. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.

R.insertAll(2, ['x','y','z'], [1,2,3,4]); //=> [1,2,'x','y','z',3,4]

intersection

Parameters
  • list1

    The first list.
  • list2

    The second list.
Returns Array The list of elements found in both `list1` and `list2`.

Added in v0.1.0

Combines two lists into a set (i.e. no duplicates) composed of those elements common to both lists.

See also innerJoin. 
R.intersection([1,2,3,4], [7,6,5,4,3]); //=> [4, 3]

intersperse

Parameters
  • separator

    The element to add to the list.
  • list

    The list to be interposed.
Returns Array The new list.

Added in v0.14.0

Creates a new list with the separator interposed between elements.

Dispatches to the intersperse method of the second argument, if present.

R.intersperse('a', ['b', 'n', 'n', 's']); //=> ['b', 'a', 'n', 'a', 'n', 'a', 's']

into

Parameters
  • acc

    The initial accumulator value.
  • xf

    The transducer function. Receives a transformer and returns a transformer.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.12.0

Transforms the items of the list with the transducer and appends the transformed items to the accumulator using an appropriate iterator function based on the accumulator type.

The accumulator can be an array, string, object or a transformer. Iterated items will be appended to arrays and concatenated to strings. Objects will be merged directly or 2-item arrays will be merged as key, value pairs.

The accumulator can also be a transformer object that provides a 2-arity reducing iterator function, step, 0-arity initial value function, init, and 1-arity result extraction function result. The step function is used as the iterator function in reduce. The result function is used to convert the final accumulator into the return type and in most cases is R.identity. The init function is used to provide the initial accumulator.

The iteration is performed with R.reduce after initializing the transducer.

See also transduce. 
const numbers = [1, 2, 3, 4];
const transducer = R.compose(R.map(R.add(1)), R.take(2));

R.into([], transducer, numbers); //=> [2, 3]

const intoArray = R.into([]);
intoArray(transducer, numbers); //=> [2, 3]

invert

Parameters
  • obj

    The object or array to invert
Returns Object out A new object with keys in an array.

Added in v0.9.0

Same as R.invertObj, however this accounts for objects with duplicate values by putting the values into an array.

See also invertObj. 
const raceResultsByFirstName = {
  first: 'alice',
  second: 'jake',
  third: 'alice',
};
R.invert(raceResultsByFirstName);
//=> { 'alice': ['first', 'third'], 'jake':['second'] }

invertObj

Parameters
  • obj

    The object or array to invert
Returns Object out A new object

Added in v0.9.0

Returns a new object with the keys of the given object as values, and the values of the given object, which are coerced to strings, as keys. Note that the last key found is preferred when handling the same value.

See also invert. 
const raceResults = {
  first: 'alice',
  second: 'jake'
};
R.invertObj(raceResults);
//=> { 'alice': 'first', 'jake':'second' }

// Alternatively:
const raceResults = ['alice', 'jake'];
R.invertObj(raceResults);
//=> { 'alice': '0', 'jake':'1' }

invoker

Parameters
  • arity

    Number of arguments the returned function should take before the target object.
  • method

    Name of any of the target object's methods to call.
Returns function A new curried function.

Added in v0.1.0

Turns a named method with a specified arity into a function that can be called directly supplied with arguments and a target object.

The returned function is curried and accepts arity + 1 parameters where the final parameter is the target object.

See also construct. 
const sliceFrom = R.invoker(1, 'slice');
sliceFrom(6, 'abcdefghijklm'); //=> 'ghijklm'
const sliceFrom6 = R.invoker(2, 'slice')(6);
sliceFrom6(8, 'abcdefghijklm'); //=> 'gh'

const dog = {
  speak: async () => 'Woof!'
};
const speak = R.invoker(0, 'speak');
speak(dog).then(console.log) //~> 'Woof!'

is

Parameters
  • ctor

    A constructor
  • val

    The value to test
Returns Boolean

Added in v0.3.0

See if an object (i.e. val) is an instance of the supplied constructor. This function will check up the inheritance chain, if any. If val was created using Object.create, R.is(Object, val) === true.

R.is(Object, {}); //=> true
R.is(Number, 1); //=> true
R.is(Object, 1); //=> false
R.is(String, 's'); //=> true
R.is(String, new String('')); //=> true
R.is(Object, new String('')); //=> true
R.is(Object, 's'); //=> false
R.is(Number, {}); //=> false

isEmpty

Parameters
  • x
Returns Boolean

Added in v0.1.0

Returns true if the given value is its type's empty value; false otherwise.

See also empty. 
R.isEmpty([1, 2, 3]);           //=> false
R.isEmpty([]);                  //=> true
R.isEmpty('');                  //=> true
R.isEmpty(null);                //=> false
R.isEmpty({});                  //=> true
R.isEmpty({length: 0});         //=> false
R.isEmpty(Uint8Array.from('')); //=> true

isNil

Parameters
  • x

    The value to test.
Returns Boolean `true` if `x` is `undefined` or `null`, otherwise `false`.

Added in v0.9.0

Checks if the input value is null or undefined.

R.isNil(null); //=> true
R.isNil(undefined); //=> true
R.isNil(0); //=> false
R.isNil([]); //=> false

join

Parameters
  • separator

    The string used to separate the elements.
  • xs

    The elements to join into a string.
Returns String str The string made by concatenating `xs` with `separator`.

Added in v0.1.0

Returns a string made by inserting the separator between each element and concatenating all the elements into a single string.

See also split. 
const spacer = R.join(' ');
spacer(['a', 2, 3.4]);   //=> 'a 2 3.4'
R.join('|', [1, 2, 3]);    //=> '1|2|3'

juxt

Parameters
  • fns

    An array of functions
Returns function A function that returns a list of values after applying each of the original `fns` to its parameters.

Added in v0.19.0

juxt applies a list of functions to a list of values.

See also applySpec. 
const getRange = R.juxt([Math.min, Math.max]);
getRange(3, 4, 9, -3); //=> [-3, 9]

keys

Parameters
  • obj

    The object to extract properties from
Returns Array An array of the object's own properties.

Added in v0.1.0

Returns a list containing the names of all the enumerable own properties of the supplied object. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.

See also keysIn, values, toPairs. 
R.keys({a: 1, b: 2, c: 3}); //=> ['a', 'b', 'c']

keysIn

Parameters
  • obj

    The object to extract properties from
Returns Array An array of the object's own and prototype properties.

Added in v0.2.0

Returns a list containing the names of all the properties of the supplied object, including prototype properties. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.

See also keys, valuesIn. 
const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.keysIn(f); //=> ['x', 'y']

last

Parameters
  • list
Returns *

Added in v0.1.4

Returns the last element of the given list or string.

See also init, head, tail. 
R.last(['fi', 'fo', 'fum']); //=> 'fum'
R.last([]); //=> undefined

R.last('abc'); //=> 'c'
R.last(''); //=> ''

lastIndexOf

Parameters
  • target

    The item to find.
  • xs

    The array to search in.
Returns Number the index of the target, or -1 if the target is not found.

Added in v0.1.0

Returns the position of the last occurrence of an item in an array, or -1 if the item is not included in the array. R.equals is used to determine equality.

See also indexOf, findLastIndex. 
R.lastIndexOf(3, [-1,3,3,0,1,2,3,4]); //=> 6
R.lastIndexOf(10, [1,2,3,4]); //=> -1

length

Parameters
  • list

    The array to inspect.
Returns Number The length of the array.

Added in v0.3.0

Returns the number of elements in the array by returning list.length.

R.length([]); //=> 0
R.length([1, 2, 3]); //=> 3

lens

Parameters
  • getter
  • setter
Returns Lens

Added in v0.8.0

Returns a lens for the given getter and setter functions. The getter "gets" the value of the focus; the setter "sets" the value of the focus. The setter should not mutate the data structure.

See also view, set, over, lensIndex, lensProp. 
const xLens = R.lens(R.prop('x'), R.assoc('x'));

R.view(xLens, {x: 1, y: 2});            //=> 1
R.set(xLens, 4, {x: 1, y: 2});          //=> {x: 4, y: 2}
R.over(xLens, R.negate, {x: 1, y: 2});  //=> {x: -1, y: 2}

lensIndex

Parameters
  • n
Returns Lens

Added in v0.14.0

Returns a lens whose focus is the specified index.

See also view, set, over, nth. 
const headLens = R.lensIndex(0);

R.view(headLens, ['a', 'b', 'c']);            //=> 'a'
R.set(headLens, 'x', ['a', 'b', 'c']);        //=> ['x', 'b', 'c']
R.over(headLens, R.toUpper, ['a', 'b', 'c']); //=> ['A', 'b', 'c']

lensPath

Parameters
  • path

    The path to use.
Returns Lens

Added in v0.19.0

Returns a lens whose focus is the specified path.

See also view, set, over. 
const xHeadYLens = R.lensPath(['x', 0, 'y']);

R.view(xHeadYLens, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> 2
R.set(xHeadYLens, 1, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> {x: [{y: 1, z: 3}, {y: 4, z: 5}]}
R.over(xHeadYLens, R.negate, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> {x: [{y: -2, z: 3}, {y: 4, z: 5}]}

lensProp

Parameters
  • k
Returns Lens

Added in v0.14.0

Returns a lens whose focus is the specified property.

See also view, set, over. 
const xLens = R.lensProp('x');

R.view(xLens, {x: 1, y: 2});            //=> 1
R.set(xLens, 4, {x: 1, y: 2});          //=> {x: 4, y: 2}
R.over(xLens, R.negate, {x: 1, y: 2});  //=> {x: -1, y: 2}

lift

Parameters
  • fn

    The function to lift into higher context
Returns function The lifted function.

Added in v0.7.0

"lifts" a function of arity >= 1 so that it may "map over" a list, Function or other object that satisfies the FantasyLand Apply spec.

See also liftN. 
const madd3 = R.lift((a, b, c) => a + b + c);

madd3([100, 200], [30, 40], [5, 6, 7]); //=> [135, 136, 137, 145, 146, 147, 235, 236, 237, 245, 246, 247]

const madd5 = R.lift((a, b, c, d, e) => a + b + c + d + e);

madd5([10, 20], [1], [2, 3], [4], [100, 200]); //=> [117, 217, 118, 218, 127, 227, 128, 228]

liftN

Parameters
  • fn

    The function to lift into higher context
Returns function The lifted function.

Added in v0.7.0

"lifts" a function to be the specified arity, so that it may "map over" that many lists, Functions or other objects that satisfy the FantasyLand Apply spec.

See also lift, ap. 
const madd3 = R.liftN(3, (...args) => R.sum(args));
madd3([1,2,3], [1,2,3], [1]); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7]

lt

Parameters
  • a
  • b
Returns Boolean

Added in v0.1.0

Returns true if the first argument is less than the second; false otherwise.

See also gt. 
R.lt(2, 1); //=> false
R.lt(2, 2); //=> false
R.lt(2, 3); //=> true
R.lt('a', 'z'); //=> true
R.lt('z', 'a'); //=> false

lte

Parameters
  • a
  • b
Returns Boolean

Added in v0.1.0

Returns true if the first argument is less than or equal to the second; false otherwise.

See also gte. 
R.lte(2, 1); //=> false
R.lte(2, 2); //=> true
R.lte(2, 3); //=> true
R.lte('a', 'z'); //=> true
R.lte('z', 'a'); //=> false

map

Parameters
  • fn

    The function to be called on every element of the input list.
  • list

    The list to be iterated over.
Returns Array The new list.

Added in v0.1.0

Takes a function and a functor, applies the function to each of the functor's values, and returns a functor of the same shape.

Ramda provides suitable map implementations for Array and Object, so this function may be applied to [1, 2, 3] or {x: 1, y: 2, z: 3}.

Dispatches to the map method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

Also treats functions as functors and will compose them together.

See also transduce, addIndex, pluck, project. 
const double = x => x * 2;

R.map(double, [1, 2, 3]); //=> [2, 4, 6]

R.map(double, {x: 1, y: 2, z: 3}); //=> {x: 2, y: 4, z: 6}

mapAccum

Parameters
  • fn

    The function to be called on every element of the input list.
  • acc

    The accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.10.0

The mapAccum function behaves like a combination of map and reduce; it applies a function to each element of a list, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new list.

The iterator function receives two arguments, acc and value, and should return a tuple [acc, value].

See also scan, addIndex, mapAccumRight. 
const digits = ['1', '2', '3', '4'];
const appender = (a, b) => [a + b, a + b];

R.mapAccum(appender, 0, digits); //=> ['01234', ['01', '012', '0123', '01234']]

mapAccumRight

Parameters
  • fn

    The function to be called on every element of the input list.
  • acc

    The accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.10.0

The mapAccumRight function behaves like a combination of map and reduce; it applies a function to each element of a list, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new list.

Similar to mapAccum, except moves through the input list from the right to the left.

The iterator function receives two arguments, acc and value, and should return a tuple [acc, value].

See also addIndex, mapAccum. 
const digits = ['1', '2', '3', '4'];
const appender = (a, b) => [b + a, b + a];

R.mapAccumRight(appender, 5, digits); //=> ['12345', ['12345', '2345', '345', '45']]

mapObjIndexed

Parameters
  • fn
  • obj
Returns Object

Added in v0.9.0

An Object-specific version of map. The function is applied to three arguments: (value, key, obj). If only the value is significant, use map instead.

See also map. 
const xyz = { x: 1, y: 2, z: 3 };
const prependKeyAndDouble = (num, key, obj) => key + (num * 2);

R.mapObjIndexed(prependKeyAndDouble, xyz); //=> { x: 'x2', y: 'y4', z: 'z6' }

match

Parameters
  • rx

    A regular expression.
  • str

    The string to match against
Returns Array The list of matches or empty array.

Added in v0.1.0

Tests a regular expression against a String. Note that this function will return an empty array when there are no matches. This differs from String.prototype.match which returns null when there are no matches.

See also test. 
R.match(/([a-z]a)/g, 'bananas'); //=> ['ba', 'na', 'na']
R.match(/a/, 'b'); //=> []
R.match(/a/, null); //=> TypeError: null does not have a method named "match"

mathMod

Parameters
  • m

    The dividend.
  • p

    the modulus.
Returns Number The result of `b mod a`.

Added in v0.3.0

mathMod behaves like the modulo operator should mathematically, unlike the % operator (and by extension, R.modulo). So while -17 % 5 is -2, mathMod(-17, 5) is 3. mathMod requires Integer arguments, and returns NaN when the modulus is zero or negative.

See also modulo. 
R.mathMod(-17, 5);  //=> 3
R.mathMod(17, 5);   //=> 2
R.mathMod(17, -5);  //=> NaN
R.mathMod(17, 0);   //=> NaN
R.mathMod(17.2, 5); //=> NaN
R.mathMod(17, 5.3); //=> NaN

const clock = R.mathMod(R.__, 12);
clock(15); //=> 3
clock(24); //=> 0

const seventeenMod = R.mathMod(17);
seventeenMod(3);  //=> 2
seventeenMod(4);  //=> 1
seventeenMod(10); //=> 7

max

Parameters
  • a
  • b
Returns *

Added in v0.1.0

Returns the larger of its two arguments.

See also maxBy, min. 
R.max(789, 123); //=> 789
R.max('a', 'b'); //=> 'b'

maxBy

Parameters
  • f
  • a
  • b
Returns *

Added in v0.8.0

Takes a function and two values, and returns whichever value produces the larger result when passed to the provided function.

See also max, minBy. 
//  square :: Number -> Number
const square = n => n * n;

R.maxBy(square, -3, 2); //=> -3

R.reduce(R.maxBy(square), 0, [3, -5, 4, 1, -2]); //=> -5
R.reduce(R.maxBy(square), 0, []); //=> 0

mean

Parameters
  • list
Returns Number

Added in v0.14.0

Returns the mean of the given list of numbers.

See also median. 
R.mean([2, 7, 9]); //=> 6
R.mean([]); //=> NaN

median

Parameters
  • list
Returns Number

Added in v0.14.0

Returns the median of the given list of numbers.

See also mean. 
R.median([2, 9, 7]); //=> 7
R.median([7, 2, 10, 9]); //=> 8
R.median([]); //=> NaN

memoizeWith

Parameters
  • fn

    The function to generate the cache key.
  • fn

    The function to memoize.
Returns function Memoized version of `fn`.

Added in v0.24.0

Creates a new function that, when invoked, caches the result of calling fn for a given argument set and returns the result. Subsequent calls to the memoized fn with the same argument set will not result in an additional call to fn; instead, the cached result for that set of arguments will be returned.

Care must be taken when implementing key generation to avoid key collision, or if tracking references, memory leaks and mutating arguments.

let count = 0;
const factorial = R.memoizeWith(Number, n => {
  count += 1;
  return R.product(R.range(1, n + 1));
});
factorial(5); //=> 120
factorial(5); //=> 120
factorial(5); //=> 120
count; //=> 1

mergeAll

Parameters
  • list

    An array of objects
Returns Object A merged object.

Added in v0.10.0

Creates one new object with the own properties from a list of objects. If a key exists in more than one object, the value from the last object it exists in will be used.

See also reduce. 
R.mergeAll([{foo:1},{bar:2},{baz:3}]); //=> {foo:1,bar:2,baz:3}
R.mergeAll([{foo:1},{foo:2},{bar:2}]); //=> {foo:2,bar:2}

mergeDeepLeft

Parameters
  • lObj
  • rObj
Returns Object

Added in v0.24.0

Creates a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects:

  • and both values are objects, the two values will be recursively merged
  • otherwise the value from the first object will be used.
See also merge, mergeDeepRight, mergeDeepWith, mergeDeepWithKey. 
R.mergeDeepLeft({ name: 'fred', age: 10, contact: { email: 'moo@example.com' }},
                { age: 40, contact: { email: 'baa@example.com' }});
//=> { name: 'fred', age: 10, contact: { email: 'moo@example.com' }}

mergeDeepRight

Parameters
  • lObj
  • rObj
Returns Object

Added in v0.24.0

Creates a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects:

  • and both values are objects, the two values will be recursively merged
  • otherwise the value from the second object will be used.
See also merge, mergeDeepLeft, mergeDeepWith, mergeDeepWithKey. 
R.mergeDeepRight({ name: 'fred', age: 10, contact: { email: 'moo@example.com' }},
                 { age: 40, contact: { email: 'baa@example.com' }});
//=> { name: 'fred', age: 40, contact: { email: 'baa@example.com' }}

mergeDeepWith

Parameters
  • fn
  • lObj
  • rObj
Returns Object

Added in v0.24.0

Creates a new object with the own properties of the two provided objects. If a key exists in both objects:

  • and both associated values are also objects then the values will be recursively merged.
  • otherwise the provided function is applied to associated values using the resulting value as the new value associated with the key. If a key only exists in one object, the value will be associated with the key of the resulting object.
See also mergeWith, mergeDeepWithKey. 
R.mergeDeepWith(R.concat,
                { a: true, c: { values: [10, 20] }},
                { b: true, c: { values: [15, 35] }});
//=> { a: true, b: true, c: { values: [10, 20, 15, 35] }}

mergeDeepWithKey

Parameters
  • fn
  • lObj
  • rObj
Returns Object

Added in v0.24.0

Creates a new object with the own properties of the two provided objects. If a key exists in both objects:

  • and both associated values are also objects then the values will be recursively merged.
  • otherwise the provided function is applied to the key and associated values using the resulting value as the new value associated with the key. If a key only exists in one object, the value will be associated with the key of the resulting object.
See also mergeWithKey, mergeDeepWith. 
let concatValues = (k, l, r) => k == 'values' ? R.concat(l, r) : r
R.mergeDeepWithKey(concatValues,
                   { a: true, c: { thing: 'foo', values: [10, 20] }},
                   { b: true, c: { thing: 'bar', values: [15, 35] }});
//=> { a: true, b: true, c: { thing: 'bar', values: [10, 20, 15, 35] }}

mergeLeft

Parameters
  • l
  • r
Returns Object

Added in v0.26.0

Create a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects, the value from the first object will be used.

See also mergeRight, mergeDeepLeft, mergeWith, mergeWithKey. 
R.mergeLeft({ 'age': 40 }, { 'name': 'fred', 'age': 10 });
//=> { 'name': 'fred', 'age': 40 }

const resetToDefault = R.mergeLeft({x: 0});
resetToDefault({x: 5, y: 2}); //=> {x: 0, y: 2}

mergeRight

Parameters
  • l
  • r
Returns Object

Added in v0.26.0

Create a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects, the value from the second object will be used.

See also mergeLeft, mergeDeepRight, mergeWith, mergeWithKey. 
R.mergeRight({ 'name': 'fred', 'age': 10 }, { 'age': 40 });
//=> { 'name': 'fred', 'age': 40 }

const withDefaults = R.mergeRight({x: 0, y: 0});
withDefaults({y: 2}); //=> {x: 0, y: 2}

mergeWith

Parameters
  • fn
  • l
  • r
Returns Object

Added in v0.19.0

Creates a new object with the own properties of the two provided objects. If a key exists in both objects, the provided function is applied to the values associated with the key in each object, with the result being used as the value associated with the key in the returned object.

See also mergeDeepWith, merge, mergeWithKey. 
R.mergeWith(R.concat,
            { a: true, values: [10, 20] },
            { b: true, values: [15, 35] });
//=> { a: true, b: true, values: [10, 20, 15, 35] }

mergeWithKey

Parameters
  • fn
  • l
  • r
Returns Object

Added in v0.19.0

Creates a new object with the own properties of the two provided objects. If a key exists in both objects, the provided function is applied to the key and the values associated with the key in each object, with the result being used as the value associated with the key in the returned object.

See also mergeDeepWithKey, merge, mergeWith. 
let concatValues = (k, l, r) => k == 'values' ? R.concat(l, r) : r
R.mergeWithKey(concatValues,
               { a: true, thing: 'foo', values: [10, 20] },
               { b: true, thing: 'bar', values: [15, 35] });
//=> { a: true, b: true, thing: 'bar', values: [10, 20, 15, 35] }

min

Parameters
  • a
  • b
Returns *

Added in v0.1.0

Returns the smaller of its two arguments.

See also minBy, max. 
R.min(789, 123); //=> 123
R.min('a', 'b'); //=> 'a'

minBy

Parameters
  • f
  • a
  • b
Returns *

Added in v0.8.0

Takes a function and two values, and returns whichever value produces the smaller result when passed to the provided function.

See also min, maxBy. 
//  square :: Number -> Number
const square = n => n * n;

R.minBy(square, -3, 2); //=> 2

R.reduce(R.minBy(square), Infinity, [3, -5, 4, 1, -2]); //=> 1
R.reduce(R.minBy(square), Infinity, []); //=> Infinity

modify

Parameters
  • prop

    The property to be modified.
  • fn

    The function to apply to the property.
  • object

    The object to be transformed.
Returns Object The transformed object.

Added in v0.28.0

Creates a copy of the passed object by applying an fn function to the given prop property.

The function will not be invoked, and the object will not change if its corresponding property does not exist in the object. All non-primitive properties are copied to the new object by reference.

const person = {name: 'James', age: 20, pets: ['dog', 'cat']};
R.modify('age', R.add(1), person); //=> {name: 'James', age: 21, pets: ['dog', 'cat']}
R.modify('pets', R.append('turtle'), person); //=> {name: 'James', age: 20, pets: ['dog', 'cat', 'turtle']}

modifyPath

Parameters
  • path

    The path to be modified.
  • fn

    The function to apply to the path.
  • object

    The object to be transformed.
Returns Object The transformed object.

Added in v0.28.0

Creates a shallow clone of the passed object by applying an fn function to the value at the given path.

The function will not be invoked, and the object will not change if its corresponding path does not exist in the object. All non-primitive properties are copied to the new object by reference.

const person = {name: 'James', address: { zipCode: '90216' }};
R.modifyPath(['address', 'zipCode'], R.reverse, person); //=> {name: 'James', address: { zipCode: '61209' }}

// Can handle arrays too
const person = {name: 'James', addresses: [{ zipCode: '90216' }]};
R.modifyPath(['addresses', 0, 'zipCode'], R.reverse, person); //=> {name: 'James', addresses: [{ zipCode: '61209' }]}

module.exports

Parameters
  • f
  • props
Returns function

Added in v0.28.0

Takes a function f and an object, and returns a function g. When applied, g returns the result of applying f to the object provided initially merged deeply (right) with the object provided as an argument to g.

See also partial, partialRight, curry, mergeDeepRight. 
const multiply2 = ({ a, b }) => a * b;
const double = R.partialObject(multiply2, { a: 2 });
double({ b: 2 }); //=> 4

const greet = ({ salutation, title, firstName, lastName }) =>
  salutation + ', ' + title + ' ' + firstName + ' ' + lastName + '!';

const sayHello = R.partialObject(greet, { salutation: 'Hello' });
const sayHelloToMs = R.partialObject(sayHello, { title: 'Ms.' });
sayHelloToMs({ firstName: 'Jane', lastName: 'Jones' }); //=> 'Hello, Ms. Jane Jones!'

modulo

Parameters
  • a

    The value to the divide.
  • b

    The pseudo-modulus
Returns Number The result of `b % a`.

Added in v0.1.1

Divides the first parameter by the second and returns the remainder. Note that this function preserves the JavaScript-style behavior for modulo. For mathematical modulo see mathMod.

See also mathMod. 
R.modulo(17, 3); //=> 2
// JS behavior:
R.modulo(-17, 3); //=> -2
R.modulo(17, -3); //=> 2

const isOdd = R.modulo(R.__, 2);
isOdd(42); //=> 0
isOdd(21); //=> 1

move

Parameters
  • from

    The source index
  • to

    The destination index
  • list

    The list which will serve to realise the move
Returns Array The new list reordered

Added in v0.27.1

Move an item, at index from, to index to, in a list of elements. A new list will be created containing the new elements order.

R.move(0, 2, ['a', 'b', 'c', 'd', 'e', 'f']); //=> ['b', 'c', 'a', 'd', 'e', 'f']
R.move(-1, 0, ['a', 'b', 'c', 'd', 'e', 'f']); //=> ['f', 'a', 'b', 'c', 'd', 'e'] list rotation

multiply

Parameters
  • a

    The first value.
  • b

    The second value.
Returns Number The result of `a * b`.

Added in v0.1.0

Multiplies two numbers. Equivalent to a * b but curried.

See also divide. 
const double = R.multiply(2);
const triple = R.multiply(3);
double(3);       //=>  6
triple(4);       //=> 12
R.multiply(2, 5);  //=> 10

nAry

Parameters
  • n

    The desired arity of the new function.
  • fn

    The function to wrap.
Returns function A new function wrapping `fn`. The new function is guaranteed to be of arity `n`.

Added in v0.1.0

Wraps a function of any arity (including nullary) in a function that accepts exactly n parameters. Any extraneous parameters will not be passed to the supplied function.

See also binary, unary. 
const takesTwoArgs = (a, b) => [a, b];

takesTwoArgs.length; //=> 2
takesTwoArgs(1, 2); //=> [1, 2]

const takesOneArg = R.nAry(1, takesTwoArgs);
takesOneArg.length; //=> 1
// Only `n` arguments are passed to the wrapped function
takesOneArg(1, 2); //=> [1, undefined]

negate

Parameters
  • n
Returns Number

Added in v0.9.0

Negates its argument.

R.negate(42); //=> -42

none

Parameters
  • fn

    The predicate function.
  • list

    The array to consider.
Returns Boolean `true` if the predicate is not satisfied by every element, `false` otherwise.

Added in v0.12.0

Returns true if no elements of the list match the predicate, false otherwise.

Dispatches to the all method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also all, any. 
const isEven = n => n % 2 === 0;
const isOdd = n => n % 2 !== 0;

R.none(isEven, [1, 3, 5, 7, 9, 11]); //=> true
R.none(isOdd, [1, 3, 5, 7, 8, 11]); //=> false

not

Parameters
  • a

    any value
Returns Boolean the logical inverse of passed argument.

Added in v0.1.0

A function that returns the ! of its argument. It will return true when passed false-y value, and false when passed a truth-y one.

See also complement. 
R.not(true); //=> false
R.not(false); //=> true
R.not(0); //=> true
R.not(1); //=> false

nth

Parameters
  • offset
  • list
Returns *

Added in v0.1.0

Returns the nth element of the given list or string. If n is negative the element at index length + n is returned.

const list = ['foo', 'bar', 'baz', 'quux'];
R.nth(1, list); //=> 'bar'
R.nth(-1, list); //=> 'quux'
R.nth(-99, list); //=> undefined

R.nth(2, 'abc'); //=> 'c'
R.nth(3, 'abc'); //=> ''

nthArg

Parameters
  • n
Returns function

Added in v0.9.0

Returns a function which returns its nth argument.

R.nthArg(1)('a', 'b', 'c'); //=> 'b'
R.nthArg(-1)('a', 'b', 'c'); //=> 'c'

o

Parameters
  • f
  • g
Returns function

Added in v0.24.0

o is a curried composition function that returns a unary function. Like compose, o performs right-to-left function composition. Unlike compose, the rightmost function passed to o will be invoked with only one argument. Also, unlike compose, o is limited to accepting only 2 unary functions. The name o was chosen because of its similarity to the mathematical composition operator ∘.

See also compose, pipe. 
const classyGreeting = name => "The name's " + name.last + ", " + name.first + " " + name.last
const yellGreeting = R.o(R.toUpper, classyGreeting);
yellGreeting({first: 'James', last: 'Bond'}); //=> "THE NAME'S BOND, JAMES BOND"

R.o(R.multiply(10), R.add(10))(-4) //=> 60

objOf

Parameters
  • key
  • val
Returns Object

Added in v0.18.0

Creates an object containing a single key:value pair.

See also pair. 
const matchPhrases = R.compose(
  R.objOf('must'),
  R.map(R.objOf('match_phrase'))
);
matchPhrases(['foo', 'bar', 'baz']); //=> {must: [{match_phrase: 'foo'}, {match_phrase: 'bar'}, {match_phrase: 'baz'}]}

of

Parameters
  • x

    any value
Returns Array An array wrapping `x`.

Added in v0.3.0

Returns a singleton array containing the value provided.

Note this of is different from the ES6 of; See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/of

R.of(null); //=> [null]
R.of([42]); //=> [[42]]

omit

Parameters
  • names

    an array of String property names to omit from the new object
  • obj

    The object to copy from
Returns Object A new object with properties from `names` not on it.

Added in v0.1.0

Returns a partial copy of an object omitting the keys specified.

See also pick. 
R.omit(['a', 'd'], {a: 1, b: 2, c: 3, d: 4}); //=> {b: 2, c: 3}

on

Parameters
  • f

    a binary function
  • g

    a unary function
  • a

    any value
  • b

    any value
Returns any The result of `f`

Added in v0.28.0

Takes a binary function f, a unary function g, and two values. Applies g to each value, then applies the result of each to f.

Also known as the P combinator.

const eqBy = R.on((a, b) => a === b);
eqBy(R.prop('a'), {b:0, a:1}, {a:1}) //=> true;

const containsInsensitive = R.on(R.contains, R.toLower);
containsInsensitive('o', 'FOO'); //=> true

once

Parameters
  • fn

    The function to wrap in a call-only-once wrapper.
Returns function The wrapped function.

Added in v0.1.0

Accepts a function fn and returns a function that guards invocation of fn such that fn can only ever be called once, no matter how many times the returned function is invoked. The first value calculated is returned in subsequent invocations.

const addOneOnce = R.once(x => x + 1);
addOneOnce(10); //=> 11
addOneOnce(addOneOnce(50)); //=> 11

or

Parameters
  • a
  • b
Returns Any

Added in v0.1.0

Returns the first argument if it is truthy, otherwise the second argument. Acts as the boolean or statement if both inputs are Booleans.

See also either, and. 
R.or(true, true); //=> true
R.or(true, false); //=> true
R.or(false, true); //=> true
R.or(false, false); //=> false

otherwise

Parameters
  • onFailure

    The function to apply. Can return a value or a promise of a value.
  • p
Returns Promise The result of calling `p.then(null, onFailure)`

Added in v0.26.0

Returns the result of applying the onFailure function to the value inside a failed promise. This is useful for handling rejected promises inside function compositions.

See also andThen. 
const failedFetch = id => Promise.reject('bad ID');
const useDefault = () => ({ firstName: 'Bob', lastName: 'Loblaw' });

//recoverFromFailure :: String -> Promise ({ firstName, lastName })
const recoverFromFailure = R.pipe(
  failedFetch,
  R.otherwise(useDefault),
  R.andThen(R.pick(['firstName', 'lastName'])),
);
recoverFromFailure(12345).then(console.log);

over

Parameters
  • lens
  • v
  • x
Returns *

Added in v0.16.0

Returns the result of "setting" the portion of the given data structure focused by the given lens to the result of applying the given function to the focused value.

See also view, set, lens, lensIndex, lensProp, lensPath. 
const headLens = R.lensIndex(0);

R.over(headLens, R.toUpper, ['foo', 'bar', 'baz']); //=> ['FOO', 'bar', 'baz']

pair

Parameters
  • fst
  • snd
Returns Array

Added in v0.18.0

Takes two arguments, fst and snd, and returns [fst, snd].

See also objOf, of. 
R.pair('foo', 'bar'); //=> ['foo', 'bar']

partial

Parameters
  • f
  • args
Returns function

Added in v0.10.0

Takes a function f and a list of arguments, and returns a function g. When applied, g returns the result of applying f to the arguments provided initially followed by the arguments provided to g.

See also partialRight, curry. 
const multiply2 = (a, b) => a * b;
const double = R.partial(multiply2, [2]);
double(3); //=> 6

const greet = (salutation, title, firstName, lastName) =>
  salutation + ', ' + title + ' ' + firstName + ' ' + lastName + '!';

const sayHello = R.partial(greet, ['Hello']);
const sayHelloToMs = R.partial(sayHello, ['Ms.']);
sayHelloToMs('Jane', 'Jones'); //=> 'Hello, Ms. Jane Jones!'

partialRight

Parameters
  • f
  • args
Returns function

Added in v0.10.0

Takes a function f and a list of arguments, and returns a function g. When applied, g returns the result of applying f to the arguments provided to g followed by the arguments provided initially.

See also partial. 
const greet = (salutation, title, firstName, lastName) =>
  salutation + ', ' + title + ' ' + firstName + ' ' + lastName + '!';

const greetMsJaneJones = R.partialRight(greet, ['Ms.', 'Jane', 'Jones']);

greetMsJaneJones('Hello'); //=> 'Hello, Ms. Jane Jones!'

partition

Parameters
  • pred

    A predicate to determine which side the element belongs to.
  • filterable

    the list (or other filterable) to partition.
Returns Array An array, containing first the subset of elements that satisfy the predicate, and second the subset of elements that do not satisfy.

Added in v0.1.4

Takes a predicate and a list or other Filterable object and returns the pair of filterable objects of the same type of elements which do and do not satisfy, the predicate, respectively. Filterable objects include plain objects or any object that has a filter method such as Array.

See also filter, reject. 
R.partition(R.includes('s'), ['sss', 'ttt', 'foo', 'bars']);
// => [ [ 'sss', 'bars' ],  [ 'ttt', 'foo' ] ]

R.partition(R.includes('s'), { a: 'sss', b: 'ttt', foo: 'bars' });
// => [ { a: 'sss', foo: 'bars' }, { b: 'ttt' }  ]

path

Parameters
  • path

    The path to use.
  • obj

    The object to retrieve the nested property from.
Returns * The data at `path`.

Added in v0.2.0

Retrieve the value at a given path.

See also prop, nth. 
R.path(['a', 'b'], {a: {b: 2}}); //=> 2
R.path(['a', 'b'], {c: {b: 2}}); //=> undefined
R.path(['a', 'b', 0], {a: {b: [1, 2, 3]}}); //=> 1
R.path(['a', 'b', -2], {a: {b: [1, 2, 3]}}); //=> 2

pathEq

Parameters
  • path

    The path of the nested property to use
  • val

    The value to compare the nested property with
  • obj

    The object to check the nested property in
Returns Boolean `true` if the value equals the nested object property, `false` otherwise.

Added in v0.7.0

Determines whether a nested path on an object has a specific value, in R.equals terms. Most likely used to filter a list.

const user1 = { address: { zipCode: 90210 } };
const user2 = { address: { zipCode: 55555 } };
const user3 = { name: 'Bob' };
const users = [ user1, user2, user3 ];
const isFamous = R.pathEq(['address', 'zipCode'], 90210);
R.filter(isFamous, users); //=> [ user1 ]

pathOr

Parameters
  • d

    The default value.
  • p

    The path to use.
  • obj

    The object to retrieve the nested property from.
Returns * The data at `path` of the supplied object or the default value.

Added in v0.18.0

If the given, non-null object has a value at the given path, returns the value at that path. Otherwise returns the provided default value.

R.pathOr('N/A', ['a', 'b'], {a: {b: 2}}); //=> 2
R.pathOr('N/A', ['a', 'b'], {c: {b: 2}}); //=> "N/A"

paths

Parameters
  • pathsArray

    The array of paths to be fetched.
  • obj

    The object to retrieve the nested properties from.
Returns Array A list consisting of values at paths specified by "pathsArray".

Added in v0.27.1

Retrieves the values at given paths of an object.

See also path. 
R.paths([['a', 'b'], ['p', 0, 'q']], {a: {b: 2}, p: [{q: 3}]}); //=> [2, 3]
R.paths([['a', 'b'], ['p', 'r']], {a: {b: 2}, p: [{q: 3}]}); //=> [2, undefined]

pathSatisfies

Parameters
  • pred
  • propPath
  • obj
Returns Boolean

Added in v0.19.0

Returns true if the specified object property at given path satisfies the given predicate; false otherwise.

See also propSatisfies, path. 
R.pathSatisfies(y => y > 0, ['x', 'y'], {x: {y: 2}}); //=> true
R.pathSatisfies(R.is(Object), [], {x: {y: 2}}); //=> true

pick

Parameters
  • names

    an array of String property names to copy onto a new object
  • obj

    The object to copy from
Returns Object A new object with only properties from `names` on it.

Added in v0.1.0

Returns a partial copy of an object containing only the keys specified. If the key does not exist, the property is ignored.

See also omit, props. 
R.pick(['a', 'd'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1, d: 4}
R.pick(['a', 'e', 'f'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1}

pickAll

Parameters
  • names

    an array of String property names to copy onto a new object
  • obj

    The object to copy from
Returns Object A new object with only properties from `names` on it.

Added in v0.1.0

Similar to pick except that this one includes a key: undefined pair for properties that don't exist.

See also pick. 
R.pickAll(['a', 'd'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1, d: 4}
R.pickAll(['a', 'e', 'f'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1, e: undefined, f: undefined}

pickBy

Parameters
  • pred

    A predicate to determine whether or not a key should be included on the output object.
  • obj

    The object to copy from
Returns Object A new object with only properties that satisfy `pred` on it.

Added in v0.8.0

Returns a partial copy of an object containing only the keys that satisfy the supplied predicate.

See also pick, filter. 
const isUpperCase = (val, key) => key.toUpperCase() === key;
R.pickBy(isUpperCase, {a: 1, b: 2, A: 3, B: 4}); //=> {A: 3, B: 4}

pipe

Parameters
  • functions
Returns function

Added in v0.1.0

Performs left-to-right function composition. The first argument may have any arity; the remaining arguments must be unary.

In some libraries this function is named sequence.

Note: The result of pipe is not automatically curried.

See also compose. 
const f = R.pipe(Math.pow, R.negate, R.inc);

f(3, 4); // -(3^4) + 1

pipeWith

Parameters
  • transformer

    The transforming function
  • functions

    The functions to pipe
Returns function

Added in v0.26.0

Performs left-to-right function composition using transforming function. The first function may have any arity; the remaining functions must be unary.

Note: The result of pipeWith is not automatically curried. Transforming function is not used on the first argument.

See also composeWith, pipe. 
const pipeWhileNotNil = R.pipeWith((f, res) => R.isNil(res) ? res : f(res));
const f = pipeWhileNotNil([Math.pow, R.negate, R.inc])

f(3, 4); // -(3^4) + 1

pluck

Parameters
  • key

    The key name to pluck off of each object.
  • f

    The array or functor to consider.
Returns Array The list of values for the given key.

Added in v0.1.0

Returns a new list by plucking the same named property off all objects in the list supplied.

pluck will work on any functor in addition to arrays, as it is equivalent to R.map(R.prop(k), f).

See also project, prop, props. 
var getAges = R.pluck('age');
getAges([{name: 'fred', age: 29}, {name: 'wilma', age: 27}]); //=> [29, 27]

R.pluck(0, [[1, 2], [3, 4]]);               //=> [1, 3]
R.pluck('val', {a: {val: 3}, b: {val: 5}}); //=> {a: 3, b: 5}

prepend

Parameters
  • el

    The item to add to the head of the output list.
  • list

    The array to add to the tail of the output list.
Returns Array A new array.

Added in v0.1.0

Returns a new list with the given element at the front, followed by the contents of the list.

See also append. 
R.prepend('fee', ['fi', 'fo', 'fum']); //=> ['fee', 'fi', 'fo', 'fum']

product

Parameters
  • list

    An array of numbers
Returns Number The product of all the numbers in the list.

Added in v0.1.0

Multiplies together all the elements of a list.

See also reduce. 
R.product([2,4,6,8,100,1]); //=> 38400

project

Parameters
  • props

    The property names to project
  • objs

    The objects to query
Returns Array An array of objects with just the `props` properties.

Added in v0.1.0

Reasonable analog to SQL select statement.

See also pluck, props, prop. 
const abby = {name: 'Abby', age: 7, hair: 'blond', grade: 2};
const fred = {name: 'Fred', age: 12, hair: 'brown', grade: 7};
const kids = [abby, fred];
R.project(['name', 'grade'], kids); //=> [{name: 'Abby', grade: 2}, {name: 'Fred', grade: 7}]

promap

Parameters
  • f

    The preprocessor function, a -> b
  • g

    The postprocessor function, c -> d
  • profunctor

    The profunctor instance to be promapped, e.g. b -> c
Returns Profunctor The new profunctor instance, e.g. a -> d

Added in v0.28.0

Takes two functions as pre- and post- processors respectively for a third function, i.e. promap(f, g, h)(x) === g(h(f(x))).

Dispatches to the promap method of the third argument, if present, according to the FantasyLand Profunctor spec.

Acts as a transducer if a transformer is given in profunctor position.

See also transduce. 
const decodeChar = R.promap(s => s.charCodeAt(), String.fromCharCode, R.add(-8))
const decodeString = R.promap(R.split(''), R.join(''), R.map(decodeChar))
decodeString("ziuli") //=> "ramda"

prop

Parameters
  • p

    The property name or array index
  • obj

    The object to query
Returns * The value at `obj.p`.

Added in v0.1.0

Returns a function that when supplied an object returns the indicated property of that object, if it exists.

See also path, props, pluck, project, nth. 
R.prop('x', {x: 100}); //=> 100
R.prop('x', {}); //=> undefined
R.prop(0, [100]); //=> 100
R.compose(R.inc, R.prop('x'))({ x: 3 }) //=> 4

propEq

Parameters
  • name
  • val
  • obj
Returns Boolean

Added in v0.1.0

Returns true if the specified object property is equal, in R.equals terms, to the given value; false otherwise. You can test multiple properties with R.whereEq.

See also whereEq, propSatisfies, equals. 
const abby = {name: 'Abby', age: 7, hair: 'blond'};
const fred = {name: 'Fred', age: 12, hair: 'brown'};
const rusty = {name: 'Rusty', age: 10, hair: 'brown'};
const alois = {name: 'Alois', age: 15, disposition: 'surly'};
const kids = [abby, fred, rusty, alois];
const hasBrownHair = R.propEq('hair', 'brown');
R.filter(hasBrownHair, kids); //=> [fred, rusty]

propIs

Parameters
  • type
  • name
  • obj
Returns Boolean

Added in v0.16.0

Returns true if the specified object property is of the given type; false otherwise.

See also is, propSatisfies. 
R.propIs(Number, 'x', {x: 1, y: 2});  //=> true
R.propIs(Number, 'x', {x: 'foo'});    //=> false
R.propIs(Number, 'x', {});            //=> false

propOr

Parameters
  • val

    The default value.
  • p

    The name of the property to return.
  • obj

    The object to query.
Returns * The value of given property of the supplied object or the default value.

Added in v0.6.0

Return the specified property of the given non-null object if the property is present and it's value is not null, undefined or NaN.

Otherwise the first argument is returned.

const alice = {
  name: 'ALICE',
  age: 101
};
const favorite = R.prop('favoriteLibrary');
const favoriteWithDefault = R.propOr('Ramda', 'favoriteLibrary');

favorite(alice);  //=> undefined
favoriteWithDefault(alice);  //=> 'Ramda'

props

Parameters
  • ps

    The property names to fetch
  • obj

    The object to query
Returns Array The corresponding values or partially applied function.

Added in v0.1.0

Acts as multiple prop: array of keys in, array of values out. Preserves order.

See also prop, pluck, project. 
R.props(['x', 'y'], {x: 1, y: 2}); //=> [1, 2]
R.props(['c', 'a', 'b'], {b: 2, a: 1}); //=> [undefined, 1, 2]

const fullName = R.compose(R.join(' '), R.props(['first', 'last']));
fullName({last: 'Bullet-Tooth', age: 33, first: 'Tony'}); //=> 'Tony Bullet-Tooth'

propSatisfies

Parameters
  • pred
  • name
  • obj
Returns Boolean

Added in v0.16.0

Returns true if the specified object property satisfies the given predicate; false otherwise. You can test multiple properties with R.where.

See also where, propEq, propIs. 
R.propSatisfies(x => x > 0, 'x', {x: 1, y: 2}); //=> true

range

Parameters
  • from

    The first number in the list.
  • to

    One more than the last number in the list.
Returns Array The list of numbers in the set `[a, b)`.

Added in v0.1.0

Returns a list of numbers from from (inclusive) to to (exclusive).

R.range(1, 5);    //=> [1, 2, 3, 4]
R.range(50, 53);  //=> [50, 51, 52]

reduce

Parameters
  • fn

    The iterator function. Receives two values, the accumulator and the current element from the array.
  • acc

    The accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.1.0

Returns a single item by iterating through the list, successively calling the iterator function and passing it an accumulator value and the current value from the array, and then passing the result to the next call.

The iterator function receives two values: (acc, value). It may use R.reduced to shortcut the iteration.

The arguments' order of reduceRight's iterator function is (value, acc).

Note: R.reduce does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.reduce method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce#Description

Dispatches to the reduce method of the third argument, if present. When doing so, it is up to the user to handle the R.reduced shortcuting, as this is not implemented by reduce.

See also reduced, addIndex, reduceRight. 
R.reduce(R.subtract, 0, [1, 2, 3, 4]) // => ((((0 - 1) - 2) - 3) - 4) = -10
//          -               -10
//         / \              / \
//        -   4           -6   4
//       / \              / \
//      -   3   ==>     -3   3
//     / \              / \
//    -   2           -1   2
//   / \              / \
//  0   1            0   1

reduceBy

Parameters
  • valueFn

    The function that reduces the elements of each group to a single value. Receives two values, accumulator for a particular group and the current element.
  • acc

    The (initial) accumulator value for each group.
  • keyFn

    The function that maps the list's element into a key.
  • list

    The array to group.
Returns Object An object with the output of `keyFn` for keys, mapped to the output of `valueFn` for elements which produced that key when passed to `keyFn`.

Added in v0.20.0

Groups the elements of the list according to the result of calling the String-returning function keyFn on each element and reduces the elements of each group to a single value via the reducer function valueFn.

The value function receives two values: (acc, value). It may use R.reduced to short circuit the iteration.

This function is basically a more general groupBy function.

Acts as a transducer if a transformer is given in list position.

See also groupBy, reduce, reduced. 
const groupNames = (acc, {name}) => acc.concat(name)
const toGrade = ({score}) =>
  score < 65 ? 'F' :
  score < 70 ? 'D' :
  score < 80 ? 'C' :
  score < 90 ? 'B' : 'A'

var students = [
  {name: 'Abby', score: 83},
  {name: 'Bart', score: 62},
  {name: 'Curt', score: 88},
  {name: 'Dora', score: 92},
]

reduceBy(groupNames, [], toGrade, students)
//=> {"A": ["Dora"], "B": ["Abby", "Curt"], "F": ["Bart"]}

reduced

Parameters
  • x

    The final value of the reduce.
Returns * The wrapped value.

Added in v0.15.0

Returns a value wrapped to indicate that it is the final value of the reduce and transduce functions. The returned value should be considered a black box: the internal structure is not guaranteed to be stable.

This optimization is available to the below functions:

See also reduce, reduceWhile, reduceBy, reduceRight, transduce. 
R.reduce(
 (acc, item) => item > 3 ? R.reduced(acc) : acc.concat(item),
 [],
 [1, 2, 3, 4, 5]) // [1, 2, 3]

reduceRight

Parameters
  • fn

    The iterator function. Receives two values, the current element from the array and the accumulator.
  • acc

    The accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.1.0

Returns a single item by iterating through the list, successively calling the iterator function and passing it an accumulator value and the current value from the array, and then passing the result to the next call.

Similar to reduce, except moves through the input list from the right to the left.

The iterator function receives two values: (value, acc), while the arguments' order of reduce's iterator function is (acc, value). reduceRight may use reduced to short circuit the iteration.

Note: R.reduceRight does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.reduceRight method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight#Description

See also reduce, addIndex, reduced. 
R.reduceRight(R.subtract, 0, [1, 2, 3, 4]) // => (1 - (2 - (3 - (4 - 0)))) = -2
//    -               -2
//   / \              / \
//  1   -            1   3
//     / \              / \
//    2   -     ==>    2  -1
//       / \              / \
//      3   -            3   4
//         / \              / \
//        4   0            4   0

reduceWhile

Parameters
  • pred

    The predicate. It is passed the accumulator and the current element.
  • fn

    The iterator function. Receives two values, the accumulator and the current element.
  • a

    The accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.22.0

Like reduce, reduceWhile returns a single item by iterating through the list, successively calling the iterator function. reduceWhile also takes a predicate that is evaluated before each step. If the predicate returns false, it "short-circuits" the iteration and returns the current value of the accumulator. reduceWhile may alternatively be short-circuited via reduced.

See also reduce, reduced. 
const isOdd = (acc, x) => x % 2 !== 0;
const xs = [1, 3, 5, 60, 777, 800];
R.reduceWhile(isOdd, R.add, 0, xs); //=> 9

const ys = [2, 4, 6]
R.reduceWhile(isOdd, R.add, 111, ys); //=> 111

reject

Parameters
  • pred
  • filterable
Returns Array

Added in v0.1.0

The complement of filter.

Acts as a transducer if a transformer is given in list position. Filterable objects include plain objects or any object that has a filter method such as Array.

See also filter, transduce, addIndex. 
const isOdd = (n) => n % 2 !== 0;

R.reject(isOdd, [1, 2, 3, 4]); //=> [2, 4]

R.reject(isOdd, {a: 1, b: 2, c: 3, d: 4}); //=> {b: 2, d: 4}

remove

Parameters
  • start

    The position to start removing elements
  • count

    The number of elements to remove
  • list

    The list to remove from
Returns Array A new Array with `count` elements from `start` removed.

Added in v0.2.2

Removes the sub-list of list starting at index start and containing count elements. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.

See also without. 
R.remove(2, 3, [1,2,3,4,5,6,7,8]); //=> [1,2,6,7,8]

repeat

Parameters
  • value

    The value to repeat.
  • n

    The desired size of the output list.
Returns Array A new array containing `n` `value`s.

Added in v0.1.1

Returns a fixed list of size n containing a specified identical value.

See also times. 
R.repeat('hi', 5); //=> ['hi', 'hi', 'hi', 'hi', 'hi']

const obj = {};
const repeatedObjs = R.repeat(obj, 5); //=> [{}, {}, {}, {}, {}]
repeatedObjs[0] === repeatedObjs[1]; //=> true

replace

Parameters
  • pattern

    A regular expression or a substring to match.
  • replacement

    The string to replace the matches with.
  • str

    The String to do the search and replacement in.
Returns String The result.

Added in v0.7.0

Replace a substring or regex match in a string with a replacement.

The first two parameters correspond to the parameters of the String.prototype.replace() function, so the second parameter can also be a function.

R.replace('foo', 'bar', 'foo foo foo'); //=> 'bar foo foo'
R.replace(/foo/, 'bar', 'foo foo foo'); //=> 'bar foo foo'

// Use the "g" (global) flag to replace all occurrences:
R.replace(/foo/g, 'bar', 'foo foo foo'); //=> 'bar bar bar'

reverse

Parameters
  • list
Returns Array

Added in v0.1.0

Returns a new list or string with the elements or characters in reverse order.

R.reverse([1, 2, 3]);  //=> [3, 2, 1]
R.reverse([1, 2]);     //=> [2, 1]
R.reverse([1]);        //=> [1]
R.reverse([]);         //=> []

R.reverse('abc');      //=> 'cba'
R.reverse('ab');       //=> 'ba'
R.reverse('a');        //=> 'a'
R.reverse('');         //=> ''

scan

Parameters
  • fn

    The iterator function. Receives two values, the accumulator and the current element from the array
  • acc

    The accumulator value.
  • list

    The list to iterate over.
Returns Array A list of all intermediately reduced values.

Added in v0.10.0

Scan is similar to reduce, but returns a list of successively reduced values from the left

See also reduce, mapAccum. 
const numbers = [1, 2, 3, 4];
const factorials = R.scan(R.multiply, 1, numbers); //=> [1, 1, 2, 6, 24]

sequence

Parameters
  • of
  • traversable
Returns *

Added in v0.19.0

Transforms a Traversable of Applicative into an Applicative of Traversable.

Dispatches to the sequence method of the second argument, if present.

See also traverse. 
R.sequence(Maybe.of, [Just(1), Just(2), Just(3)]);   //=> Just([1, 2, 3])
R.sequence(Maybe.of, [Just(1), Just(2), Nothing()]); //=> Nothing()

R.sequence(R.of, Just([1, 2, 3])); //=> [Just(1), Just(2), Just(3)]
R.sequence(R.of, Nothing());       //=> [Nothing()]

set

Parameters
  • lens
  • v
  • x
Returns *

Added in v0.16.0

Returns the result of "setting" the portion of the given data structure focused by the given lens to the given value.

See also view, over, lens, lensIndex, lensProp, lensPath. 
const xLens = R.lensProp('x');

R.set(xLens, 4, {x: 1, y: 2});  //=> {x: 4, y: 2}
R.set(xLens, 8, {x: 1, y: 2});  //=> {x: 8, y: 2}

slice

Parameters
  • fromIndex

    The start index (inclusive).
  • toIndex

    The end index (exclusive).
  • list
Returns *

Added in v0.1.4

Returns the elements of the given list or string (or object with a slice method) from fromIndex (inclusive) to toIndex (exclusive).

Dispatches to the slice method of the third argument, if present.

R.slice(1, 3, ['a', 'b', 'c', 'd']);        //=> ['b', 'c']
R.slice(1, Infinity, ['a', 'b', 'c', 'd']); //=> ['b', 'c', 'd']
R.slice(0, -1, ['a', 'b', 'c', 'd']);       //=> ['a', 'b', 'c']
R.slice(-3, -1, ['a', 'b', 'c', 'd']);      //=> ['b', 'c']
R.slice(0, 3, 'ramda');                     //=> 'ram'

sort

Parameters
  • comparator

    A sorting function :: a -> b -> Int
  • list

    The list to sort
Returns Array a new array with its elements sorted by the comparator function.

Added in v0.1.0

Returns a copy of the list, sorted according to the comparator function, which should accept two values at a time and return a negative number if the first value is smaller, a positive number if it's larger, and zero if they are equal. Please note that this is a copy of the list. It does not modify the original.

See also ascend, descend. 
const diff = function(a, b) { return a - b; };
R.sort(diff, [4,2,7,5]); //=> [2, 4, 5, 7]

sortBy

Parameters
  • fn
  • list

    The list to sort.
Returns Array A new list sorted by the keys generated by `fn`.

Added in v0.1.0

Sorts the list according to the supplied function.

const sortByFirstItem = R.sortBy(R.prop(0));
const pairs = [[-1, 1], [-2, 2], [-3, 3]];
sortByFirstItem(pairs); //=> [[-3, 3], [-2, 2], [-1, 1]]

const sortByNameCaseInsensitive = R.sortBy(R.compose(R.toLower, R.prop('name')));
const alice = {
  name: 'ALICE',
  age: 101
};
const bob = {
  name: 'Bob',
  age: -10
};
const clara = {
  name: 'clara',
  age: 314.159
};
const people = [clara, bob, alice];
sortByNameCaseInsensitive(people); //=> [alice, bob, clara]

sortWith

Parameters
  • functions

    A list of comparator functions.
  • list

    The list to sort.
Returns Array A new list sorted according to the comarator functions.

Added in v0.23.0

Sorts a list according to a list of comparators.

See also ascend, descend. 
const alice = {
  name: 'alice',
  age: 40
};
const bob = {
  name: 'bob',
  age: 30
};
const clara = {
  name: 'clara',
  age: 40
};
const people = [clara, bob, alice];
const ageNameSort = R.sortWith([
  R.descend(R.prop('age')),
  R.ascend(R.prop('name'))
]);
ageNameSort(people); //=> [alice, clara, bob]

split

Parameters
  • sep

    The pattern.
  • str

    The string to separate into an array.
Returns Array The array of strings from `str` separated by `sep`.

Added in v0.1.0

Splits a string into an array of strings based on the given separator.

See also join. 
const pathComponents = R.split('/');
R.tail(pathComponents('/usr/local/bin/node')); //=> ['usr', 'local', 'bin', 'node']

R.split('.', 'a.b.c.xyz.d'); //=> ['a', 'b', 'c', 'xyz', 'd']

splitAt

Parameters
  • index

    The index where the array/string is split.
  • array

    The array/string to be split.
Returns Array

Added in v0.19.0

Splits a given list or string at a given index.

R.splitAt(1, [1, 2, 3]);          //=> [[1], [2, 3]]
R.splitAt(5, 'hello world');      //=> ['hello', ' world']
R.splitAt(-1, 'foobar');          //=> ['fooba', 'r']

splitEvery

Parameters
  • n
  • list
Returns Array

Added in v0.16.0

Splits a collection into slices of the specified length.

R.splitEvery(3, [1, 2, 3, 4, 5, 6, 7]); //=> [[1, 2, 3], [4, 5, 6], [7]]
R.splitEvery(3, 'foobarbaz'); //=> ['foo', 'bar', 'baz']

splitWhen

Parameters
  • pred

    The predicate that determines where the array is split.
  • list

    The array to be split.
Returns Array

Added in v0.19.0

Takes a list and a predicate and returns a pair of lists with the following properties:

  • the result of concatenating the two output lists is equivalent to the input list;
  • none of the elements of the first output list satisfies the predicate; and
  • if the second output list is non-empty, its first element satisfies the predicate.
R.splitWhen(R.equals(2), [1, 2, 3, 1, 2, 3]);   //=> [[1], [2, 3, 1, 2, 3]]

splitWhenever

Parameters
  • pred

    The predicate that determines where the array is split.
  • list

    The array to be split.
Returns Array

Added in v0.26.1

Splits an array into slices on every occurrence of a value.

R.splitWhenever(R.equals(2), [1, 2, 3, 2, 4, 5, 2, 6, 7]); //=> [[1], [3], [4, 5], [6, 7]]

startsWith

Parameters
  • prefix
  • list
Returns Boolean

Added in v0.24.0

Checks if a list starts with the provided sublist.

Similarly, checks if a string starts with the provided substring.

See also endsWith. 
R.startsWith('a', 'abc')                //=> true
R.startsWith('b', 'abc')                //=> false
R.startsWith(['a'], ['a', 'b', 'c'])    //=> true
R.startsWith(['b'], ['a', 'b', 'c'])    //=> false

subtract

Parameters
  • a

    The first value.
  • b

    The second value.
Returns Number The result of `a - b`.

Added in v0.1.0

Subtracts its second argument from its first argument.

See also add. 
R.subtract(10, 8); //=> 2

const minus5 = R.subtract(R.__, 5);
minus5(17); //=> 12

const complementaryAngle = R.subtract(90);
complementaryAngle(30); //=> 60
complementaryAngle(72); //=> 18

sum

Parameters
  • list

    An array of numbers
Returns Number The sum of all the numbers in the list.

Added in v0.1.0

Adds together all the elements of a list.

See also reduce. 
R.sum([2,4,6,8,100,1]); //=> 121

symmetricDifference

Parameters
  • list1

    The first list.
  • list2

    The second list.
Returns Array The elements in `list1` or `list2`, but not both.

Added in v0.19.0

Finds the set (i.e. no duplicates) of all elements contained in the first or second list, but not both.

See also symmetricDifferenceWith, difference, differenceWith. 
R.symmetricDifference([1,2,3,4], [7,6,5,4,3]); //=> [1,2,7,6,5]
R.symmetricDifference([7,6,5,4,3], [1,2,3,4]); //=> [7,6,5,1,2]

symmetricDifferenceWith

Parameters
  • pred

    A predicate used to test whether two items are equal.
  • list1

    The first list.
  • list2

    The second list.
Returns Array The elements in `list1` or `list2`, but not both.

Added in v0.19.0

Finds the set (i.e. no duplicates) of all elements contained in the first or second list, but not both. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.

See also symmetricDifference, difference, differenceWith. 
const eqA = R.eqBy(R.prop('a'));
const l1 = [{a: 1}, {a: 2}, {a: 3}, {a: 4}];
const l2 = [{a: 3}, {a: 4}, {a: 5}, {a: 6}];
R.symmetricDifferenceWith(eqA, l1, l2); //=> [{a: 1}, {a: 2}, {a: 5}, {a: 6}]

T

Parameters
Returns Boolean

Added in v0.9.0

A function that always returns true. Any passed in parameters are ignored.

See also F. 
R.T(); //=> true

tail

Parameters
  • list
Returns *

Added in v0.1.0

Returns all but the first element of the given list or string (or object with a tail method).

Dispatches to the slice method of the first argument, if present.

See also head, init, last. 
R.tail([1, 2, 3]);  //=> [2, 3]
R.tail([1, 2]);     //=> [2]
R.tail([1]);        //=> []
R.tail([]);         //=> []

R.tail('abc');  //=> 'bc'
R.tail('ab');   //=> 'b'
R.tail('a');    //=> ''
R.tail('');     //=> ''

take

Parameters
  • n
  • list
Returns *

Added in v0.1.0

Returns the first n elements of the given list, string, or transducer/transformer (or object with a take method).

Dispatches to the take method of the second argument, if present.

See also drop. 
R.take(1, ['foo', 'bar', 'baz']); //=> ['foo']
R.take(2, ['foo', 'bar', 'baz']); //=> ['foo', 'bar']
R.take(3, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz']
R.take(4, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz']
R.take(3, 'ramda');               //=> 'ram'

const personnel = [
  'Dave Brubeck',
  'Paul Desmond',
  'Eugene Wright',
  'Joe Morello',
  'Gerry Mulligan',
  'Bob Bates',
  'Joe Dodge',
  'Ron Crotty'
];

const takeFive = R.take(5);
takeFive(personnel);
//=> ['Dave Brubeck', 'Paul Desmond', 'Eugene Wright', 'Joe Morello', 'Gerry Mulligan']

takeLast

Parameters
  • n

    The number of elements to return.
  • xs

    The collection to consider.
Returns Array

Added in v0.16.0

Returns a new list containing the last n elements of the given list. If n > list.length, returns a list of list.length elements.

See also dropLast. 
R.takeLast(1, ['foo', 'bar', 'baz']); //=> ['baz']
R.takeLast(2, ['foo', 'bar', 'baz']); //=> ['bar', 'baz']
R.takeLast(3, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz']
R.takeLast(4, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz']
R.takeLast(3, 'ramda');               //=> 'mda'

takeLastWhile

Parameters
  • fn

    The function called per iteration.
  • xs

    The collection to iterate over.
Returns Array A new array.

Added in v0.16.0

Returns a new list containing the last n elements of a given list, passing each value to the supplied predicate function, and terminating when the predicate function returns false. Excludes the element that caused the predicate function to fail. The predicate function is passed one argument: (value).

See also dropLastWhile, addIndex. 
const isNotOne = x => x !== 1;

R.takeLastWhile(isNotOne, [1, 2, 3, 4]); //=> [2, 3, 4]

R.takeLastWhile(x => x !== 'R' , 'Ramda'); //=> 'amda'

takeWhile

Parameters
  • fn

    The function called per iteration.
  • xs

    The collection to iterate over.
Returns Array A new array.

Added in v0.1.0

Returns a new list containing the first n elements of a given list, passing each value to the supplied predicate function, and terminating when the predicate function returns false. Excludes the element that caused the predicate function to fail. The predicate function is passed one argument: (value).

Dispatches to the takeWhile method of the second argument, if present.

Acts as a transducer if a transformer is given in list position.

See also dropWhile, transduce, addIndex. 
const isNotFour = x => x !== 4;

R.takeWhile(isNotFour, [1, 2, 3, 4, 3, 2, 1]); //=> [1, 2, 3]

R.takeWhile(x => x !== 'd' , 'Ramda'); //=> 'Ram'

tap

Parameters
  • fn

    The function to call with x. The return value of fn will be thrown away.
  • x
Returns * `x`.

Added in v0.1.0

Runs the given function with the supplied object, then returns the object.

Acts as a transducer if a transformer is given as second parameter.

const sayX = x => console.log('x is ' + x);
R.tap(sayX, 100); //=> 100
// logs 'x is 100'

test

Parameters
  • pattern
  • str
Returns Boolean

Added in v0.12.0

Determines whether a given string matches a given regular expression.

See also match. 
R.test(/^x/, 'xyz'); //=> true
R.test(/^y/, 'xyz'); //=> false

thunkify

Parameters
  • fn

    A function to wrap in a thunk
Returns function Expects arguments for `fn` and returns a new function that, when called, applies those arguments to `fn`.

Added in v0.26.0

Creates a thunk out of a function. A thunk delays a calculation until its result is needed, providing lazy evaluation of arguments.

See also partial, partialRight. 
R.thunkify(R.identity)(42)(); //=> 42
R.thunkify((a, b) => a + b)(25, 17)(); //=> 42

times

Parameters
  • fn

    The function to invoke. Passed one argument, the current value of n.
  • n

    A value between 0 and n - 1. Increments after each function call.
Returns Array An array containing the return values of all calls to `fn`.

Added in v0.2.3

Calls an input function n times, returning an array containing the results of those function calls.

fn is passed one argument: The current value of n, which begins at 0 and is gradually incremented to n - 1.

See also repeat. 
R.times(R.identity, 5); //=> [0, 1, 2, 3, 4]

toLower

Parameters
  • str

    The string to lower case.
Returns String The lower case version of `str`.

Added in v0.9.0

The lower case version of a string.

See also toUpper. 
R.toLower('XYZ'); //=> 'xyz'

toPairs

Parameters
  • obj

    The object to extract from
Returns Array An array of key, value arrays from the object's own properties.

Added in v0.4.0

Converts an object into an array of key, value arrays. Only the object's own properties are used. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.

See also fromPairs, keys, values. 
R.toPairs({a: 1, b: 2, c: 3}); //=> [['a', 1], ['b', 2], ['c', 3]]

toPairsIn

Parameters
  • obj

    The object to extract from
Returns Array An array of key, value arrays from the object's own and prototype properties.

Added in v0.4.0

Converts an object into an array of key, value arrays. The object's own properties and prototype properties are used. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.

const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.toPairsIn(f); //=> [['x','X'], ['y','Y']]

toString

Parameters
  • val
Returns String

Added in v0.14.0

Returns the string representation of the given value. eval'ing the output should result in a value equivalent to the input value. Many of the built-in toString methods do not satisfy this requirement.

If the given value is an [object Object] with a toString method other than Object.prototype.toString, this method is invoked with no arguments to produce the return value. This means user-defined constructor functions can provide a suitable toString method. For example:

function Point(x, y) {
  this.x = x;
  this.y = y;
}

Point.prototype.toString = function() {
  return 'new Point(' + this.x + ', ' + this.y + ')';
};

R.toString(new Point(1, 2)); //=> 'new Point(1, 2)'

R.toString(42); //=> '42'
R.toString('abc'); //=> '"abc"'
R.toString([1, 2, 3]); //=> '[1, 2, 3]'
R.toString({foo: 1, bar: 2, baz: 3}); //=> '{"bar": 2, "baz": 3, "foo": 1}'
R.toString(new Date('2001-02-03T04:05:06Z')); //=> 'new Date("2001-02-03T04:05:06.000Z")'

toUpper

Parameters
  • str

    The string to upper case.
Returns String The upper case version of `str`.

Added in v0.9.0

The upper case version of a string.

See also toLower. 
R.toUpper('abc'); //=> 'ABC'

transduce

Parameters
  • xf

    The transducer function. Receives a transformer and returns a transformer.
  • fn

    The iterator function. Receives two values, the accumulator and the current element from the array. Wrapped as transformer, if necessary, and used to initialize the transducer
  • acc

    The initial accumulator value.
  • list

    The list to iterate over.
Returns * The final, accumulated value.

Added in v0.12.0

Initializes a transducer using supplied iterator function. Returns a single item by iterating through the list, successively calling the transformed iterator function and passing it an accumulator value and the current value from the array, and then passing the result to the next call.

The iterator function receives two values: (acc, value). It will be wrapped as a transformer to initialize the transducer. A transformer can be passed directly in place of an iterator function. In both cases, iteration may be stopped early with the R.reduced function.

A transducer is a function that accepts a transformer and returns a transformer and can be composed directly.

A transformer is an object that provides a 2-arity reducing iterator function, step, 0-arity initial value function, init, and 1-arity result extraction function, result. The step function is used as the iterator function in reduce. The result function is used to convert the final accumulator into the return type and in most cases is R.identity. The init function can be used to provide an initial accumulator, but is ignored by transduce.

The iteration is performed with R.reduce after initializing the transducer.

See also reduce, reduced, into. 
const numbers = [1, 2, 3, 4];
const transducer = R.compose(R.map(R.add(1)), R.take(2));
R.transduce(transducer, R.flip(R.append), [], numbers); //=> [2, 3]

const isOdd = (x) => x % 2 !== 0;
const firstOddTransducer = R.compose(R.filter(isOdd), R.take(1));
R.transduce(firstOddTransducer, R.flip(R.append), [], R.range(0, 100)); //=> [1]

transpose

Parameters
  • list

    A 2D list
Returns Array A 2D list

Added in v0.19.0

Transposes the rows and columns of a 2D list. When passed a list of n lists of length x, returns a list of x lists of length n.

R.transpose([[1, 'a'], [2, 'b'], [3, 'c']]) //=> [[1, 2, 3], ['a', 'b', 'c']]
R.transpose([[1, 2, 3], ['a', 'b', 'c']]) //=> [[1, 'a'], [2, 'b'], [3, 'c']]

// If some of the rows are shorter than the following rows, their elements are skipped:
R.transpose([[10, 11], [20], [], [30, 31, 32]]) //=> [[10, 20, 30], [11, 31], [32]]

traverse

Parameters
  • of
  • f
  • traversable
Returns *

Added in v0.19.0

Maps an Applicative-returning function over a Traversable, then uses sequence to transform the resulting Traversable of Applicative into an Applicative of Traversable.

Dispatches to the traverse method of the third argument, if present.

See also sequence. 
// Returns `Maybe.Nothing` if the given divisor is `0`
const safeDiv = n => d => d === 0 ? Maybe.Nothing() : Maybe.Just(n / d)

R.traverse(Maybe.of, safeDiv(10), [2, 4, 5]); //=> Maybe.Just([5, 2.5, 2])
R.traverse(Maybe.of, safeDiv(10), [2, 0, 5]); //=> Maybe.Nothing

trim

Parameters
  • str

    The string to trim.
Returns String Trimmed version of `str`.

Added in v0.6.0

Removes (strips) whitespace from both ends of the string.

R.trim('   xyz  '); //=> 'xyz'
R.map(R.trim, R.split(',', 'x, y, z')); //=> ['x', 'y', 'z']

tryCatch

Parameters
  • tryer

    The function that may throw.
  • catcher

    The function that will be evaluated if tryer throws.
Returns function A new function that will catch exceptions and send them to the catcher.

Added in v0.20.0

tryCatch takes two functions, a tryer and a catcher. The returned function evaluates the tryer; if it does not throw, it simply returns the result. If the tryer does throw, the returned function evaluates the catcher function and returns its result. Note that for effective composition with this function, both the tryer and catcher functions must return the same type of results.

R.tryCatch(R.prop('x'), R.F)({x: true}); //=> true
R.tryCatch(() => { throw 'foo'}, R.always('caught'))('bar') // =>
'caught'
R.tryCatch(R.times(R.identity), R.always([]))('s') // => []
R.tryCatch(() => { throw 'this is not a valid value'}, (err, value)=>({error : err,  value }))('bar') // => {'error': 'this is not a valid value', 'value': 'bar'}

type

Parameters
  • val

    The value to test
Returns String

Added in v0.8.0

Gives a single-word string description of the (native) type of a value, returning such answers as 'Object', 'Number', 'Array', or 'Null'. Does not attempt to distinguish user Object types any further, reporting them all as 'Object'.

R.type({}); //=> "Object"
R.type(1); //=> "Number"
R.type(false); //=> "Boolean"
R.type('s'); //=> "String"
R.type(null); //=> "Null"
R.type([]); //=> "Array"
R.type(/[A-z]/); //=> "RegExp"
R.type(() => {}); //=> "Function"
R.type(undefined); //=> "Undefined"

unapply

Parameters
  • fn
Returns function

Added in v0.8.0

Takes a function fn, which takes a single array argument, and returns a function which:

  • takes any number of positional arguments;
  • passes these arguments to fn as an array; and
  • returns the result.

In other words, R.unapply derives a variadic function from a function which takes an array. R.unapply is the inverse of R.apply.

See also apply. 
R.unapply(JSON.stringify)(1, 2, 3); //=> '[1,2,3]'

unary

Parameters
  • fn

    The function to wrap.
Returns function A new function wrapping `fn`. The new function is guaranteed to be of arity 1.

Added in v0.2.0

Wraps a function of any arity (including nullary) in a function that accepts exactly 1 parameter. Any extraneous parameters will not be passed to the supplied function.

See also binary, nAry. 
const takesTwoArgs = function(a, b) {
  return [a, b];
};
takesTwoArgs.length; //=> 2
takesTwoArgs(1, 2); //=> [1, 2]

const takesOneArg = R.unary(takesTwoArgs);
takesOneArg.length; //=> 1
// Only 1 argument is passed to the wrapped function
takesOneArg(1, 2); //=> [1, undefined]

uncurryN

Parameters
  • length

    The arity for the returned function.
  • fn

    The function to uncurry.
Returns function A new function.

Added in v0.14.0

Returns a function of arity n from a (manually) curried function. Note that, the returned function is actually a ramda style curryied function, which can accept one or more arguments in each function calling.

See also curry, curryN. 
const addFour = a => b => c => d => a + b + c + d;

const uncurriedAddFour = R.uncurryN(4, addFour);
uncurriedAddFour(1, 2, 3, 4); //=> 10

unfold

Parameters
  • fn

    The iterator function. receives one argument, seed, and returns either false to quit iteration or an array of length two to proceed. The element at index 0 of this array will be added to the resulting array, and the element at index 1 will be passed to the next call to fn.
  • seed

    The seed value.
Returns Array The final list.

Added in v0.10.0

Builds a list from a seed value. Accepts an iterator function, which returns either false to stop iteration or an array of length 2 containing the value to add to the resulting list and the seed to be used in the next call to the iterator function.

The iterator function receives one argument: (seed).

const f = n => n > 50 ? false : [-n, n + 10];
R.unfold(f, 10); //=> [-10, -20, -30, -40, -50]

union

Parameters
  • as

    The first list.
  • bs

    The second list.
Returns Array The first and second lists concatenated, with duplicates removed.

Added in v0.1.0

Combines two lists into a set (i.e. no duplicates) composed of the elements of each list.

R.union([1, 2, 3], [2, 3, 4]); //=> [1, 2, 3, 4]

unionWith

Parameters
  • pred

    A predicate used to test whether two items are equal.
  • list1

    The first list.
  • list2

    The second list.
Returns Array The first and second lists concatenated, with duplicates removed.

Added in v0.1.0

Combines two lists into a set (i.e. no duplicates) composed of the elements of each list. Duplication is determined according to the value returned by applying the supplied predicate to two list elements. If an element exists in both lists, the first element from the first list will be used.

See also union. 
const l1 = [{a: 1}, {a: 2}];
const l2 = [{a: 1}, {a: 4}];
R.unionWith(R.eqBy(R.prop('a')), l1, l2); //=> [{a: 1}, {a: 2}, {a: 4}]

uniq

Parameters
  • list

    The array to consider.
Returns Array The list of unique items.

Added in v0.1.0

Returns a new list containing only one copy of each element in the original list. R.equals is used to determine equality.

R.uniq([1, 1, 2, 1]); //=> [1, 2]
R.uniq([1, '1']);     //=> [1, '1']
R.uniq([[42], [42]]); //=> [[42]]

uniqBy

Parameters
  • fn

    A function used to produce a value to use during comparisons.
  • list

    The array to consider.
Returns Array The list of unique items.

Added in v0.16.0

Returns a new list containing only one copy of each element in the original list, based upon the value returned by applying the supplied function to each list element. Prefers the first item if the supplied function produces the same value on two items. R.equals is used for comparison.

Acts as a transducer if a transformer is given in list position.

R.uniqBy(Math.abs, [-1, -5, 2, 10, 1, 2]); //=> [-1, -5, 2, 10]

uniqWith

Parameters
  • pred

    A predicate used to test whether two items are equal.
  • list

    The array to consider.
Returns Array The list of unique items.

Added in v0.2.0

Returns a new list containing only one copy of each element in the original list, based upon the value returned by applying the supplied predicate to two list elements. Prefers the first item if two items compare equal based on the predicate.

Acts as a transducer if a transformer is given in list position.

const strEq = R.eqBy(String);
R.uniqWith(strEq)([1, '1', 2, 1]); //=> [1, 2]
R.uniqWith(strEq)([{}, {}]);       //=> [{}]
R.uniqWith(strEq)([1, '1', 1]);    //=> [1]
R.uniqWith(strEq)(['1', 1, 1]);    //=> ['1']

unless

Parameters
  • pred

    A predicate function
  • whenFalseFn

    A function to invoke when the pred evaluates to a falsy value.
  • x

    An object to test with the pred function and pass to whenFalseFn if necessary.
Returns * Either `x` or the result of applying `x` to `whenFalseFn`.

Added in v0.18.0

Tests the final argument by passing it to the given predicate function. If the predicate is not satisfied, the function will return the result of calling the whenFalseFn function with the same argument. If the predicate is satisfied, the argument is returned as is.

See also ifElse, when, cond. 
let safeInc = R.unless(R.isNil, R.inc);
safeInc(null); //=> null
safeInc(1); //=> 2

unnest

Parameters
  • list
Returns *

Added in v0.3.0

Shorthand for R.chain(R.identity), which removes one level of nesting from any Chain.

See also flatten, chain. 
R.unnest([1, [2], [[3]]]); //=> [1, 2, [3]]
R.unnest([[1, 2], [3, 4], [5, 6]]); //=> [1, 2, 3, 4, 5, 6]

until

Parameters
  • pred

    A predicate function
  • fn

    The iterator function
  • init

    Initial value
Returns * Final value that satisfies predicate

Added in v0.20.0

Takes a predicate, a transformation function, and an initial value, and returns a value of the same type as the initial value. It does so by applying the transformation until the predicate is satisfied, at which point it returns the satisfactory value.

R.until(R.gt(R.__, 100), R.multiply(2))(1) // => 128

unwind

Parameters
  • key

    The key to determine which property of the object should be unwind
  • object

    The object containing list under property named as key which is to unwind
Returns List A new list of object containing the value of input key having list replaced by each element in the object.

Added in v0.28.0

Deconstructs an array field from the input documents to output a document for each element. Each output document is the input document with the value of the array field replaced by the element.

R.unwind('hobbies', {
  name: 'alice',
  hobbies: ['Golf', 'Hacking'],
  colors: ['red', 'green'],
});
// [
//   { name: 'alice', hobbies: 'Golf', colors: ['red', 'green'] },
//   { name: 'alice', hobbies: 'Hacking', colors: ['red', 'green'] }
// ]

update

Parameters
  • idx

    The index to update.
  • x

    The value to exist at the given index of the returned array.
  • list

    The source array-like object to be updated.
Returns Array A copy of `list` with the value at index `idx` replaced with `x`.

Added in v0.14.0

Returns a new copy of the array with the element at the provided index replaced with the given value.

See also adjust. 
R.update(1, '_', ['a', 'b', 'c']);      //=> ['a', '_', 'c']
R.update(-1, '_', ['a', 'b', 'c']);     //=> ['a', 'b', '_']

useWith

Parameters
  • fn

    The function to wrap.
  • transformers

    A list of transformer functions
Returns function The wrapped function.

Added in v0.1.0

Accepts a function fn and a list of transformer functions and returns a new curried function. When the new function is invoked, it calls the function fn with parameters consisting of the result of calling each supplied handler on successive arguments to the new function.

If more arguments are passed to the returned function than transformer functions, those arguments are passed directly to fn as additional parameters. If you expect additional arguments that don't need to be transformed, although you can ignore them, it's best to pass an identity function so that the new function reports the correct arity.

See also converge. 
R.useWith(Math.pow, [R.identity, R.identity])(3, 4); //=> 81
R.useWith(Math.pow, [R.identity, R.identity])(3)(4); //=> 81
R.useWith(Math.pow, [R.dec, R.inc])(3, 4); //=> 32
R.useWith(Math.pow, [R.dec, R.inc])(3)(4); //=> 32

values

Parameters
  • obj

    The object to extract values from
Returns Array An array of the values of the object's own properties.

Added in v0.1.0

Returns a list of all the enumerable own properties of the supplied object. Note that the order of the output array is not guaranteed across different JS platforms.

See also valuesIn, keys, toPairs. 
R.values({a: 1, b: 2, c: 3}); //=> [1, 2, 3]

valuesIn

Parameters
  • obj

    The object to extract values from
Returns Array An array of the values of the object's own and prototype properties.

Added in v0.2.0

Returns a list of all the properties, including prototype properties, of the supplied object. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.

See also values, keysIn. 
const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.valuesIn(f); //=> ['X', 'Y']

view

Parameters
  • lens
  • x
Returns *

Added in v0.16.0

Returns a "view" of the given data structure, determined by the given lens. The lens's focus determines which portion of the data structure is visible.

See also set, over, lens, lensIndex, lensProp, lensPath. 
const xLens = R.lensProp('x');

R.view(xLens, {x: 1, y: 2});  //=> 1
R.view(xLens, {x: 4, y: 2});  //=> 4

when

Parameters
  • pred

    A predicate function
  • whenTrueFn

    A function to invoke when the condition evaluates to a truthy value.
  • x

    An object to test with the pred function and pass to whenTrueFn if necessary.
Returns * Either `x` or the result of applying `x` to `whenTrueFn`.

Added in v0.18.0

Tests the final argument by passing it to the given predicate function. If the predicate is satisfied, the function will return the result of calling the whenTrueFn function with the same argument. If the predicate is not satisfied, the argument is returned as is.

See also ifElse, unless, cond. 
// truncate :: String -> String
const truncate = R.when(
  R.propSatisfies(R.gt(R.__, 10), 'length'),
  R.pipe(R.take(10), R.append('…'), R.join(''))
);
truncate('12345');         //=> '12345'
truncate('0123456789ABC'); //=> '0123456789…'

where

Parameters
  • spec
  • testObj
Returns Boolean

Added in v0.1.1

Takes a spec object and a test object; returns true if the test satisfies the spec. Each of the spec's own properties must be a predicate function. Each predicate is applied to the value of the corresponding property of the test object. where returns true if all the predicates return true, false otherwise.

where is well suited to declaratively expressing constraints for other functions such as filter and find.

See also propSatisfies, whereEq. 
// pred :: Object -> Boolean
const pred = R.where({
  a: R.equals('foo'),
  b: R.complement(R.equals('bar')),
  x: R.gt(R.__, 10),
  y: R.lt(R.__, 20)
});

pred({a: 'foo', b: 'xxx', x: 11, y: 19}); //=> true
pred({a: 'xxx', b: 'xxx', x: 11, y: 19}); //=> false
pred({a: 'foo', b: 'bar', x: 11, y: 19}); //=> false
pred({a: 'foo', b: 'xxx', x: 10, y: 19}); //=> false
pred({a: 'foo', b: 'xxx', x: 11, y: 20}); //=> false

whereAny

Parameters
  • spec
  • testObj
Returns Boolean

Added in v0.28.0

Takes a spec object and a test object; each of the spec's own properties must be a predicate function. Each predicate is applied to the value of the corresponding property of the test object. whereAny returns true if at least one of the predicates return true, false otherwise.

whereAny is well suited to declaratively expressing constraints for other functions such as filter and find.

See also propSatisfies, where. 
// pred :: Object -> Boolean
const pred = R.whereAny({
  a: R.equals('foo'),
  b: R.complement(R.equals('xxx')),
  x: R.gt(R.__, 10),
  y: R.lt(R.__, 20)
});

pred({a: 'foo', b: 'xxx', x: 8, y: 34}); //=> true
pred({a: 'xxx', b: 'xxx', x: 9, y: 21}); //=> false
pred({a: 'bar', b: 'xxx', x: 10, y: 20}); //=> false
pred({a: 'foo', b: 'bar', x: 10, y: 20}); //=> true
pred({a: 'foo', b: 'xxx', x: 11, y: 20}); //=> true

whereEq

Parameters
  • spec
  • testObj
Returns Boolean

Added in v0.14.0

Takes a spec object and a test object; returns true if the test satisfies the spec, false otherwise. An object satisfies the spec if, for each of the spec's own properties, accessing that property of the object gives the same value (in R.equals terms) as accessing that property of the spec.

whereEq is a specialization of where.

See also propEq, where. 
// pred :: Object -> Boolean
const pred = R.whereEq({a: 1, b: 2});

pred({a: 1});              //=> false
pred({a: 1, b: 2});        //=> true
pred({a: 1, b: 2, c: 3});  //=> true
pred({a: 1, b: 1});        //=> false

without

Parameters
  • list1

    The values to be removed from list2.
  • list2

    The array to remove values from.
Returns Array The new array without values in `list1`.

Added in v0.19.0

Returns a new list without values in the first argument. R.equals is used to determine equality.

Acts as a transducer if a transformer is given in list position.

See also transduce, difference, remove. 
R.without([1, 2], [1, 2, 1, 3, 4]); //=> [3, 4]

xor

Parameters
  • a
  • b
Returns Boolean true if one of the arguments is truthy and the other is falsy

Added in v0.27.1

Exclusive disjunction logical operation. Returns true if one of the arguments is truthy and the other is falsy. Otherwise, it returns false.

See also or, and. 
R.xor(true, true); //=> false
R.xor(true, false); //=> true
R.xor(false, true); //=> true
R.xor(false, false); //=> false

xprod

Parameters
  • as

    The first list.
  • bs

    The second list.
Returns Array The list made by combining each possible pair from `as` and `bs` into pairs (`[a, b]`).

Added in v0.1.0

Creates a new list out of the two supplied by creating each possible pair from the lists.

R.xprod([1, 2], ['a', 'b']); //=> [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]

zip

Parameters
  • list1

    The first array to consider.
  • list2

    The second array to consider.
Returns Array The list made by pairing up same-indexed elements of `list1` and `list2`.

Added in v0.1.0

Creates a new list out of the two supplied by pairing up equally-positioned items from both lists. The returned list is truncated to the length of the shorter of the two input lists. Note: zip is equivalent to zipWith(function(a, b) { return [a, b] }).

R.zip([1, 2, 3], ['a', 'b', 'c']); //=> [[1, 'a'], [2, 'b'], [3, 'c']]

zipObj

Parameters
  • keys

    The array that will be properties on the output object.
  • values

    The list of values on the output object.
Returns Object The object made by pairing up same-indexed elements of `keys` and `values`.

Added in v0.3.0

Creates a new object out of a list of keys and a list of values. Key/value pairing is truncated to the length of the shorter of the two lists. Note: zipObj is equivalent to pipe(zip, fromPairs).

R.zipObj(['a', 'b', 'c'], [1, 2, 3]); //=> {a: 1, b: 2, c: 3}

zipWith

Parameters
  • fn

    The function used to combine the two elements into one value.
  • list1

    The first array to consider.
  • list2

    The second array to consider.
Returns Array The list made by combining same-indexed elements of `list1` and `list2` using `fn`.

Added in v0.1.0

Creates a new list out of the two supplied by applying the function to each equally-positioned pair in the lists. The returned list is truncated to the length of the shorter of the two input lists.

const f = (x, y) => {
  // ...
};
R.zipWith(f, [1, 2, 3], ['a', 'b', 'c']);
//=> [f(1, 'a'), f(2, 'b'), f(3, 'c')]