## __ Function

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)`
``````var greet = R.replace('{name}', R.__, 'Hello, {name}!');
greet('Alice'); //=> 'Hello, Alice!'``````

`Number → Number → Number`
Parameters
• Number a
• Number b
Returns Number

``````R.add(2, 3);       //=>  5

`((a … → b) … → [a] → *) → (a …, Int, [a] → b) … → [a] → *)`
Parameters
• function 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

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, Ramda's simple `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.)

``````var 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']``````

`(a → a) → Number → [a] → [a]`
Parameters
• function fn

The function to apply.

• Number idx

The index.

• Array 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.

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.

``````R.adjust(R.add(10), 1, [0, 1, 2]);     //=> [0, 11, 2]

## all List

`(a → Boolean) → [a] → Boolean`
Parameters
• function fn

The predicate function.

• Array list

The array to consider.

Returns Boolean

`true` if the predicate is satisfied by every element, `false` otherwise.

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.

``````var lessThan2 = R.flip(R.lt)(2);
var lessThan3 = R.flip(R.lt)(3);
R.all(lessThan2)([1, 2]); //=> false
R.all(lessThan3)([1, 2]); //=> true``````

## allPass Logic

`[(*… → Boolean)] → (*… → Boolean)`
Parameters
• Array preds
Returns function

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.

``````var isQueen = R.propEq('rank', 'Q');

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

## always Function

`a → (* → a)`
Parameters
• * val

The value to wrap in a function

Returns function

A Function :: * -> val.

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.

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

## and Logic

`* → * → *`
Parameters
• Boolean a

A boolean value

• Boolean b

A boolean value

Returns Boolean

`true` if both arguments are `true`, `false` otherwise

Returns `true` if both arguments are `true`; `false` otherwise.

``````R.and(true, true); //=> true
R.and(true, false); //=> false
R.and(false, true); //=> false
R.and(false, false); //=> false``````

## any List

`(a → Boolean) → [a] → Boolean`
Parameters
• function fn

The predicate function.

• Array list

The array to consider.

Returns Boolean

`true` if the predicate is satisfied by at least one element, `false` otherwise.

Returns `true` if at least one of 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.

``````var lessThan0 = R.flip(R.lt)(0);
var lessThan2 = R.flip(R.lt)(2);
R.any(lessThan0)([1, 2]); //=> false
R.any(lessThan2)([1, 2]); //=> true``````

## anyPass Logic

`[(*… → Boolean)] → (*… → Boolean)`
Parameters
• Array preds
Returns function

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.

``````var gte = R.anyPass([R.gt, R.equals]);

gte(3, 2); //=> true
gte(2, 2); //=> true
gte(2, 3); //=> false``````

## ap Function

`[a → b] → [a] → [b]`
`Apply f => f (a → b) → f a → f b`
Parameters
• Array fns

An array of functions

• Array vs

An array of values

Returns Array

An array of results of applying each of `fns` to all of `vs` in turn.

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]``

## aperture List

`Number → [a] → [[a]]`
Parameters
• Number n

The size of the tuples to create

• Array list

The list to split into `n`-tuples

Returns Array

The new list.

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.

Dispatches to the `aperture` method of the second argument, if present.

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

``````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 List

`a → [a] → [a]`
Parameters
• * el

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

• Array list

The list whose contents will be added to the beginning of the output list.

Returns Array

A new list containing the contents of the old list followed by `el`.

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

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

## apply Function

`(*… → a) → [*] → a`
Parameters
• function fn
• Array args
Returns *

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.

``````var nums = [1, 2, 3, -99, 42, 6, 7];
R.apply(Math.max, nums); //=> 42``````

## applySpec Function

`{k: ((a, b, …, m) → v)} → ((a, b, …, m) → {k: v})`
Parameters
• Object 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.

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.

``````var getMetrics = R.applySpec({
nested: { mul: R.multiply }
});
getMetrics(2, 4); // => { sum: 6, nested: { mul: 8 } }``````

## assoc Object

`String → a → {k: v} → {k: v}`
Parameters
• String prop

the property name to set

• * val

the new value

• Object obj

the object to clone

Returns Object

a new object similar to the original except for the specified property.

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.

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

## assocPath Object

`[String] → a → {k: v} → {k: v}`
Parameters
• Array path

the path to set

• * val

the new value

• Object obj

the object to clone

Returns Object

a new object similar to the original except along the specified path.

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.

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

## binary Function

`(* → c) → (a, b → c)`
Parameters
• function fn

The function to wrap.

Returns function

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

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.

``````var takesThreeArgs = function(a, b, c) {
return [a, b, c];
};
takesThreeArgs.length; //=> 3
takesThreeArgs(1, 2, 3); //=> [1, 2, 3]

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

## bind Function

`(* → *) → {*} → (* → *)`
Parameters
• function fn

The function to bind to context

• Object thisObj

The context to bind `fn` to

Returns function

A function that will execute in the context of `thisObj`.

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

``````var 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 Logic

`(*… → Boolean) → (*… → Boolean) → (*… → Boolean)`
Parameters
• function f

a predicate

• function g

another predicate

Returns function

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

A function wrapping calls to the two functions in an `&&` operation, returning 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.

``````var gt10 = x => x > 10;
var even = x => x % 2 === 0;
var f = R.both(gt10, even);
f(100); //=> true
f(101); //=> false``````

## call Function

`(*… → a),*… → a`
Parameters
• function fn

The function to apply to the remaining arguments.

• * args

Any number of positional arguments.

Returns *

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

``````var indentN = R.pipe(R.times(R.always(' ')),
R.join(''),
R.replace(/^(?!\$)/gm));

var 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 List

`Chain m => (a → m b) → m a → m b`
Parameters
• function fn
• Array list
Returns Array

`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.

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

## clamp Relation

`Ord a => a → a → a → a`
Parameters
• Number minimum

number

• Number maximum

number

• Number value

to be clamped

Returns Number

Returns the clamped value

Restricts a number to be within a range.

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

``````R.clamp(1, 10, -1) // => 1
R.clamp(1, 10, 11) // => 10
R.clamp(1, 10, 4)  // => 4``````

## clone Object

`{*} → {*}`
Parameters
• * value

The object or array to clone

Returns *

A new object or array.

Creates a deep copy of the value which may contain (nested) `Array`s and `Object`s, `Number`s, `String`s, `Boolean`s and `Date`s. `Function`s are not copied, but assigned by their reference.

Dispatches to a `clone` method if present.

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

## comparator Function

`(a, b → Boolean) → (a, b → Number)`
Parameters
• function pred

A predicate function of arity two.

Returns function

A Function :: a -> b -> Int that returns `-1` if a < b, `1` if b < a, otherwise `0`.

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

``````var cmp = R.comparator((a, b) => a.age < b.age);
var people = [
// ...
];
R.sort(cmp, people);``````

## complement Logic

`(*… → *) → (*… → Boolean)`
Parameters
• function f
Returns function

Takes a function `f` and returns a function `g` such that:

• applying `g` to zero or more arguments will give true if applying the same arguments to `f` gives a logical false value; and

• applying `g` to zero or more arguments will give false if applying the same arguments to `f` gives a logical true value.

`R.complement` will work on all other functors as well.

``````var isEven = n => n % 2 === 0;
var isOdd = R.complement(isEven);
isOdd(21); //=> true
isOdd(42); //=> false``````

## compose Function

`((y → z), (x → y), …, (o → p), ((a, b, …, n) → o)) → ((a, b, …, n) → z)`
Parameters
• function functions
Returns function

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

Note: The result of compose is not automatically curried.

``````var f = R.compose(R.inc, R.negate, Math.pow);

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

## composeK Function

`Chain m => ((y → m z), (x → m y), …, (a → m b)) → (m a → m z)`
Parameters
• function
Returns function

Returns the right-to-left Kleisli composition of the provided functions, each of which must return a value of a type supported by `chain`.

`R.composeK(h, g, f)` is equivalent to `R.compose(R.chain(h), R.chain(g), R.chain(f))`.

``````//  parseJson :: String -> Maybe *
//  get :: String -> Object -> Maybe *

//  getStateCode :: Maybe String -> Maybe String
var getStateCode = R.composeK(
R.compose(Maybe.of, R.toUpper),
get('state'),
get('user'),
parseJson
);

//=> Just('NY')
getStateCode(Maybe.of('[Invalid JSON]'));
//=> Nothing()``````

## composeP Function

`((y → Promise z), (x → Promise y), …, (a → Promise b)) → (a → Promise z)`
Parameters
• function functions
Returns function

Performs right-to-left composition of one or more Promise-returning functions. The rightmost function may have any arity; the remaining functions must be unary.

``````//  followersForUser :: String -> Promise [User]
var followersForUser = R.composeP(db.getFollowers, db.getUserById);``````

## concat List

`[a] → [a] → [a]`
`String → String → String`
Parameters
• Array a
• Array b
Returns Array

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.

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

## cond Logic

`[[(*… → Boolean),(*… → *)]] → (*… → *)`
Parameters
• Array pairs
Returns function

Returns a function, `fn`, which encapsulates if/else-if/else logic. `R.cond` takes a list of [predicate, transform] 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.

``````var 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 Function

`(* → {*}) → (* → {*})`
Parameters
• function Fn

The constructor function to wrap.

Returns function

A wrapped, curried constructor function.

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

``````// Constructor function
var Widget = config => {
// ...
};
Widget.prototype = {
// ...
};
var allConfigs = [
// ...
];
R.map(R.construct(Widget), allConfigs); // a list of Widgets``````

## constructN Function

`Number → (* → {*}) → (* → {*})`
Parameters
• Number n

The arity of the constructor function.

• function Fn

The constructor function to wrap.

Returns function

A wrapped, curried constructor function.

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
var Widget = () => {
this.children = Array.prototype.slice.call(arguments);
// ...
};
Widget.prototype = {
// ...
};
var allConfigs = [
// ...
];
R.map(R.constructN(1, Widget), allConfigs); // a list of Widgets``````

## contains List

`a → [a] → Boolean`
Parameters
• Object a

The item to compare against.

• Array list

The array to consider.

Returns Boolean

`true` if the item is in the list, `false` otherwise.

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

``````R.contains(3, [1, 2, 3]); //=> true
R.contains(4, [1, 2, 3]); //=> false
R.contains(, []); //=> true``````

## converge Function

`(x1 → x2 → … → z) → [(a → b → … → x1), (a → b → … → x2), …] → (a → b → … → z)`
Parameters
• function after

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

• Array functions

A list of functions.

Returns function

A new function.

Accepts a converging function and a list of branching functions and returns a new function. When invoked, this new function is applied to some arguments, 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.

``````var add = (a, b) => a + b;
var multiply = (a, b) => a * b;
var subtract = (a, b) => a - b;

//≅ multiply( add(1, 2), subtract(1, 2) );
R.converge(multiply, [add, subtract])(1, 2); //=> -3

var add3 = (a, b, c) => a + b + c;

## countBy Relation

`(a → String) → [a] → {*}`
Parameters
• function fn

The function used to map values to keys.

• Array list

The list to count elements from.

Returns Object

An object mapping keys to number of occurrences in the list.

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.

``````var numbers = [1.0, 1.1, 1.2, 2.0, 3.0, 2.2];
var letters = R.split('', 'abcABCaaaBBc');
R.countBy(Math.floor)(numbers);    //=> {'1': 3, '2': 2, '3': 1}
R.countBy(R.toLower)(letters);   //=> {'a': 5, 'b': 4, 'c': 3}``````

## curry Function

`(* → a) → (* → a)`
Parameters
• function fn

The function to curry.

Returns function

A new, curried function.

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)`
``````var addFourNumbers = (a, b, c, d) => a + b + c + d;

var g = f(3);
g(4); //=> 10``````

## curryN Function

`Number → (* → a) → (* → a)`
Parameters
• Number length

The arity for the returned function.

• function fn

The function to curry.

Returns function

A new, curried function.

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)`
``````var sumArgs = (...args) => R.sum(args);

var g = f(3);
g(4); //=> 10``````

## dec Math

`Number → Number`
Parameters
• Number n
Returns Number

Decrements its argument.

``R.dec(42); //=> 41``

## defaultTo Logic

`a → b → a | b`
Parameters
• a val

The default value.

• b val

The value to return if it is not null or undefined

Returns *

The the second value or the default value

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

``````var defaultTo42 = R.defaultTo(42);

defaultTo42(null);  //=> 42
defaultTo42(undefined);  //=> 42
defaultTo42('Ramda');  //=> 'Ramda'
defaultTo42(parseInt('string')); //=> 42``````

## difference Relation

`[*] → [*] → [*]`
Parameters
• Array list1

The first list.

• Array list2

The second list.

Returns Array

The elements in `list1` that are not in `list2`.

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

``````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]``````

## differenceWith Relation

`(a → a → Boolean) → [*] → [*] → [*]`
Parameters
• function pred

A predicate used to test whether two items are equal.

• Array list1

The first list.

• Array list2

The second list.

Returns Array

The elements in `list1` that are not in `list2`.

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.

``````var cmp = (x, y) => x.a === y.a;
var l1 = [{a: 1}, {a: 2}, {a: 3}];
var l2 = [{a: 3}, {a: 4}];
R.differenceWith(cmp, l1, l2); //=> [{a: 1}, {a: 2}]``````

## dissoc Object

`String → {k: v} → {k: v}`
Parameters
• String prop

the name of the property to dissociate

• Object obj

the object to clone

Returns Object

a new object similar to the original but without the specified property

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

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

## dissocPath Object

`[String] → {k: v} → {k: v}`
Parameters
• Array path

the path to set

• Object obj

the object to clone

Returns Object

a new object without the property at path

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.

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

## divide Math

`Number → Number → Number`
Parameters
• Number a

The first value.

• Number b

The second value.

Returns Number

The result of `a / b`.

Divides two numbers. Equivalent to `a / b`.

``````R.divide(71, 100); //=> 0.71

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

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

## drop List

`Number → [a] → [a]`
`Number → String → String`
Parameters
• Number n
• * list
Returns *

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.

``````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 List

`Number → [a] → [a]`
`Number → String → String`
Parameters
• Number n

The number of elements of `xs` to skip.

• Array xs

The collection to consider.

Returns Array

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

``````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 List

`(a → Boolean) → [a] → [a]`
Parameters
• function fn

The function called per iteration.

• Array list

The collection to iterate over.

Returns Array

A new array.

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 while the predicate function returns `true`. The predicate function is applied to one argument: (value).

``````var lteThree = x => x <= 3;

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

## dropRepeats List

`[a] → [a]`
Parameters
• Array list

The array to consider.

Returns Array

`list` without repeating elements.

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

Dispatches to the `dropRepeats` method of the first argument, if present.

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

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

## dropRepeatsWith List

`(a, a → Boolean) → [a] → [a]`
Parameters
• function pred

A predicate used to test whether two items are equal.

• Array list

The array to consider.

Returns Array

`list` without repeating elements.

Returns a new list without any consecutively repeating elements. Equality is determined by applying the supplied predicate two consecutive elements. The first element in a series of equal element is the one being preserved.

Dispatches to the `dropRepeatsWith` method of the second argument, if present.

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

``````var 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 List

`(a → Boolean) → [a] → [a]`
Parameters
• function fn

The function called per iteration.

• Array list

The collection to iterate over.

Returns Array

A new array.

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.

``````var lteTwo = x => x <= 2;

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

## either Logic

`(*… → Boolean) → (*… → Boolean) → (*… → Boolean)`
Parameters
• function f

a predicate

• function g

another predicate

Returns function

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

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.

``````var gt10 = x => x > 10;
var even = x => x % 2 === 0;
var f = R.either(gt10, even);
f(101); //=> true
f(8); //=> true``````

## empty Function

`a → a`
Parameters
• * x
Returns *

Returns the empty value of its argument's type. Ramda defines the empty value of Array (`[]`), Object (`{}`), String (`''`), and Arguments. Other types are supported if they define `<Type>.empty` and/or `<Type>.prototype.empty`.

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});  //=> {}``````

## eqBy Relation

`(a → b) → a → a → Boolean`
Parameters
• function f
• * x
• * y
Returns Boolean

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 Object

`k → {k: v} → {k: v} → Boolean`
Parameters
• String prop

The name of the property to compare

• Object obj1
• Object obj2
Returns Boolean

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

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

## equals Relation

`a → b → Boolean`
Parameters
• * a
• * b
Returns Boolean

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

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

## evolve Object

`{k: (v → v)} → {k: v} → {k: v}`
Parameters
• Object transformations

The object specifying transformation functions to apply to the object.

• Object object

The object to be transformed.

Returns Object

The transformed object.

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.

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

## F Function

`* → Boolean`
Parameters
• *
Returns Boolean

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

``R.F(); //=> false``

## filter List

`Filterable f => (a → Boolean) → f a → f a`
Parameters
• function pred
• Array filterable
Returns Array

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.

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

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

``````var 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 List

`(a → Boolean) → [a] → a | undefined`
Parameters
• function fn

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

• Array list

The array to consider.

Returns Object

The element found, or `undefined`.

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.

``````var 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 List

`(a → Boolean) → [a] → Number`
Parameters
• function fn

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

• Array list

The array to consider.

Returns Number

The index of the element found, or `-1`.

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

Dispatches to the `findIndex` method of the second argument, if present.

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

``````var xs = [{a: 1}, {a: 2}, {a: 3}];
R.findIndex(R.propEq('a', 2))(xs); //=> 1
R.findIndex(R.propEq('a', 4))(xs); //=> -1``````

## findLast List

`(a → Boolean) → [a] → a | undefined`
Parameters
• function fn

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

• Array list

The array to consider.

Returns Object

The element found, or `undefined`.

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

Dispatches to the `findLast` method of the second argument, if present.

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

``````var 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 List

`(a → Boolean) → [a] → Number`
Parameters
• function fn

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

• Array list

The array to consider.

Returns Number

The index of the element found, or `-1`.

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

Dispatches to the `findLastIndex` method of the second argument, if present.

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

``````var 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 List

`[a] → [b]`
Parameters
• Array list

The array to consider.

Returns Array

The flattened list.

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.

``````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 Function

`(a → b → c → … → z) → (b → a → c → … → z)`
Parameters
• function fn

The function to invoke with its first two parameters reversed.

Returns *

The result of invoking `fn` with its first two parameters' order reversed.

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

``````var 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 List

`(a → *) → [a] → [a]`
Parameters
• function fn

The function to invoke. Receives one argument, `value`.

• Array list

The list to iterate over.

Returns Array

The original list.

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.

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

## fromPairs List

`[[k,v]] → {k: v}`
Parameters
• Array 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`.

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.

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

## groupBy List

`(a → String) → [a] → {String: [a]}`
Parameters
• function fn

Function :: a -> String

• Array 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`.

Splits a list into sub-lists stored in an object, based on the result of calling a String-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.

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

## groupWith List

`((a, a) → Boolean) → [a] → [[a]]`
Parameters
• function fn

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

• Array 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 equal elements, whose concatenations are equal to the original list.

Takes a list and returns a list of lists where each sublist's elements are all "equal" according to the provided equality function.

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

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

R.groupWith(R.eqBy(isVowel), 'aestiou')
//=> ['ae', 'st', 'iou']``````

## gt Relation

`Ord a => a → a → Boolean`
Parameters
• * a
• * b
Returns Boolean

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

``````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 Relation

`Ord a => a → a → Boolean`
Parameters
• Number a
• Number b
Returns Boolean

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

``````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 Object

`s → {s: x} → Boolean`
Parameters
• String prop

The name of the property to check for.

• Object obj

The object to query.

Returns Boolean

Whether the property exists.

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

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

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

## hasIn Object

`s → {s: x} → Boolean`
Parameters
• String prop

The name of the property to check for.

• Object obj

The object to query.

Returns Boolean

Whether the property exists.

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;
};

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

## identical Relation

`a → a → Boolean`
Parameters
• * a
• * b
Returns Boolean

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.

``````var 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 Function

`a → a`
Parameters
• * x

The value to return.

Returns *

The input value, `x`.

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

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

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

## ifElse Logic

`(*… → Boolean) → (*… → *) → (*… → *) → (*… → *)`
Parameters
• function condition

A predicate function

• function onTrue

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

• function onFalse

A function to invoke when the `condition` evaluates to a falsy value.

Returns function

A new unary function that will process either the `onTrue` or the `onFalse` function depending upon the result of the `condition` predicate.

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

``````var incCount = R.ifElse(
R.has('count'),
R.over(R.lensProp('count'), R.inc),
R.assoc('count', 1)
);
incCount({});           //=> { count: 1 }
incCount({ count: 1 }); //=> { count: 2 }``````

## inc Math

`Number → Number`
Parameters
• Number n
Returns Number

Increments its argument.

``R.inc(42); //=> 43``

## indexBy List

`(a → String) → [{k: v}] → {k: {k: v}}`
Parameters
• function fn

Function :: a -> String

• Array array

The array of objects to index

Returns Object

An object indexing each array element by the given property.

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.

``````var 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 List

`a → [a] → Number`
Parameters
• * target

The item to find.

• Array xs

The array to search in.

Returns Number

the index of the target, or -1 if the target is not found.

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.

``````R.indexOf(3, [1,2,3,4]); //=> 2
R.indexOf(10, [1,2,3,4]); //=> -1``````

## init List

`[a] → [a]`
`String → String`
Parameters
• * list
Returns *

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

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

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

## insert List

`Number → a → [a] → [a]`
Parameters
• Number index

The position to insert the element

• * elt

The element to insert into the Array

• Array list

The list to insert into

Returns Array

A new Array with `elt` inserted at `index`.

Inserts the supplied element into the list, at index `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 List

`Number → [a] → [a] → [a]`
Parameters
• Number index

The position to insert the sub-list

• Array elts

The sub-list to insert into the Array

• Array list

The list to insert the sub-list into

Returns Array

A new Array with `elts` inserted starting at `index`.

Inserts the sub-list into the list, at index `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 Relation

`[*] → [*] → [*]`
Parameters
• Array list1

The first list.

• Array list2

The second list.

Returns Array

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

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

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

## intersectionWith Relation

`(a → a → Boolean) → [*] → [*] → [*]`
Parameters
• function pred

A predicate function that determines whether the two supplied elements are equal.

• Array list1

One list of items to compare

• Array list2

A second list of items to compare

Returns Array

A new list containing those elements common to both lists.

Combines two lists into a set (i.e. no duplicates) composed of those elements common to both lists. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.

``````var buffaloSpringfield = [
{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'}
];
var csny = [
{id: 204, name: 'David Crosby'},
{id: 456, name: 'Stephen Stills'},
{id: 539, name: 'Graham Nash'},
{id: 177, name: 'Neil Young'}
];

R.intersectionWith(R.eqBy(R.prop('id')), buffaloSpringfield, csny);
//=> [{id: 456, name: 'Stephen Stills'}, {id: 177, name: 'Neil Young'}]``````

## intersperse List

`a → [a] → [a]`
Parameters
• * separator

The element to add to the list.

• Array list

The list to be interposed.

Returns Array

The new list.

Creates a new list with the separator interposed between elements.

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

``R.intersperse('n', ['ba', 'a', 'a']); //=> ['ba', 'n', 'a', 'n', 'a']``

## into List

`a → (b → b) → [c] → a`
Parameters
• * acc

The initial accumulator value.

• function xf

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

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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.

``````var numbers = [1, 2, 3, 4];

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

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

## invert Object

`{s: x} → {x: [ s, … ]}`
Parameters
• Object obj

The object or array to invert

Returns Object

out A new object with keys in an array.

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

``````var raceResultsByFirstName = {
first: 'alice',
second: 'jake',
third: 'alice',
};
R.invert(raceResultsByFirstName);
//=> { 'alice': ['first', 'third'], 'jake':['second'] }``````

## invertObj Object

`{s: x} → {x: s}`
Parameters
• Object obj

The object or array to invert

Returns Object

out A new object

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.

``````var raceResults = {
first: 'alice',
second: 'jake'
};
R.invertObj(raceResults);
//=> { 'alice': 'first', 'jake':'second' }

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

## invoker Function

`Number → String → (a → b → … → n → Object → *)`
Parameters
• Number arity

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

• String method

Name of the method to call.

Returns function

A new curried function.

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.

``````var sliceFrom = R.invoker(1, 'slice');
sliceFrom(6, 'abcdefghijklm'); //=> 'ghijklm'
var sliceFrom6 = R.invoker(2, 'slice')(6);
sliceFrom6(8, 'abcdefghijklm'); //=> 'gh'``````

## is Type

`(* → {*}) → a → Boolean`
Parameters
• Object ctor

A constructor

• * val

The value to test

Returns Boolean

See if an object (`val`) is an instance of the supplied constructor. This function will check up the inheritance chain, if any.

``````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``````

## isArrayLike Type

`* → Boolean`
Parameters
• * x

The object to test.

Returns Boolean

`true` if `x` has a numeric length property and extreme indices defined; `false` otherwise.

Tests whether or not an object is similar to an array.

``````R.isArrayLike([]); //=> true
R.isArrayLike(true); //=> false
R.isArrayLike({}); //=> false
R.isArrayLike({length: 10}); //=> false
R.isArrayLike({0: 'zero', 9: 'nine', length: 10}); //=> true``````

## isEmpty Logic

`a → Boolean`
Parameters
• * x
Returns Boolean

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

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

## isNil Type

`* → Boolean`
Parameters
• * x

The value to test.

Returns Boolean

`true` if `x` is `undefined` or `null`, otherwise `false`.

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 List

`String → [a] → String`
Parameters
• Number separator

The string used to separate the elements.

• Array xs

The elements to join into a string.

Returns String

str The string made by concatenating `xs` with `separator`.

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

``````var spacer = R.join(' ');
spacer(['a', 2, 3.4]);   //=> 'a 2 3.4'
R.join('|', [1, 2, 3]);    //=> '1|2|3'``````

## juxt Function

`[(a, b, …, m) → n] → ((a, b, …, m) → [n])`
Parameters
• Array 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.

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

``````var getRange = R.juxt([Math.min, Math.max]);
getRange(3, 4, 9, -3); //=> [-3, 9]``````

## keys Object

`{k: v} → [k]`
Parameters
• Object obj

The object to extract properties from

Returns Array

An array of the object's own properties.

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.

``R.keys({a: 1, b: 2, c: 3}); //=> ['a', 'b', 'c']``

## keysIn Object

`{k: v} → [k]`
Parameters
• Object obj

The object to extract properties from

Returns Array

An array of the object's own and prototype properties.

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.

``````var F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
var f = new F();
R.keysIn(f); //=> ['x', 'y']``````

## last List

`[a] → a | Undefined`
`String → String`
Parameters
• * list
Returns *

Returns the last element of the given list or string.

``````R.last(['fi', 'fo', 'fum']); //=> 'fum'
R.last([]); //=> undefined

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

## lastIndexOf List

`a → [a] → Number`
Parameters
• * target

The item to find.

• Array xs

The array to search in.

Returns Number

the index of the target, or -1 if the target is not found.

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.

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

## length List

`[a] → Number`
Parameters
• Array list

The array to inspect.

Returns Number

The length of the array.

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

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

## lens Object

`(s → a) → ((a, s) → s) → Lens s a`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• function getter
• function setter
Returns Lens

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.

``````var 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 Object

`Number → Lens s a`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• Number n
Returns Lens

Returns a lens whose focus is the specified index.

``````var 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 Object

`[String] → Lens s a`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• Array path

The path to use.

Returns Lens

Returns a lens whose focus is the specified path.

``````var xyLens = R.lensPath(['x', 'y']);

R.view(xyLens, {x: {y: 2, z: 3}});            //=> 2
R.set(xyLens, 4, {x: {y: 2, z: 3}});          //=> {x: {y: 4, z: 3}}
R.over(xyLens, R.negate, {x: {y: 2, z: 3}});  //=> {x: {y: -2, z: 3}}``````

## lensProp Object

`String → Lens s a`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• String k
Returns Lens

Returns a lens whose focus is the specified property.

``````var 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 Function

`(*… → *) → ([*]… → [*])`
Parameters
• function fn

The function to lift into higher context

Returns function

The lifted function.

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

``````var madd3 = R.lift(R.curry((a, b, c) => a + b + c));

madd3([1,2,3], [1,2,3], ); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7]

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

madd5([1,2], , [4, 5], , [7, 8]); //=> [21, 22, 22, 23, 22, 23, 23, 24]``````

## liftN Function

`Number → (*… → *) → ([*]… → [*])`
Parameters
• function fn

The function to lift into higher context

Returns function

The lifted function.

"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.

``````var madd3 = R.liftN(3, R.curryN(3, (...args) => R.sum(args)));
madd3([1,2,3], [1,2,3], ); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7]``````

## lt Relation

`Ord a => a → a → Boolean`
Parameters
• * a
• * b
Returns Boolean

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

``````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 Relation

`Ord a => a → a → Boolean`
Parameters
• Number a
• Number b
Returns Boolean

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

``````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 List

`Functor f => (a → b) → f a → f b`
Parameters
• function fn

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

• Array list

The list to be iterated over.

Returns Array

The new list.

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.

``````var 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 List

`(acc → x → (acc, y)) → acc → [x] → (acc, [y])`
Parameters
• function fn

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

• * acc

The accumulator value.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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].

``````var digits = ['1', '2', '3', '4'];
var appender = (a, b) => [a + b, a + b];

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

## mapAccumRight List

`(acc → x → (acc, y)) → acc → [x] → (acc, [y])`
Parameters
• function fn

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

• * acc

The accumulator value.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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].

``````var digits = ['1', '2', '3', '4'];
var append = (a, b) => [a + b, a + b];

R.mapAccumRight(append, 0, digits); //=> ['04321', ['04321', '0432', '043', '04']]``````

## mapObjIndexed Object

`((*, String, Object) → *) → Object → Object`
Parameters
• function fn
• Object obj
Returns Object

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.

``````var values = { x: 1, y: 2, z: 3 };
var prependKeyAndDouble = (num, key, obj) => key + (num * 2);

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

## match String

`RegExp → String → [String | Undefined]`
Parameters
• RegExp rx

A regular expression.

• String str

The string to match against

Returns Array

The list of matches or empty array.

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.

``````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 Math

`Number → Number → Number`
Parameters
• Number m

The dividend.

• Number p

the modulus.

Returns Number

The result of `b mod a`.

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.

``````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

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

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

## max Relation

`Ord a => a → a → a`
Parameters
• * a
• * b
Returns *

Returns the larger of its two arguments.

``````R.max(789, 123); //=> 789
R.max('a', 'b'); //=> 'b'``````

## maxBy Relation

`Ord b => (a → b) → a → a → a`
Parameters
• function f
• * a
• * b
Returns *

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

``````//  square :: Number -> Number
var 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 Math

`[Number] → Number`
Parameters
• Array list
Returns Number

Returns the mean of the given list of numbers.

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

## median Math

`[Number] → Number`
Parameters
• Array list
Returns Number

Returns the median of the given list of numbers.

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

## memoize Function

`(*… → a) → (*… → a)`
Parameters
• function fn

The function to memoize.

Returns function

Memoized version of `fn`.

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.

``````var count = 0;
var factorial = R.memoize(n => {
count += 1;
return R.product(R.range(1, n + 1));
});
factorial(5); //=> 120
factorial(5); //=> 120
factorial(5); //=> 120
count; //=> 1``````

## merge Object

`{k: v} → {k: v} → {k: v}`
Parameters
• Object l
• Object r
Returns Object

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.

``````R.merge({ 'name': 'fred', 'age': 10 }, { 'age': 40 });
//=> { 'name': 'fred', 'age': 40 }

var resetToDefault = R.merge(R.__, {x: 0});
resetToDefault({x: 5, y: 2}); //=> {x: 0, y: 2}``````

## mergeAll List

`[{k: v}] → {k: v}`
Parameters
• Array list

An array of objects

Returns Object

A merged object.

Merges a list of objects together into one object.

``````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}``````

## mergeWith Object

`(a → a → a) → {a} → {a} → {a}`
Parameters
• function fn
• Object l
• Object r
Returns Object

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. The key will be excluded from the returned object if the resulting value is `undefined`.

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

## mergeWithKey Object

`(String → a → a → a) → {a} → {a} → {a}`
Parameters
• function fn
• Object l
• Object r
Returns Object

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. The key will be excluded from the returned object if the resulting value is `undefined`.

``````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 Relation

`Ord a => a → a → a`
Parameters
• * a
• * b
Returns *

Returns the smaller of its two arguments.

``````R.min(789, 123); //=> 123
R.min('a', 'b'); //=> 'a'``````

## minBy Relation

`Ord b => (a → b) → a → a → a`
Parameters
• function f
• * a
• * b
Returns *

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

``````//  square :: Number -> Number
var 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``````

## modulo Math

`Number → Number → Number`
Parameters
• Number a

The value to the divide.

• Number b

The pseudo-modulus

Returns Number

The result of `b % a`.

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`.

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

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

## multiply Math

`Number → Number → Number`
Parameters
• Number a

The first value.

• Number b

The second value.

Returns Number

The result of `a * b`.

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

``````var double = R.multiply(2);
var triple = R.multiply(3);
double(3);       //=>  6
triple(4);       //=> 12
R.multiply(2, 5);  //=> 10``````

## nAry Function

`Number → (* → a) → (* → a)`
Parameters
• Number n

The desired arity of the new function.

• function fn

The function to wrap.

Returns function

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

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.

``````var takesTwoArgs = (a, b) => [a, b];

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

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

## negate Math

`Number → Number`
Parameters
• Number n
Returns Number

Negates its argument.

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

## none List

`(a → Boolean) → [a] → Boolean`
Parameters
• function fn

The predicate function.

• Array list

The array to consider.

Returns Boolean

`true` if the predicate is not satisfied by every element, `false` otherwise.

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

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

``````var isEven = n => n % 2 === 0;

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

## not Logic

`* → Boolean`
Parameters
• * a

any value

Returns Boolean

the logical inverse of passed argument.

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.

``````R.not(true); //=> false
R.not(false); //=> true
R.not(0); //=> true
R.not(1); //=> false``````

## nth List

`Number → [a] → a | Undefined`
`Number → String → String`
Parameters
• Number offset
• * list
Returns *

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

``````var 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 Function

`Number → *… → *`
Parameters
• Number n
Returns function

Returns a function which returns its nth argument.

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

## objOf Object

`String → a → {String:a}`
Parameters
• String key
• * val
Returns Object

Creates an object containing a single key:value pair.

``````var 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 Function

`a → [a]`
Parameters
• * x

any value

Returns Array

An array wrapping `x`.

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(); //=> []``````

## omit Object

`[String] → {String: *} → {String: *}`
Parameters
• Array names

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

• Object obj

The object to copy from

Returns Object

A new object with properties from `names` not on it.

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

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

## once Function

`(a… → b) → (a… → b)`
Parameters
• function fn

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

Returns function

The wrapped function.

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.

``````var addOneOnce = R.once(x => x + 1);

## or Logic

`* → * → *`
Parameters
• Boolean a

A boolean value

• Boolean b

A boolean value

Returns Boolean

`true` if one or both arguments are `true`, `false` otherwise

Returns `true` if one or both of its arguments are `true`. Returns `false` if both arguments are `false`.

``````R.or(true, true); //=> true
R.or(true, false); //=> true
R.or(false, true); //=> true
R.or(false, false); //=> false``````

## over Object

`Lens s a → (a → a) → s → s`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• Lens lens
• * v
• * x
Returns *

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.

``````var headLens = R.lensIndex(0);

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

## pair List

`a → b → (a,b)`
Parameters
• * fst
• * snd
Returns Array

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

``R.pair('foo', 'bar'); //=> ['foo', 'bar']``

## partial Function

`((a, b, c, …, n) → x) → [a, b, c, …] → ((d, e, f, …, n) → x)`
Parameters
• function f
• Array args
Returns function

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`.

``````var multiply = (a, b) => a * b;
var double = R.partial(multiply, );
double(2); //=> 4

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

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

## partialRight Function

`((a, b, c, …, n) → x) → [d, e, f, …, n] → ((a, b, c, …) → x)`
Parameters
• function f
• Array args
Returns function

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.

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

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

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

## partition List

`Filterable f => (a → Boolean) → f a → [f a, f a]`
Parameters
• function pred

A predicate to determine which side the element belongs to.

• Array 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.

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.

``````R.partition(R.contains('s'), ['sss', 'ttt', 'foo', 'bars']);
// => [ [ 'sss', 'bars' ],  [ 'ttt', 'foo' ] ]

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

## path Object

`[String] → {k: v} → v | Undefined`
Parameters
• Array path

The path to use.

• Object obj

The object to retrieve the nested property from.

Returns *

The data at `path`.

Retrieve the value at a given path.

``````R.path(['a', 'b'], {a: {b: 2}}); //=> 2
R.path(['a', 'b'], {c: {b: 2}}); //=> undefined``````

## pathEq Relation

`[String] → * → {String: *} → Boolean`
Parameters
• Array path

The path of the nested property to use

• * val

The value to compare the nested property with

• Object obj

The object to check the nested property in

Returns Boolean

`true` if the value equals the nested object property, `false` otherwise.

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

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

## pathOr Object

`a → [String] → Object → a`
Parameters
• * d

The default value.

• Array p

The path to use.

• Object obj

The object to retrieve the nested property from.

Returns *

The data at `path` of the supplied object or the default value.

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"``````

## pathSatisfies Logic

`(a → Boolean) → [String] → Object → Boolean`
Parameters
• function pred
• Array propPath
• * obj
Returns Boolean

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

``R.pathSatisfies(y => y > 0, ['x', 'y'], {x: {y: 2}}); //=> true``

## pick Object

`[k] → {k: v} → {k: v}`
Parameters
• Array names

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

• Object obj

The object to copy from

Returns Object

A new object with only properties from `names` on it.

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

``````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 Object

`[k] → {k: v} → {k: v}`
Parameters
• Array names

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

• Object obj

The object to copy from

Returns Object

A new object with only properties from `names` on it.

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

``````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 Object

`(v, k → Boolean) → {k: v} → {k: v}`
Parameters
• function pred

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

• Object obj

The object to copy from

Returns Object

A new object with only properties that satisfy `pred` on it.

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

``````var isUpperCase = (val, key) => key.toUpperCase() === key;
R.pickBy(isUpperCase, {a: 1, b: 2, A: 3, B: 4}); //=> {A: 3, B: 4}``````

## pipe Function

`(((a, b, …, n) → o), (o → p), …, (x → y), (y → z)) → ((a, b, …, n) → z)`
Parameters
• function functions
Returns function

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

In some libraries this function is named `sequence`.

Note: The result of pipe is not automatically curried.

``````var f = R.pipe(Math.pow, R.negate, R.inc);

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

## pipeK Function

`Chain m => ((a → m b), (b → m c), …, (y → m z)) → (m a → m z)`
Parameters
• function
Returns function

Returns the left-to-right Kleisli composition of the provided functions, each of which must return a value of a type supported by `chain`.

`R.pipeK(f, g, h)` is equivalent to `R.pipe(R.chain(f), R.chain(g), R.chain(h))`.

``````//  parseJson :: String -> Maybe *
//  get :: String -> Object -> Maybe *

//  getStateCode :: Maybe String -> Maybe String
var getStateCode = R.pipeK(
parseJson,
get('user'),
get('state'),
R.compose(Maybe.of, R.toUpper)
);

//=> Just('NY')
getStateCode(Maybe.of('[Invalid JSON]'));
//=> Nothing()``````

## pipeP Function

`((a → Promise b), (b → Promise c), …, (y → Promise z)) → (a → Promise z)`
Parameters
• function functions
Returns function

Performs left-to-right composition of one or more Promise-returning functions. The leftmost function may have any arity; the remaining functions must be unary.

``````//  followersForUser :: String -> Promise [User]
var followersForUser = R.pipeP(db.getUserById, db.getFollowers);``````

## pluck List

`k → [{k: v}] → [v]`
Parameters
• Number key

The key name to pluck off of each object.

• Array list

The array to consider.

Returns Array

The list of values for the given key.

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

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

## prepend List

`a → [a] → [a]`
Parameters
• * el

• Array list

The array to add to the tail of the output list.

Returns Array

A new array.

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

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

## product Math

`[Number] → Number`
Parameters
• Array list

An array of numbers

Returns Number

The product of all the numbers in the list.

Multiplies together all the elements of a list.

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

## project Object

`[k] → [{k: v}] → [{k: v}]`
Parameters
• Array props

The property names to project

• Array objs

The objects to query

Returns Array

An array of objects with just the `props` properties.

Reasonable analog to SQL `select` statement.

``````var abby = {name: 'Abby', age: 7, hair: 'blond', grade: 2};
var fred = {name: 'Fred', age: 12, hair: 'brown', grade: 7};
var kids = [abby, fred];

## prop Object

`s → {s: a} → a | Undefined`
Parameters
• String p

The property name

• Object obj

The object to query

Returns *

The value at `obj.p`.

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

``````R.prop('x', {x: 100}); //=> 100
R.prop('x', {}); //=> undefined``````

## propEq Relation

`String → a → Object → Boolean`
Parameters
• String name
• * val
• * obj
Returns Boolean

Returns `true` if the specified object property is equal, in `R.equals` terms, to the given value; `false` otherwise.

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

## propIs Type

`Type → String → Object → Boolean`
Parameters
• function type
• String name
• * obj
Returns Boolean

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

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

## propOr Object

`a → String → Object → a`
Parameters
• * val

The default value.

• String p

The name of the property to return.

• Object obj

The object to query.

Returns *

The value of given property of the supplied object or the default value.

If the given, non-null object has an own property with the specified name, returns the value of that property. Otherwise returns the provided default value.

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

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

## props Object

`[k] → {k: v} → [v]`
Parameters
• Array ps

The property names to fetch

• Object obj

The object to query

Returns Array

The corresponding values or partially applied function.

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

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

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

## propSatisfies Logic

`(a → Boolean) → String → {String: a} → Boolean`
Parameters
• function pred
• String name
• * obj
Returns Boolean

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

``R.propSatisfies(x => x > 0, 'x', {x: 1, y: 2}); //=> true``

## range List

`Number → Number → [Number]`
Parameters
• Number from

The first number in the list.

• Number to

One more than the last number in the list.

Returns Array

The list of numbers in tthe set `[a, b)`.

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 List

`((a, b) → a) → a → [b] → a`
Parameters
• function fn

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

• * acc

The accumulator value.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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.

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.

``````var numbers = [1, 2, 3];
var plus = (a, b) => a + b;

R.reduce(plus, 10, numbers); //=> 16``````

## reduceBy List

`((a, b) → a) → a → (b → String) → [b] → {String: a}`
Parameters
• function 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.

• function keyFn

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

• Array 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`.

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`.

This function is basically a more general `groupBy` function.

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

``````var reduceToNamesBy = R.reduceBy((acc, student) => acc.concat(student.name), []);
var score = student.score;
return score < 65 ? 'F' :
score < 70 ? 'D' :
score < 80 ? 'C' :
score < 90 ? 'B' : 'A';
});
var students = [{name: 'Lucy', score: 92},
{name: 'Drew', score: 85},
// ...
{name: 'Bart', score: 62}];
// {
//   'A': ['Lucy'],
//   'B': ['Drew']
//   // ...,
//   'F': ['Bart']
// }``````

## reduced List

`a → *`
Parameters
• * x

The final value of the reduce.

Returns *

The wrapped value.

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.

Note: this optimization is unavailable to functions not explicitly listed above. For instance, it is not currently supported by reduceRight.

``````R.reduce(
0,
[1, 2, 3, 4, 5]) // 10``````

## reduceRight List

`(a,b → a) → a → [b] → a`
Parameters
• function fn

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

• * acc

The accumulator value.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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: (acc, value)

Note: `R.reduceRight` 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/reduceRight#Description

``````var pairs = [ ['a', 1], ['b', 2], ['c', 3] ];
var flattenPairs = (acc, pair) => acc.concat(pair);

R.reduceRight(flattenPairs, [], pairs); //=> [ 'c', 3, 'b', 2, 'a', 1 ]``````

## reduceWhile List

`((a, b) → Boolean) → ((a, b) → a) → a → [b] → a`
Parameters
• function pred

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

• function fn

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

• * a

The accumulator value.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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.

``````var isOdd = (acc, x) => x % 2 === 1;
var xs = [1, 3, 5, 60, 777, 800];
R.reduceWhile(isOdd, R.add, 0, xs); //=> 9

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

## reject List

`Filterable f => (a → Boolean) → f a → f a`
Parameters
• function pred
• Array filterable
Returns Array

The complement of `filter`.

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

``````var isOdd = (n) => n % 2 === 1;

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 List

`Number → Number → [a] → [a]`
Parameters
• Number start

The position to start removing elements

• Number count

The number of elements to remove

• Array list

The list to remove from

Returns Array

A new Array with `count` elements from `start` removed.

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.

``R.remove(2, 3, [1,2,3,4,5,6,7,8]); //=> [1,2,6,7,8]``

## repeat List

`a → n → [a]`
Parameters
• * value

The value to repeat.

• Number n

The desired size of the output list.

Returns Array

A new array containing `n` `value`s.

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

``````R.repeat('hi', 5); //=> ['hi', 'hi', 'hi', 'hi', 'hi']

var obj = {};
var repeatedObjs = R.repeat(obj, 5); //=> [{}, {}, {}, {}, {}]
repeatedObjs === repeatedObjs; //=> true``````

## replace String

`RegExp|String → String → String → String`
Parameters
• RegExp pattern

A regular expression or a substring to match.

• String replacement

The string to replace the matches with.

• String str

The String to do the search and replacement in.

Returns String

The result.

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

``````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 List

`[a] → [a]`
`String → String`
Parameters
• Array list
Returns Array

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();        //=> 
R.reverse([]);         //=> []

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

## scan List

`(a,b → a) → a → [b] → [a]`
Parameters
• function fn

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

• * acc

The accumulator value.

• Array list

The list to iterate over.

Returns Array

A list of all intermediately reduced values.

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

``````var numbers = [1, 2, 3, 4];
var factorials = R.scan(R.multiply, 1, numbers); //=> [1, 1, 2, 6, 24]``````

## sequence List

`(Applicative f, Traversable t) => (a → f a) → t (f a) → f (t a)`
Parameters
• function of
• * traversable
Returns *

Transforms a Traversable of Applicative into an Applicative of Traversable.

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

``````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 Object

`Lens s a → a → s → s`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• Lens lens
• * v
• * x
Returns *

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

``````var 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 List

`Number → Number → [a] → [a]`
`Number → Number → String → String`
Parameters
• Number fromIndex

The start index (inclusive).

• Number toIndex

The end index (exclusive).

• * list
Returns *

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 List

`(a,a → Number) → [a] → [a]`
Parameters
• function comparator

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

• Array list

The list to sort

Returns Array

a new array with its elements sorted by the comparator function.

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.

``````var diff = function(a, b) { return a - b; };
R.sort(diff, [4,2,7,5]); //=> [2, 4, 5, 7]``````

## sortBy Relation

`Ord b => (a → b) → [a] → [a]`
Parameters
• function fn
• Array list

The list to sort.

Returns Array

A new list sorted by the keys generated by `fn`.

Sorts the list according to the supplied function.

``````var sortByFirstItem = R.sortBy(R.prop(0));
var sortByNameCaseInsensitive = R.sortBy(R.compose(R.toLower, R.prop('name')));
var pairs = [[-1, 1], [-2, 2], [-3, 3]];
sortByFirstItem(pairs); //=> [[-3, 3], [-2, 2], [-1, 1]]
var alice = {
name: 'ALICE',
age: 101
};
var bob = {
name: 'Bob',
age: -10
};
var clara = {
name: 'clara',
age: 314.159
};
var people = [clara, bob, alice];
sortByNameCaseInsensitive(people); //=> [alice, bob, clara]``````

## split String

`(String | RegExp) → String → [String]`
Parameters
• String sep

The pattern.

• String str

The string to separate into an array.

Returns Array

The array of strings from `str` separated by `str`.

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

``````var 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 List

`Number → [a] → [[a], [a]]`
`Number → String → [String, String]`
Parameters
• Number index

The index where the array/string is split.

• Array array

The array/string to be split.

Returns Array

Splits a given list or string at a given index.

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

## splitEvery List

`Number → [a] → [[a]]`
`Number → String → [String]`
Parameters
• Number n
• Array list
Returns Array

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], ]
R.splitEvery(3, 'foobarbaz'); //=> ['foo', 'bar', 'baz']``````

## splitWhen List

`(a → Boolean) → [a] → [[a], [a]]`
Parameters
• function pred

The predicate that determines where the array is split.

• Array list

The array to be split.

Returns Array

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]);   //=> [, [2, 3, 1, 2, 3]]``

## subtract Math

`Number → Number → Number`
Parameters
• Number a

The first value.

• Number b

The second value.

Returns Number

The result of `a - b`.

Subtracts its second argument from its first argument.

``````R.subtract(10, 8); //=> 2

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

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

## sum Math

`[Number] → Number`
Parameters
• Array list

An array of numbers

Returns Number

The sum of all the numbers in the list.

Adds together all the elements of a list.

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

## symmetricDifference Relation

`[*] → [*] → [*]`
Parameters
• Array list1

The first list.

• Array list2

The second list.

Returns Array

The elements in `list1` or `list2`, but not both.

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

``````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 Relation

`(a → a → Boolean) → [a] → [a] → [a]`
Parameters
• function pred

A predicate used to test whether two items are equal.

• Array list1

The first list.

• Array list2

The second list.

Returns Array

The elements in `list1` or `list2`, but not both.

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.

``````var eqA = R.eqBy(R.prop('a'));
var l1 = [{a: 1}, {a: 2}, {a: 3}, {a: 4}];
var l2 = [{a: 3}, {a: 4}, {a: 5}, {a: 6}];
R.symmetricDifferenceWith(eqA, l1, l2); //=> [{a: 1}, {a: 2}, {a: 5}, {a: 6}]``````

## T Function

`* → Boolean`
Parameters
• *
Returns Boolean

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

``R.T(); //=> true``

## tail List

`[a] → [a]`
`String → String`
Parameters
• * list
Returns *

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.

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

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

## take List

`Number → [a] → [a]`
`Number → String → String`
Parameters
• Number n
• * list
Returns *

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.

``````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'

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

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

## takeLast List

`Number → [a] → [a]`
`Number → String → String`
Parameters
• Number n

The number of elements to return.

• Array xs

The collection to consider.

Returns Array

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

``````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 List

`(a → Boolean) → [a] → [a]`
Parameters
• function fn

The function called per iteration.

• Array list

The collection to iterate over.

Returns Array

A new array.

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).

``````var isNotOne = x => x !== 1;

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

## takeWhile List

`(a → Boolean) → [a] → [a]`
Parameters
• function fn

The function called per iteration.

• Array list

The collection to iterate over.

Returns Array

A new array.

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.

``````var isNotFour = x => x !== 4;

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

## tap Function

`(a → *) → a → a`
Parameters
• function fn

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

• * x
Returns *

`x`.

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

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

## test String

`RegExp → String → Boolean`
Parameters
• RegExp pattern
• String str
Returns Boolean

Determines whether a given string matches a given regular expression.

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

## times List

`(Number → a) → Number → [a]`
Parameters
• function fn

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

• Number 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`.

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`.

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

## toLower String

`String → String`
Parameters
• String str

The string to lower case.

Returns String

The lower case version of `str`.

The lower case version of a string.

``R.toLower('XYZ'); //=> 'xyz'``

## toPairs Object

`{String: *} → [[String,*]]`
Parameters
• Object obj

The object to extract from

Returns Array

An array of key, value arrays from the object's own properties.

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.

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

## toPairsIn Object

`{String: *} → [[String,*]]`
Parameters
• Object obj

The object to extract from

Returns Array

An array of key, value arrays from the object's own and prototype properties.

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.

``````var F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
var f = new F();
R.toPairsIn(f); //=> [['x','X'], ['y','Y']]``````

## toString String

`* → String`
Parameters
• * val
Returns String

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 String

`String → String`
Parameters
• String str

The string to upper case.

Returns String

The upper case version of `str`.

The upper case version of a string.

``R.toUpper('abc'); //=> 'ABC'``

## transduce List

`(c → c) → (a,b → a) → a → [b] → a`
Parameters
• function xf

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

• function 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.

• Array list

The list to iterate over.

Returns *

The final, accumulated value.

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 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.

``````var numbers = [1, 2, 3, 4];

R.transduce(transducer, R.flip(R.append), [], numbers); //=> [2, 3]``````

## transpose List

`[[a]] → [[a]]`
Parameters
• Array list

A 2D list

Returns Array

A 2D list

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], , [], [30, 31, 32]]) //=> [[10, 20, 30], [11, 31], ]``````

## traverse List

`(Applicative f, Traversable t) => (a → f a) → (a → f b) → t a → f (t b)`
Parameters
• function of
• function f
• * traversable
Returns *

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 `sequence` method of the third argument, if present.

``````// Returns `Nothing` if the given divisor is `0`
safeDiv = n => d => d === 0 ? Nothing() : Just(n / d)

R.traverse(Maybe.of, safeDiv(10), [2, 4, 5]); //=> Just([5, 2.5, 2])
R.traverse(Maybe.of, safeDiv(10), [2, 0, 5]); //=> Nothing``````

## trim String

`String → String`
Parameters
• String str

The string to trim.

Returns String

Trimmed version of `str`.

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 Function

`(…x → a) → ((e, …x) → a) → (…x → a)`
Parameters
• function tryer

The function that may throw.

• function catcher

The function that will be evaluated if `tryer` throws.

Returns function

A new function that will catch exceptions and send then to the catcher.

`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(R.prop('x'), R.F)(null);      //=> false``````

## type Type

`(* → {*}) → String`
Parameters
• * val

The value to test

Returns String

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"``````

## unapply Function

`([*…] → a) → (*… → a)`
Parameters
• function fn
Returns function

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.

``R.unapply(JSON.stringify)(1, 2, 3); //=> '[1,2,3]'``

## unary Function

`(* → b) → (a → b)`
Parameters
• function fn

The function to wrap.

Returns function

A new function wrapping `fn`. The new function is guaranteed to be of arity 1.

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.

``````var takesTwoArgs = function(a, b) {
return [a, b];
};
takesTwoArgs.length; //=> 2
takesTwoArgs(1, 2); //=> [1, 2]

var takesOneArg = R.unary(takesTwoArgs);
takesOneArg.length; //=> 1
// Only 1 argument is passed to the wrapped function
takesOneArg(1, 2); //=> [1, undefined]``````

## uncurryN Function

`Number → (a → b) → (a → c)`
Parameters
• Number length

The arity for the returned function.

• function fn

The function to uncurry.

Returns function

A new function.

Returns a function of arity `n` from a (manually) curried function.

``````var addFour = a => b => c => d => a + b + c + d;

uncurriedAddFour(1, 2, 3, 4); //=> 10``````

## unfold List

`(a → [b]) → * → [b]`
Parameters
• function 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.

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).

``````var f = n => n > 50 ? false : [-n, n + 10];
R.unfold(f, 10); //=> [-10, -20, -30, -40, -50]``````

## union Relation

`[*] → [*] → [*]`
Parameters
• Array as

The first list.

• Array bs

The second list.

Returns Array

The first and second lists concatenated, with duplicates removed.

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 Relation

`(a → a → Boolean) → [*] → [*] → [*]`
Parameters
• function pred

A predicate used to test whether two items are equal.

• Array list1

The first list.

• Array list2

The second list.

Returns Array

The first and second lists concatenated, with duplicates removed.

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.

``````var l1 = [{a: 1}, {a: 2}];
var l2 = [{a: 1}, {a: 4}];
R.unionWith(R.eqBy(R.prop('a')), l1, l2); //=> [{a: 1}, {a: 2}, {a: 4}]``````

## uniq List

`[a] → [a]`
Parameters
• Array list

The array to consider.

Returns Array

The list of unique items.

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([, ]); //=> []``````

## uniqBy List

`(a → b) → [a] → [a]`
Parameters
• function fn

A function used to produce a value to use during comparisons.

• Array list

The array to consider.

Returns Array

The list of unique items.

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.

``R.uniqBy(Math.abs, [-1, -5, 2, 10, 1, 2]); //=> [-1, -5, 2, 10]``

## uniqWith List

`(a, a → Boolean) → [a] → [a]`
Parameters
• function pred

A predicate used to test whether two items are equal.

• Array list

The array to consider.

Returns Array

The list of unique items.

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.

``````var strEq = R.eqBy(String);
R.uniqWith(strEq)([1, '1', 2, 1]); //=> [1, 2]
R.uniqWith(strEq)([{}, {}]);       //=> [{}]
R.uniqWith(strEq)([1, '1', 1]);    //=> 
R.uniqWith(strEq)(['1', 1, 1]);    //=> ['1']``````

## unless Logic

`(a → Boolean) → (a → a) → a → a`
Parameters
• function pred

A predicate function

• 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`.

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.

``````// coerceArray :: (a|[a]) -> [a]
var coerceArray = R.unless(R.isArrayLike, R.of);
coerceArray([1, 2, 3]); //=> [1, 2, 3]
coerceArray(1);         //=> ``````

## unnest List

`Chain c => c (c a) → c a`
Parameters
• * list
Returns *

Shorthand for `R.chain(R.identity)`, which removes one level of nesting from any Chain.

``````R.unnest([1, , []]); //=> [1, 2, ]
R.unnest([[1, 2], [3, 4], [5, 6]]); //=> [1, 2, 3, 4, 5, 6]``````

## until Logic

`(a → Boolean) → (a → a) → a → a`
Parameters
• function pred

A predicate function

• function fn

The iterator function

• * init

Initial value

Returns *

Final value that satisfies predicate

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``

## update List

`Number → a → [a] → [a]`
Parameters
• Number idx

The index to update.

• * x

The value to exist at the given index of the returned array.

• 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`.

Returns a new copy of the array with the element at the provided index replaced with the given value.

``````R.update(1, 11, [0, 1, 2]);     //=> [0, 11, 2]
R.update(1)(11)([0, 1, 2]);     //=> [0, 11, 2]``````

## useWith Function

`(x1 → x2 → … → z) → [(a → x1), (b → x2), …] → (a → b → … → z)`
Parameters
• function fn

The function to wrap.

• Array transformers

A list of transformer functions

Returns function

The wrapped function.

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.

``````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 Object

`{k: v} → [v]`
Parameters
• Object obj

The object to extract values from

Returns Array

An array of the values of the object's own properties.

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.

``R.values({a: 1, b: 2, c: 3}); //=> [1, 2, 3]``

## valuesIn Object

`{k: v} → [v]`
Parameters
• Object obj

The object to extract values from

Returns Array

An array of the values of the object's own and prototype properties.

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.

``````var F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
var f = new F();
R.valuesIn(f); //=> ['X', 'Y']``````

## view Object

`Lens s a → s → a`
`Lens s a = Functor f => (a → f a) → s → f s`
Parameters
• Lens lens
• * x
Returns *

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.

``````var xLens = R.lensProp('x');

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

## when Logic

`(a → Boolean) → (a → a) → a → a`
Parameters
• function pred

A predicate function

• 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`.

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.

``````// truncate :: String -> String
var 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 Object

`{String: (* → Boolean)} → {String: *} → Boolean`
Parameters
• Object spec
• Object testObj
Returns Boolean

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`.

``````// pred :: Object -> Boolean
var pred = where({
a: equals('foo'),
b: complement(equals('bar')),
x: gt(__, 10),
y: lt(__, 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``````

## whereEq Object

`{String: *} → {String: *} → Boolean`
Parameters
• Object spec
• Object testObj
Returns Boolean

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`.

``````// pred :: Object -> Boolean
var 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 List

`[a] → [a] → [a]`
Parameters
• Array list1

The values to be removed from `list2`.

• Array list2

The array to remove values from.

Returns Array

The new array without values in `list1`.

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.

``R.without([1, 2], [1, 2, 1, 3, 4]); //=> [3, 4]``

## wrap Function

Deprecated since v0.22.0
`(a… → b) → ((a… → b) → a… → c) → (a… → c)`
Parameters
• function fn

The function to wrap.

• function wrapper

The wrapper function.

Returns function

The wrapped function.

Wrap a function inside another to allow you to make adjustments to the parameters, or do other processing either before the internal function is called or with its results.

``````var greet = name => 'Hello ' + name;

var shoutedGreet = R.wrap(greet, (gr, name) => gr(name).toUpperCase());

shoutedGreet("Kathy"); //=> "HELLO KATHY"

var shortenedGreet = R.wrap(greet, function(gr, name) {
return gr(name.substring(0, 3));
});
shortenedGreet("Robert"); //=> "Hello Rob"``````

## xprod List

`[a] → [b] → [[a,b]]`
Parameters
• Array as

The first list.

• Array bs

The second list.

Returns Array

The list made by combining each possible pair from `as` and `bs` into pairs (`[a, b]`).

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 List

`[a] → [b] → [[a,b]]`
Parameters
• Array list1

The first array to consider.

• Array list2

The second array to consider.

Returns Array

The list made by pairing up same-indexed elements of `list1` and `list2`.

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 List

`[String] → [*] → {String: *}`
Parameters
• Array keys

The array that will be properties on the output object.

• Array values

The list of values on the output object.

Returns Object

The object made by pairing up same-indexed elements of `keys` and `values`.

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(zipWith(pair), fromPairs)`.

``R.zipObj(['a', 'b', 'c'], [1, 2, 3]); //=> {a: 1, b: 2, c: 3}``

## zipWith List

`(a,b → c) → [a] → [b] → [c]`
Parameters
• function fn

The function used to combine the two elements into one value.

• Array list1

The first array to consider.

• Array list2

The second array to consider.

Returns Array

The list made by combining same-indexed elements of `list1` and `list2` using `fn`.

``````var f = (x, y) => {