A special placeholder value used to specify "gaps" within curried functions, allowing partial application of any combination of arguments, regardless of their positions.
If g is a curried ternary function and _ is R.__, the following are equivalent:
g(1, 2, 3)g(_, 2, 3)(1)g(_, _, 3)(1)(2)g(_, _, 3)(1, 2)g(_, 2, _)(1, 3)g(_, 2)(1)(3)g(_, 2)(1, 3)g(_, 2)(_, 3)(1)const greet = R.replace('{name}', R.__, 'Hello, {name}!');
greet('Alice'); //=> 'Hello, Alice!' Number → Number → Number
a b Adds two values.
See also subtract.
R.add(2, 3); //=> 5 R.add(7)(10); //=> 17
((a … → b) … → [a] → *) → ((a …, Int, [a] → b) … → [a] → *)
fn A list iteration function that does not pass index or 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, R.map function into one that more closely resembles Array.prototype.map. Note that this will only work for functions in which the iteration callback function is the first parameter, and where the list is the last parameter. (This latter might be unimportant if the list parameter is not used.)
const mapIndexed = R.addIndex(R.map); mapIndexed((val, idx) => idx + '-' + val, ['f', 'o', 'o', 'b', 'a', 'r']); //=> ['0-f', '1-o', '2-o', '3-b', '4-a', '5-r']
Number → (a → a) → [a] → [a]
idx The index.
fn The function to apply.
list An array-like object whose value at the supplied index will be replaced.
Applies a function to the value at the given index of an array, returning a new copy of the array with the element at the given index replaced with the result of the function application.
See also update.
R.adjust(1, R.toUpper, ['a', 'b', 'c', 'd']); //=> ['a', 'B', 'c', 'd'] R.adjust(-1, R.toUpper, ['a', 'b', 'c', 'd']); //=> ['a', 'b', 'c', 'D']
(a → Boolean) → [a] → Boolean
fn The predicate function.
list The array to consider.
Returns true if all elements of the list match the predicate, false if there are any that don't.
Dispatches to the all method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also any, none, transduce.
const equals3 = R.equals(3); R.all(equals3)([3, 3, 3, 3]); //=> true R.all(equals3)([3, 3, 1, 3]); //=> false
[(*… → Boolean)] → (*… → Boolean)
predicates An array of predicates to check
Takes a list of predicates and returns a predicate that returns true for a given list of arguments if every one of the provided predicates is satisfied by those arguments.
The function returned is a curried function whose arity matches that of the highest-arity predicate.
See also anyPass.
const isQueen = R.propEq('rank', 'Q');
const isSpade = R.propEq('suit', '♠︎');
const isQueenOfSpades = R.allPass([isQueen, isSpade]);
isQueenOfSpades({rank: 'Q', suit: '♣︎'}); //=> false
isQueenOfSpades({rank: 'Q', suit: '♠︎'}); //=> true a → (* → a)
val The value to wrap in a function
Returns a function that always returns the given value. Note that for non-primitives the value returned is a reference to the original value.
This function is known as const, constant, or K (for K combinator) in other languages and libraries.
const t = R.always('Tee');
t(); //=> 'Tee' a → b → a | b
a b Returns true if both arguments are true; false otherwise.
See also both.
R.and(true, true); //=> true R.and(true, false); //=> false R.and(false, true); //=> false R.and(false, false); //=> false
(a → Boolean) → [a] → Boolean
fn The predicate function.
list The array to consider.
Returns true if at least one of the elements of the list match the predicate, false otherwise.
Dispatches to the any method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also all, none, transduce.
const lessThan0 = R.flip(R.lt)(0); const lessThan2 = R.flip(R.lt)(2); R.any(lessThan0)([1, 2]); //=> false R.any(lessThan2)([1, 2]); //=> true
[(*… → Boolean)] → (*… → Boolean)
predicates An array of predicates to check
Takes a list of predicates and returns a predicate that returns true for a given list of arguments if at least one of the provided predicates is satisfied by those arguments.
The function returned is a curried function whose arity matches that of the highest-arity predicate.
See also allPass.
const isClub = R.propEq('suit', '♣');
const isSpade = R.propEq('suit', '♠');
const isBlackCard = R.anyPass([isClub, isSpade]);
isBlackCard({rank: '10', suit: '♣'}); //=> true
isBlackCard({rank: 'Q', suit: '♠'}); //=> true
isBlackCard({rank: 'Q', suit: '♦'}); //=> false [a → b] → [a] → [b]
Apply f => f (a → b) → f a → f b(r → a → b) → (r → a) → (r → b)applyF applyX ap applies a list of functions to a list of values.
Dispatches to the ap method of the second argument, if present. Also treats curried functions as applicatives.
R.ap([R.multiply(2), R.add(3)], [1,2,3]); //=> [2, 4, 6, 4, 5, 6]
R.ap([R.concat('tasty '), R.toUpper], ['pizza', 'salad']); //=> ["tasty pizza", "tasty salad", "PIZZA", "SALAD"]
// R.ap can also be used as S combinator
// when only two functions are passed
R.ap(R.concat, R.toUpper)('Ramda') //=> 'RamdaRAMDA' Number → [a] → [[a]]
n The size of the tuples to create
list The list to split into n-length tuples
Returns a new list, composed of n-tuples of consecutive elements. If n is greater than the length of the list, an empty list is returned.
Acts as a transducer if a transformer is given in list position.
See also transduce.
R.aperture(2, [1, 2, 3, 4, 5]); //=> [[1, 2], [2, 3], [3, 4], [4, 5]] R.aperture(3, [1, 2, 3, 4, 5]); //=> [[1, 2, 3], [2, 3, 4], [3, 4, 5]] R.aperture(7, [1, 2, 3, 4, 5]); //=> []
a → [a] → [a]
el The element to add to the end of the new list.
list The list of elements to add a new item to. list.
Returns a new list containing the contents of the given list, followed by the given element.
See also prepend.
R.append('tests', ['write', 'more']); //=> ['write', 'more', 'tests']
R.append('tests', []); //=> ['tests']
R.append(['tests'], ['write', 'more']); //=> ['write', 'more', ['tests']] (*… → a) → [*] → a
fn The function which will be called with args
args The arguments to call fn with
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.
const nums = [1, 2, 3, -99, 42, 6, 7]; R.apply(Math.max, nums); //=> 42
{k: ((a, b, …, m) → v)} → ((a, b, …, m) → {k: v}) spec an object recursively mapping properties to functions for producing the values for these properties.
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.
const getMetrics = R.applySpec({
sum: R.add,
nested: { mul: R.multiply }
});
getMetrics(2, 4); // => { sum: 6, nested: { mul: 8 } } a → (a → b) → b
x The value
f The function to apply
Takes a value and applies a function to it.
This function is also known as the thrush combinator.
const t42 = R.applyTo(42); t42(R.identity); //=> 42 t42(R.add(1)); //=> 43
Ord b => (a → b) → a → a → Number
fn A function of arity one that returns a value that can be compared
a The first item to be compared.
b The second item to be compared.
Makes an ascending comparator function out of a function that returns a value that can be compared with < and >.
See also descend.
const byAge = R.ascend(R.prop('age'));
const people = [
{ name: 'Emma', age: 70 },
{ name: 'Peter', age: 78 },
{ name: 'Mikhail', age: 62 },
];
const peopleByYoungestFirst = R.sort(byAge, people);
//=> [{ name: 'Mikhail', age: 62 },{ name: 'Emma', age: 70 }, { name: 'Peter', age: 78 }] String → a → {k: v} → {k: v} prop The property name to set
val The new value
obj The object to clone
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} [Idx] → a → {a} → {a} Idx = String | Intpath the path to set
val The new value
obj The object to clone
Makes a shallow clone of an object, setting or overriding the nodes required to create the given path, and placing the specific value at the tail end of that path. Note that this copies and flattens prototype properties onto the new object as well. All non-primitive properties are copied by reference.
See also dissocPath.
R.assocPath(['a', 'b', 'c'], 42, {a: {b: {c: 0}}}); //=> {a: {b: {c: 42}}}
// Any missing or non-object keys in path will be overridden
R.assocPath(['a', 'b', 'c'], 42, {a: 5}); //=> {a: {b: {c: 42}}} (* → c) → (a, b → c)
fn The function to wrap.
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.
const takesThreeArgs = function(a, b, c) {
return [a, b, c];
};
takesThreeArgs.length; //=> 3
takesThreeArgs(1, 2, 3); //=> [1, 2, 3]
const takesTwoArgs = R.binary(takesThreeArgs);
takesTwoArgs.length; //=> 2
// Only 2 arguments are passed to the wrapped function
takesTwoArgs(1, 2, 3); //=> [1, 2, undefined] (* → *) → {*} → (* → *) fn The function to bind to context
thisObj The context to bind fn to
Creates a function that is bound to a context. Note: R.bind does not provide the additional argument-binding capabilities of Function.prototype.bind.
See also partial.
const log = R.bind(console.log, console);
R.pipe(R.assoc('a', 2), R.tap(log), R.assoc('a', 3))({a: 1}); //=> {a: 3}
// logs {a: 2} (*… → Boolean) → (*… → Boolean) → (*… → Boolean)
f A predicate
g Another predicate
A function which calls the two provided functions and returns the && of the results. It returns the result of the first function if it is false-y and the result of the second function otherwise. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a false-y value.
In addition to functions, R.both also accepts any fantasy-land compatible applicative functor.
See also and.
const gt10 = R.gt(R.__, 10) const lt20 = R.lt(R.__, 20) const f = R.both(gt10, lt20); f(15); //=> true f(30); //=> false R.both(Maybe.Just(false), Maybe.Just(55)); // => Maybe.Just(false) R.both([false, false, 'a'], [11]); //=> [false, false, 11]
(*… → a),*… → a
fn The function to apply to the remaining arguments.
args Any number of positional arguments.
Returns the result of calling its first argument with the remaining arguments. This is occasionally useful as a converging function for R.converge: the first branch can produce a function while the remaining branches produce values to be passed to that function as its arguments.
See also apply.
R.call(R.add, 1, 2); //=> 3
const indentN = R.pipe(R.repeat(' '),
R.join(''),
R.replace(/^(?!$)/gm));
const format = R.converge(R.call, [
R.pipe(R.prop('indent'), indentN),
R.prop('value')
]);
format({indent: 2, value: 'foo\nbar\nbaz\n'}); //=> ' foo\n bar\n baz\n' Chain m => (a → m b) → m a → m b
fn The function to map with
list The list to map over
chain maps a function over a list and concatenates the results. chain is also known as flatMap in some libraries.
Dispatches to the chain method of the second argument, if present, according to the FantasyLand Chain spec.
If second argument is a function, chain(f, g)(x) is equivalent to f(g(x), x).
Acts as a transducer if a transformer is given in list position.
const duplicate = n => [n, n]; R.chain(duplicate, [1, 2, 3]); //=> [1, 1, 2, 2, 3, 3] R.chain(R.append, R.head)([1, 2, 3]); //=> [1, 2, 3, 1]
Ord a => a → a → a → a
minimum The lower limit of the clamp (inclusive)
maximum The upper limit of the clamp (inclusive)
value Value to be clamped
Restricts a number to be within a range.
Also works for other ordered types such as Strings and Dates.
R.clamp(1, 10, -5) // => 1 R.clamp(1, 10, 15) // => 10 R.clamp(1, 10, 4) // => 4
{*} → {*} value The object or array to clone
Creates a deep copy of the value which may contain (nested) Arrays and Objects, Numbers, Strings, Booleans and Dates. Functions are assigned by reference rather than copied
Dispatches to a clone method if present.
const objects = [{}, {}, {}];
const objectsClone = R.clone(objects);
objects === objectsClone; //=> false
objects[0] === objectsClone[0]; //=> false ((a, b) → Boolean) → ((a, b) → Number)
pred A predicate function of arity two which will return true if the first argument is less than the second, false otherwise
Makes a comparator function out of a function that reports whether the first element is less than the second.
const byAge = R.comparator((a, b) => a.age < b.age);
const people = [
{ name: 'Emma', age: 70 },
{ name: 'Peter', age: 78 },
{ name: 'Mikhail', age: 62 },
];
const peopleByIncreasingAge = R.sort(byAge, people);
//=> [{ name: 'Mikhail', age: 62 },{ name: 'Emma', age: 70 }, { name: 'Peter', age: 78 }] (*… → *) → (*… → Boolean)
f Takes a function f and returns a function g such that if called with the same arguments when f returns a "truthy" value, g returns false and when f returns a "falsy" value g returns true.
R.complement may be applied to any functor
See also not.
const isNotNil = R.complement(R.isNil); isNil(null); //=> true isNotNil(null); //=> false isNil(7); //=> false isNotNil(7); //=> true
((y → z), (x → y), …, (o → p), ((a, b, …, n) → o)) → ((a, b, …, n) → z)
...functions The functions to compose
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.
See also pipe.
const classyGreeting = (firstName, lastName) => "The name's " + lastName + ", " + firstName + " " + lastName
const yellGreeting = R.compose(R.toUpper, classyGreeting);
yellGreeting('James', 'Bond'); //=> "THE NAME'S BOND, JAMES BOND"
R.compose(Math.abs, R.add(1), R.multiply(2))(-4) //=> 7 Chain m => ((y → m z), (x → m y), …, (a → m b)) → (a → m z)...functions The functions to compose
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), f).
See also pipeK.
// get :: String -> Object -> Maybe *
const get = R.curry((propName, obj) => Maybe(obj[propName]))
// getStateCode :: Maybe String -> Maybe String
const getStateCode = R.composeK(
R.compose(Maybe.of, R.toUpper),
get('state'),
get('address'),
get('user'),
);
getStateCode({"user":{"address":{"state":"ny"}}}); //=> Maybe.Just("NY")
getStateCode({}); //=> Maybe.Nothing() ((y → Promise z), (x → Promise y), …, (a → Promise b)) → (a → Promise z)functions The functions to compose
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.
See also pipeP.
const db = {
users: {
JOE: {
name: 'Joe',
followers: ['STEVE', 'SUZY']
}
}
}
// We'll pretend to do a db lookup which returns a promise
const lookupUser = (userId) => Promise.resolve(db.users[userId])
const lookupFollowers = (user) => Promise.resolve(user.followers)
lookupUser('JOE').then(lookupFollowers)
// followersForUser :: String -> Promise [UserId]
const followersForUser = R.composeP(lookupFollowers, lookupUser);
followersForUser('JOE').then(followers => console.log('Followers:', followers))
// Followers: ["STEVE","SUZY"] ((* → *), [(y → z), (x → y), …, (o → p), ((a, b, …, n) → o)]) → ((a, b, …, n) → z)
...functions The functions to compose
Performs right-to-left function composition using transforming function. The rightmost function may have any arity; the remaining functions must be unary.
Note: The result of compose is not automatically curried.
const composeWhileNotNil = R.composeWith((f, res) => R.isNil(res) ? res : f(res));
composeWhileNotNil([R.inc, R.prop('age')])({age: 1}) //=> 2
composeWhileNotNil([R.inc, R.prop('age')])({}) //=> undefined [a] → [a] → [a]
String → String → StringfirstList The first list
secondList The second list
Returns the result of concatenating the given lists or strings.
Note: R.concat expects both arguments to be of the same type, unlike the native Array.prototype.concat method. It will throw an error if you concat an Array with a non-Array value.
Dispatches to the concat method of the first argument, if present. Can also concatenate two members of a fantasy-land compatible semigroup.
R.concat('ABC', 'DEF'); // 'ABCDEF'
R.concat([4, 5, 6], [1, 2, 3]); //=> [4, 5, 6, 1, 2, 3]
R.concat([], []); //=> [] [[(*… → Boolean),(*… → *)]] → (*… → *)
pairs A list of [predicate, transformer]
Returns a function, fn, which encapsulates if/else, if/else, ... logic. R.cond takes a list of [predicate, transformer] pairs. All of the arguments to fn are applied to each of the predicates in turn until one returns a "truthy" value, at which point fn returns the result of applying its arguments to the corresponding transformer. If none of the predicates matches, fn returns undefined.
See also ifElse, unless, when.
const fn = R.cond([
[R.equals(0), R.always('water freezes at 0°C')],
[R.equals(100), R.always('water boils at 100°C')],
[R.T, temp => 'nothing special happens at ' + temp + '°C']
]);
fn(0); //=> 'water freezes at 0°C'
fn(50); //=> 'nothing special happens at 50°C'
fn(100); //=> 'water boils at 100°C' (* → {*}) → (* → {*}) fn The constructor function to wrap.
Wraps a constructor function inside a curried function that can be called with the same arguments and returns the same type.
See also invoker.
// Constructor function
function Animal(kind) {
this.kind = kind;
};
Animal.prototype.sighting = function() {
return "It's a " + this.kind + "!";
}
const AnimalConstructor = R.construct(Animal)
// Notice we no longer need the 'new' keyword:
AnimalConstructor('Pig'); //=> {"kind": "Pig", "sighting": function (){...}};
const animalTypes = ["Lion", "Tiger", "Bear"];
const animalSighting = R.invoker(0, 'sighting');
const sightNewAnimal = R.compose(animalSighting, AnimalConstructor);
R.map(sightNewAnimal, animalTypes); //=> ["It's a Lion!", "It's a Tiger!", "It's a Bear!"] Number → (* → {*}) → (* → {*}) n The arity of the constructor function.
Fn The constructor function to wrap.
Wraps a constructor function inside a curried function that can be called with the same arguments and returns the same type. The arity of the function returned is specified to allow using variadic constructor functions.
// Variadic Constructor function
function Salad() {
this.ingredients = arguments;
}
Salad.prototype.recipe = function() {
const instructions = R.map(ingredient => 'Add a dollop of ' + ingredient, this.ingredients);
return R.join('\n', instructions);
};
const ThreeLayerSalad = R.constructN(3, Salad);
// Notice we no longer need the 'new' keyword, and the constructor is curried for 3 arguments.
const salad = ThreeLayerSalad('Mayonnaise')('Potato Chips')('Ketchup');
console.log(salad.recipe());
// Add a dollop of Mayonnaise
// Add a dollop of Potato Chips
// Add a dollop of Ketchup a → [a] → Booleana The item to compare against.
list The array to consider.
Returns true if the specified value is equal, in R.equals terms, to at least one element of the given list; false otherwise. Works also with strings.
See also includes.
R.contains(3, [1, 2, 3]); //=> true
R.contains(4, [1, 2, 3]); //=> false
R.contains({ name: 'Fred' }, [{ name: 'Fred' }]); //=> true
R.contains([42], [[42]]); //=> true
R.contains('ba', 'banana'); //=>true ((x1, x2, …) → z) → [((a, b, …) → x1), ((a, b, …) → x2), …] → (a → b → … → z)
after A function. after will be invoked with the return values of fn1 and fn2 as its arguments.
functions A list of functions.
Accepts a converging function and a list of branching functions and returns a new function. The arity of the new function is the same as the arity of the longest branching function. When invoked, this new function is applied to some arguments, and each branching function is applied to those same arguments. The results of each branching function are passed as arguments to the converging function to produce the return value.
See also useWith.
const average = R.converge(R.divide, [R.sum, R.length])
average([1, 2, 3, 4, 5, 6, 7]) //=> 4
const strangeConcat = R.converge(R.concat, [R.toUpper, R.toLower])
strangeConcat("Yodel") //=> "YODELyodel" (a → String) → [a] → {*} fn The function used to map values to keys.
list The list to count elements from.
Counts the elements of a list according to how many match each value of a key generated by the supplied function. Returns an object mapping the keys produced by fn to the number of occurrences in the list. Note that all keys are coerced to strings because of how JavaScript objects work.
Acts as a transducer if a transformer is given in list position.
const numbers = [1.0, 1.1, 1.2, 2.0, 3.0, 2.2];
R.countBy(Math.floor)(numbers); //=> {'1': 3, '2': 2, '3': 1}
const letters = ['a', 'b', 'A', 'a', 'B', 'c'];
R.countBy(R.toLower)(letters); //=> {'a': 3, 'b': 2, 'c': 1} (* → a) → (* → a)
fn The function to curry.
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)const addFourNumbers = (a, b, c, d) => a + b + c + d; const curriedAddFourNumbers = R.curry(addFourNumbers); const f = curriedAddFourNumbers(1, 2); const g = f(3); g(4); //=> 10
Number → (* → a) → (* → a)
length The arity for the returned function.
fn The function to curry.
Returns a curried equivalent of the provided function, with the specified arity. The curried function has two unusual capabilities. First, its arguments needn't be provided one at a time. If g is R.curryN(3, f), the following are equivalent:
g(1)(2)(3)g(1)(2, 3)g(1, 2)(3)g(1, 2, 3)Secondly, the special placeholder value R.__ may be used to specify "gaps", allowing partial application of any combination of arguments, regardless of their positions. If g is as above and _ is R.__, the following are equivalent:
g(1, 2, 3)g(_, 2, 3)(1)g(_, _, 3)(1)(2)g(_, _, 3)(1, 2)g(_, 2)(1)(3)g(_, 2)(1, 3)g(_, 2)(_, 3)(1) See also curry.
const sumArgs = (...args) => R.sum(args); const curriedAddFourNumbers = R.curryN(4, sumArgs); const f = curriedAddFourNumbers(1, 2); const g = f(3); g(4); //=> 10
Number → Number
n Decrements its argument.
See also inc.
R.dec(42); //=> 41
a → b → a | b
default The default value.
val val will be returned instead of default unless val is null, undefined or NaN.
Returns the second argument if it is not null, undefined or NaN; otherwise the first argument is returned.
const defaultTo42 = R.defaultTo(42);
defaultTo42(null); //=> 42
defaultTo42(undefined); //=> 42
defaultTo42(false); //=> false
defaultTo42('Ramda'); //=> 'Ramda'
// parseInt('string') results in NaN
defaultTo42(parseInt('string')); //=> 42 Ord b => (a → b) → a → a → Number
fn A function of arity one that returns a value that can be compared
a The first item to be compared.
b The second item to be compared.
Makes a descending comparator function out of a function that returns a value that can be compared with < and >.
See also ascend.
const byAge = R.descend(R.prop('age'));
const people = [
{ name: 'Emma', age: 70 },
{ name: 'Peter', age: 78 },
{ name: 'Mikhail', age: 62 },
];
const peopleByOldestFirst = R.sort(byAge, people);
//=> [{ name: 'Peter', age: 78 }, { name: 'Emma', age: 70 }, { name: 'Mikhail', age: 62 }] [*] → [*] → [*]
list1 The first list.
list2 The second list.
Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. Objects and Arrays are compared in terms of value equality, not reference equality.
See also differenceWith, symmetricDifference, symmetricDifferenceWith, without.
R.difference([1,2,3,4], [7,6,5,4,3]); //=> [1,2]
R.difference([7,6,5,4,3], [1,2,3,4]); //=> [7,6,5]
R.difference([{a: 1}, {b: 2}], [{a: 1}, {c: 3}]) //=> [{b: 2}] ((a, a) → Boolean) → [a] → [a] → [a]
pred A predicate used to test whether two items are equal.
list1 The first list.
list2 The second list.
Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.
See also difference, symmetricDifference, symmetricDifferenceWith.
const cmp = (x, y) => x.a === y.a;
const l1 = [{a: 1}, {a: 2}, {a: 3}];
const l2 = [{a: 3}, {a: 4}];
R.differenceWith(cmp, l1, l2); //=> [{a: 1}, {a: 2}] String → {k: v} → {k: v} prop The name of the property to dissociate
obj The object to clone
Returns a new object that does not contain a prop property.
R.dissoc('b', {a: 1, b: 2, c: 3}); //=> {a: 1, c: 3} [Idx] → {k: v} → {k: v} Idx = String | Intpath The path to the value to omit
obj The object to clone
Makes a shallow clone of an object, omitting the property at the given path. Note that this copies and flattens prototype properties onto the new object as well. All non-primitive properties are copied by reference.
See also assocPath.
R.dissocPath(['a', 'b', 'c'], {a: {b: {c: 42}}}); //=> {a: {b: {}}} Number → Number → Number
a The first value.
b The second value.
Divides two numbers. Equivalent to a / b.
See also multiply.
R.divide(71, 100); //=> 0.71 const half = R.divide(R.__, 2); half(42); //=> 21 const reciprocal = R.divide(1); reciprocal(4); //=> 0.25
Number → [a] → [a]
Number → String → Stringn list Returns all but the first n elements of the given list, string, or transducer/transformer (or object with a drop method).
Dispatches to the drop method of the second argument, if present.
See also take, transduce, dropLast, dropWhile.
R.drop(1, ['foo', 'bar', 'baz']); //=> ['bar', 'baz'] R.drop(2, ['foo', 'bar', 'baz']); //=> ['baz'] R.drop(3, ['foo', 'bar', 'baz']); //=> [] R.drop(4, ['foo', 'bar', 'baz']); //=> [] R.drop(3, 'ramda'); //=> 'da'
Number → [a] → [a]
Number → String → Stringn The number of elements of list to skip.
list The list of elements to consider.
Returns a list containing all but the last n elements of the given list.
Acts as a transducer if a transformer is given in list position.
See also takeLast, drop, dropWhile, dropLastWhile.
R.dropLast(1, ['foo', 'bar', 'baz']); //=> ['foo', 'bar'] R.dropLast(2, ['foo', 'bar', 'baz']); //=> ['foo'] R.dropLast(3, ['foo', 'bar', 'baz']); //=> [] R.dropLast(4, ['foo', 'bar', 'baz']); //=> [] R.dropLast(3, 'ramda'); //=> 'ra'
(a → Boolean) → [a] → [a]
(a → Boolean) → String → Stringpredicate The function to be called on each element
xs The collection to iterate over.
Returns a new list excluding all the tailing elements of a given list which satisfy the supplied predicate function. It passes each value from the right to the supplied predicate function, skipping elements until the predicate function returns a falsy value. The predicate function is applied to one argument: (value).
Acts as a transducer if a transformer is given in list position.
See also takeLastWhile, addIndex, drop, dropWhile.
const lteThree = x => x <= 3; R.dropLastWhile(lteThree, [1, 2, 3, 4, 3, 2, 1]); //=> [1, 2, 3, 4] R.dropLastWhile(x => x !== 'd' , 'Ramda'); //=> 'Ramd'
[a] → [a]
list The array to consider.
Returns a new list without any consecutively repeating elements. R.equals is used to determine equality.
Acts as a transducer if a transformer is given in list position.
See also transduce.
R.dropRepeats([1, 1, 1, 2, 3, 4, 4, 2, 2]); //=> [1, 2, 3, 4, 2]
((a, a) → Boolean) → [a] → [a]
pred A predicate used to test whether two items are equal.
list The array to consider.
Returns a new list without any consecutively repeating elements. Equality is determined by applying the supplied predicate to each pair of consecutive elements. The first element in a series of equal elements will be preserved.
Acts as a transducer if a transformer is given in list position.
See also transduce.
const l = [1, -1, 1, 3, 4, -4, -4, -5, 5, 3, 3]; R.dropRepeatsWith(R.eqBy(Math.abs), l); //=> [1, 3, 4, -5, 3]
(a → Boolean) → [a] → [a]
(a → Boolean) → String → Stringfn The function called per iteration.
xs The collection to iterate over.
Returns a new list excluding the leading elements of a given list which satisfy the supplied predicate function. It passes each value to the supplied predicate function, skipping elements while the predicate function returns true. The predicate function is applied to one argument: (value).
Dispatches to the dropWhile method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also takeWhile, transduce, addIndex.
const lteTwo = x => x <= 2; R.dropWhile(lteTwo, [1, 2, 3, 4, 3, 2, 1]); //=> [3, 4, 3, 2, 1] R.dropWhile(x => x !== 'd' , 'Ramda'); //=> 'da'
(*… → Boolean) → (*… → Boolean) → (*… → Boolean)
f a predicate
g another predicate
A function wrapping calls to the two functions in an || operation, returning the result of the first function if it is truth-y and the result of the second function otherwise. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a truth-y value.
In addition to functions, R.either also accepts any fantasy-land compatible applicative functor.
See also or.
const gt10 = x => x > 10; const even = x => x % 2 === 0; const f = R.either(gt10, even); f(101); //=> true f(8); //=> true R.either(Maybe.Just(false), Maybe.Just(55)); // => Maybe.Just(55) R.either([false, false, 'a'], [11]) // => [11, 11, "a"]
a → a
x 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, <Type>.prototype.empty or implement the FantasyLand Monoid spec.
Dispatches to the empty method of the first argument, if present.
R.empty(Just(42)); //=> Nothing()
R.empty([1, 2, 3]); //=> []
R.empty('unicorns'); //=> ''
R.empty({x: 1, y: 2}); //=> {} [a] → [a] → Boolean
String → String → Booleansuffix list Checks if a list ends with the provided sublist.
Similarly, checks if a string ends with the provided substring.
See also startsWith.
R.endsWith('c', 'abc') //=> true
R.endsWith('b', 'abc') //=> false
R.endsWith(['c'], ['a', 'b', 'c']) //=> true
R.endsWith(['b'], ['a', 'b', 'c']) //=> false (a → b) → a → a → Boolean
f x y 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
k → {k: v} → {k: v} → Boolean prop The name of the property to compare
obj1 obj2 Reports whether two objects have the same value, in R.equals terms, for the specified property. Useful as a curried predicate.
const o1 = { a: 1, b: 2, c: 3, d: 4 };
const o2 = { a: 10, b: 20, c: 3, d: 40 };
R.eqProps('a', o1, o2); //=> false
R.eqProps('c', o1, o2); //=> true a → b → Boolean
a b Returns true if its arguments are equivalent, false otherwise. Handles cyclical data structures.
Dispatches symmetrically to the equals methods of both arguments, if present.
R.equals(1, 1); //=> true
R.equals(1, '1'); //=> false
R.equals([1, 2, 3], [1, 2, 3]); //=> true
const a = {}; a.v = a;
const b = {}; b.v = b;
R.equals(a, b); //=> true {k: (v → v)} → {k: v} → {k: v} transformations The object specifying transformation functions to apply to the object.
object The object to be transformed.
Creates a new object by recursively evolving a shallow copy of object, according to the transformation functions. All non-primitive properties are copied by reference.
A transformation function will not be invoked if its corresponding key does not exist in the evolved object.
const tomato = {firstName: ' Tomato ', data: {elapsed: 100, remaining: 1400}, id:123};
const transformations = {
firstName: R.trim,
lastName: R.trim, // Will not get invoked.
data: {elapsed: R.add(1), remaining: R.add(-1)}
};
R.evolve(transformations, tomato); //=> {firstName: 'Tomato', data: {elapsed: 101, remaining: 1399}, id:123} * → Boolean
A function that always returns false. Any passed in parameters are ignored.
See also T.
R.F(); //=> false
Filterable f => (a → Boolean) → f a → f a
pred filterable Takes a predicate and a Filterable, and returns a new filterable of the same type containing the members of the given filterable which satisfy the given predicate. Filterable objects include plain objects or any object that has a filter method such as Array.
Dispatches to the filter method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also reject, transduce, addIndex.
const isEven = n => n % 2 === 0;
R.filter(isEven, [1, 2, 3, 4]); //=> [2, 4]
R.filter(isEven, {a: 1, b: 2, c: 3, d: 4}); //=> {b: 2, d: 4} (a → Boolean) → [a] → a | undefined
fn The predicate function used to determine if the element is the desired one.
list The array to consider.
Returns the first element of the list which matches the predicate, or undefined if no element matches.
Dispatches to the find method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also transduce.
const xs = [{a: 1}, {a: 2}, {a: 3}];
R.find(R.propEq('a', 2))(xs); //=> {a: 2}
R.find(R.propEq('a', 4))(xs); //=> undefined (a → Boolean) → [a] → Number
fn The predicate function used to determine if the element is the desired one.
list The array to consider.
Returns the index of the first element of the list which matches the predicate, or -1 if no element matches.
Acts as a transducer if a transformer is given in list position.
See also transduce.
const xs = [{a: 1}, {a: 2}, {a: 3}];
R.findIndex(R.propEq('a', 2))(xs); //=> 1
R.findIndex(R.propEq('a', 4))(xs); //=> -1 (a → Boolean) → [a] → a | undefined
fn The predicate function used to determine if the element is the desired one.
list The array to consider.
Returns the last element of the list which matches the predicate, or undefined if no element matches.
Acts as a transducer if a transformer is given in list position.
See also transduce.
const xs = [{a: 1, b: 0}, {a:1, b: 1}];
R.findLast(R.propEq('a', 1))(xs); //=> {a: 1, b: 1}
R.findLast(R.propEq('a', 4))(xs); //=> undefined (a → Boolean) → [a] → Number
fn The predicate function used to determine if the element is the desired one.
list The array to consider.
Returns the index of the last element of the list which matches the predicate, or -1 if no element matches.
Acts as a transducer if a transformer is given in list position.
See also transduce.
const xs = [{a: 1, b: 0}, {a:1, b: 1}];
R.findLastIndex(R.propEq('a', 1))(xs); //=> 1
R.findLastIndex(R.propEq('a', 4))(xs); //=> -1 [a] → [b]
list The array to consider.
Returns a new list by pulling every item out of it (and all its sub-arrays) and putting them in a new array, depth-first.
See also unnest.
R.flatten([1, 2, [3, 4], 5, [6, [7, 8, [9, [10, 11], 12]]]]); //=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
((a, b, c, …) → z) → (b → a → c → … → z)
fn The function to invoke with its first two parameters reversed.
Returns a new function much like the supplied one, except that the first two arguments' order is reversed.
const mergeThree = (a, b, c) => [].concat(a, b, c); mergeThree(1, 2, 3); //=> [1, 2, 3] R.flip(mergeThree)(1, 2, 3); //=> [2, 1, 3]
(a → *) → [a] → [a]
fn The function to invoke. Receives one argument, value.
list The list to iterate over.
Iterate over an input list, calling a provided function fn for each element in the list.
fn receives one argument: (value).
Note: R.forEach does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.forEach method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach#Description
Also note that, unlike Array.prototype.forEach, Ramda's forEach returns the original array. In some libraries this function is named each.
Dispatches to the forEach method of the second argument, if present.
See also addIndex.
const printXPlusFive = x => console.log(x + 5); R.forEach(printXPlusFive, [1, 2, 3]); //=> [1, 2, 3] // logs 6 // logs 7 // logs 8
((a, String, StrMap a) → Any) → StrMap a → StrMap a
fn The function to invoke. Receives three argument, value, key, obj.
obj The object to iterate over.
Iterate over an input object, calling a provided function fn for each key and value in the object.
fn receives three argument: (value, key, obj).
const printKeyConcatValue = (value, key) => console.log(key + ':' + value);
R.forEachObjIndexed(printKeyConcatValue, {x: 1, y: 2}); //=> {x: 1, y: 2}
// logs x:1
// logs y:2 [[k,v]] → {k: v} pairs An array of two-element arrays that will be the keys and values of the output object.
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} (a → String) → [a] → {String: [a]} fn Function :: a -> String
list The array to group
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.
const byGrade = R.groupBy(function(student) {
const score = student.score;
return score < 65 ? 'F' :
score < 70 ? 'D' :
score < 80 ? 'C' :
score < 90 ? 'B' : 'A';
});
const students = [{name: 'Abby', score: 84},
{name: 'Eddy', score: 58},
// ...
{name: 'Jack', score: 69}];
byGrade(students);
// {
// 'A': [{name: 'Dianne', score: 99}],
// 'B': [{name: 'Abby', score: 84}]
// // ...,
// 'F': [{name: 'Eddy', score: 58}]
// } ((a, a) → Boolean) → [a] → [[a]]
fn Function for determining whether two given (adjacent) elements should be in the same group
list The array to group. Also accepts a string, which will be treated as a list of characters.
Takes a list and returns a list of lists where each sublist's elements are all satisfied pairwise comparison according to the provided function. Only adjacent elements are passed to the comparison function.
R.groupWith(R.equals, [0, 1, 1, 2, 3, 5, 8, 13, 21]) //=> [[0], [1, 1], [2], [3], [5], [8], [13], [21]] R.groupWith((a, b) => a + 1 === b, [0, 1, 1, 2, 3, 5, 8, 13, 21]) //=> [[0, 1], [1, 2, 3], [5], [8], [13], [21]] R.groupWith((a, b) => a % 2 === b % 2, [0, 1, 1, 2, 3, 5, 8, 13, 21]) //=> [[0], [1, 1], [2], [3, 5], [8], [13, 21]] R.groupWith(R.eqBy(isVowel), 'aestiou') //=> ['ae', 'st', 'iou']
Ord a => a → a → Boolean
a b Returns true if the first argument is greater than the second; false otherwise.
See also lt.
R.gt(2, 1); //=> true
R.gt(2, 2); //=> false
R.gt(2, 3); //=> false
R.gt('a', 'z'); //=> false
R.gt('z', 'a'); //=> true Ord a => a → a → Boolean
a b Returns true if the first argument is greater than or equal to the second; false otherwise.
See also lte.
R.gte(2, 1); //=> true
R.gte(2, 2); //=> true
R.gte(2, 3); //=> false
R.gte('a', 'z'); //=> false
R.gte('z', 'a'); //=> true s → {s: x} → Boolean prop The name of the property to check for.
obj The object to query.
Returns whether or not an object has an own property with the specified name
const hasName = R.has('name');
hasName({name: 'alice'}); //=> true
hasName({name: 'bob'}); //=> true
hasName({}); //=> false
const point = {x: 0, y: 0};
const pointHas = R.has(R.__, point);
pointHas('x'); //=> true
pointHas('y'); //=> true
pointHas('z'); //=> false s → {s: x} → Boolean prop The name of the property to check for.
obj The object to query.
Returns whether or not an object or its prototype chain has a property with the specified name
function Rectangle(width, height) {
this.width = width;
this.height = height;
}
Rectangle.prototype.area = function() {
return this.width * this.height;
};
const square = new Rectangle(2, 2);
R.hasIn('width', square); //=> true
R.hasIn('area', square); //=> true [Idx] → {a} → Boolean Idx = String | Intpath The path to use.
obj The object to check the path in.
Returns whether or not a path exists in an object. Only the object's own properties are checked.
See also has.
R.hasPath(['a', 'b'], {a: {b: 2}}); // => true
R.hasPath(['a', 'b'], {a: {b: undefined}}); // => true
R.hasPath(['a', 'b'], {a: {c: 2}}); // => false
R.hasPath(['a', 'b'], {}); // => false [a] → a | Undefined
String → Stringlist Returns the first element of the given list or string. In some libraries this function is named first.
R.head(['fi', 'fo', 'fum']); //=> 'fi'
R.head([]); //=> undefined
R.head('abc'); //=> 'a'
R.head(''); //=> '' a → a → Boolean
a b Returns true if its arguments are identical, false otherwise. Values are identical if they reference the same memory. NaN is identical to NaN; 0 and -0 are not identical.
Note this is merely a curried version of ES6 Object.is.
const o = {};
R.identical(o, o); //=> true
R.identical(1, 1); //=> true
R.identical(1, '1'); //=> false
R.identical([], []); //=> false
R.identical(0, -0); //=> false
R.identical(NaN, NaN); //=> true a → a
x The value to return.
A function that does nothing but return the parameter supplied to it. Good as a default or placeholder function.
R.identity(1); //=> 1
const obj = {};
R.identity(obj) === obj; //=> true (*… → Boolean) → (*… → *) → (*… → *) → (*… → *)
condition A predicate function
onTrue A function to invoke when the condition evaluates to a truthy value.
onFalse A function to invoke when the condition evaluates to a falsy value.
Creates a function that will process either the onTrue or the onFalse function depending upon the result of the condition predicate.
const 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 } Number → Number
n Increments its argument.
See also dec.
R.inc(42); //=> 43
a → [a] → Boolean
a The item to compare against.
list The array to consider.
Returns true if the specified value is equal, in R.equals terms, to at least one element of the given list; false otherwise. Works also with strings.
See also any.
R.includes(3, [1, 2, 3]); //=> true
R.includes(4, [1, 2, 3]); //=> false
R.includes({ name: 'Fred' }, [{ name: 'Fred' }]); //=> true
R.includes([42], [[42]]); //=> true
R.includes('ba', 'banana'); //=>true (a → String) → [{k: v}] → {k: {k: v}} fn Function :: a -> String
array The array of objects to index
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.
const list = [{id: 'xyz', title: 'A'}, {id: 'abc', title: 'B'}];
R.indexBy(R.prop('id'), list);
//=> {abc: {id: 'abc', title: 'B'}, xyz: {id: 'xyz', title: 'A'}} a → [a] → Number
target The item to find.
xs The array to search in.
Returns the position of the first occurrence of an item in an array, or -1 if the item is not included in the array. R.equals is used to determine equality.
See also lastIndexOf.
R.indexOf(3, [1,2,3,4]); //=> 2 R.indexOf(10, [1,2,3,4]); //=> -1
[a] → [a]
String → Stringlist Returns all but the last element of the given list or string.
R.init([1, 2, 3]); //=> [1, 2]
R.init([1, 2]); //=> [1]
R.init([1]); //=> []
R.init([]); //=> []
R.init('abc'); //=> 'ab'
R.init('ab'); //=> 'a'
R.init('a'); //=> ''
R.init(''); //=> '' ((a, b) → Boolean) → [a] → [b] → [a]
pred xs ys Takes a predicate pred, a list xs, and a list ys, and returns a list xs' comprising each of the elements of xs which is equal to one or more elements of ys according to pred.
pred must be a binary function expecting an element from each list.
xs, ys, and xs' are treated as sets, semantically, so ordering should not be significant, but since xs' is ordered the implementation guarantees that its values are in the same order as they appear in xs. Duplicates are not removed, so xs' may contain duplicates if xs contains duplicates.
See also intersection.
R.innerJoin(
(record, id) => record.id === id,
[{id: 824, name: 'Richie Furay'},
{id: 956, name: 'Dewey Martin'},
{id: 313, name: 'Bruce Palmer'},
{id: 456, name: 'Stephen Stills'},
{id: 177, name: 'Neil Young'}],
[177, 456, 999]
);
//=> [{id: 456, name: 'Stephen Stills'}, {id: 177, name: 'Neil Young'}] Number → a → [a] → [a]
index The position to insert the element
elt The element to insert into the Array
list The list to insert into
Inserts the supplied element into the list, at the specified index. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.
R.insert(2, 'x', [1,2,3,4]); //=> [1,2,'x',3,4]
Number → [a] → [a] → [a]
index The position to insert the sub-list
elts The sub-list to insert into the Array
list The list to insert the sub-list into
Inserts the sub-list into the list, at the specified index. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.
R.insertAll(2, ['x','y','z'], [1,2,3,4]); //=> [1,2,'x','y','z',3,4]
[*] → [*] → [*]
list1 The first list.
list2 The second list.
Combines two lists into a set (i.e. no duplicates) composed of those elements common to both lists.
See also innerJoin.
R.intersection([1,2,3,4], [7,6,5,4,3]); //=> [4, 3]
a → [a] → [a]
separator The element to add to the list.
list The list to be interposed.
Creates a new list with the separator interposed between elements.
Dispatches to the intersperse method of the second argument, if present.
R.intersperse('a', ['b', 'n', 'n', 's']); //=> ['b', 'a', 'n', 'a', 'n', 'a', 's'] a → (b → b) → [c] → a
acc The initial accumulator value.
xf The transducer function. Receives a transformer and returns a transformer.
list The list to iterate over.
Transforms the items of the list with the transducer and appends the transformed items to the accumulator using an appropriate iterator function based on the accumulator type.
The accumulator can be an array, string, object or a transformer. Iterated items will be appended to arrays and concatenated to strings. Objects will be merged directly or 2-item arrays will be merged as key, value pairs.
The accumulator can also be a transformer object that provides a 2-arity reducing iterator function, step, 0-arity initial value function, init, and 1-arity result extraction function result. The step function is used as the iterator function in reduce. The result function is used to convert the final accumulator into the return type and in most cases is R.identity. The init function is used to provide the initial accumulator.
The iteration is performed with R.reduce after initializing the transducer.
See also transduce.
const numbers = [1, 2, 3, 4]; const transducer = R.compose(R.map(R.add(1)), R.take(2)); R.into([], transducer, numbers); //=> [2, 3] const intoArray = R.into([]); intoArray(transducer, numbers); //=> [2, 3]
{s: x} → {x: [ s, … ]} obj The object or array to invert
Same as R.invertObj, however this accounts for objects with duplicate values by putting the values into an array.
See also invertObj.
const raceResultsByFirstName = {
first: 'alice',
second: 'jake',
third: 'alice',
};
R.invert(raceResultsByFirstName);
//=> { 'alice': ['first', 'third'], 'jake':['second'] } {s: x} → {x: s} obj The object or array to invert
Returns a new object with the keys of the given object as values, and the values of the given object, which are coerced to strings, as keys. Note that the last key found is preferred when handling the same value.
See also invert.
const raceResults = {
first: 'alice',
second: 'jake'
};
R.invertObj(raceResults);
//=> { 'alice': 'first', 'jake':'second' }
// Alternatively:
const raceResults = ['alice', 'jake'];
R.invertObj(raceResults);
//=> { 'alice': '0', 'jake':'1' } Number → String → (a → b → … → n → Object → *)
arity Number of arguments the returned function should take before the target object.
method Name of the method to call.
Turns a named method with a specified arity into a function that can be called directly supplied with arguments and a target object.
The returned function is curried and accepts arity + 1 parameters where the final parameter is the target object.
See also construct.
const sliceFrom = R.invoker(1, 'slice'); sliceFrom(6, 'abcdefghijklm'); //=> 'ghijklm' const sliceFrom6 = R.invoker(2, 'slice')(6); sliceFrom6(8, 'abcdefghijklm'); //=> 'gh'
(* → {*}) → a → Boolean ctor A constructor
val The value to test
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 a → Boolean
x Returns true if the given value is its type's empty value; false otherwise.
See also empty.
R.isEmpty([1, 2, 3]); //=> false
R.isEmpty([]); //=> true
R.isEmpty(''); //=> true
R.isEmpty(null); //=> false
R.isEmpty({}); //=> true
R.isEmpty({length: 0}); //=> false * → Boolean
x The value to test.
Checks if the input value is null or undefined.
R.isNil(null); //=> true R.isNil(undefined); //=> true R.isNil(0); //=> false R.isNil([]); //=> false
String → [a] → String
separator The string used to separate the elements.
xs The elements to join into a string.
Returns a string made by inserting the separator between each element and concatenating all the elements into a single string.
See also split.
const spacer = R.join(' ');
spacer(['a', 2, 3.4]); //=> 'a 2 3.4'
R.join('|', [1, 2, 3]); //=> '1|2|3' [(a, b, …, m) → n] → ((a, b, …, m) → [n])
fns An array of functions
juxt applies a list of functions to a list of values.
See also applySpec.
const getRange = R.juxt([Math.min, Math.max]); getRange(3, 4, 9, -3); //=> [-3, 9]
{k: v} → [k] obj The object to extract properties from
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'] {k: v} → [k] obj The object to extract properties from
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.
const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.keysIn(f); //=> ['x', 'y'] [a] → a | Undefined
String → Stringlist 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(''); //=> '' a → [a] → Number
target The item to find.
xs The array to search in.
Returns the position of the last occurrence of an item in an array, or -1 if the item is not included in the array. R.equals is used to determine equality.
See also indexOf.
R.lastIndexOf(3, [-1,3,3,0,1,2,3,4]); //=> 6 R.lastIndexOf(10, [1,2,3,4]); //=> -1
[a] → Number
list The array to inspect.
Returns the number of elements in the array by returning list.length.
R.length([]); //=> 0 R.length([1, 2, 3]); //=> 3
(s → a) → ((a, s) → s) → Lens s a
Lens s a = Functor f => (a → f a) → s → f sgetter setter Returns a lens for the given getter and setter functions. The getter "gets" the value of the focus; the setter "sets" the value of the focus. The setter should not mutate the data structure.
See also view, set, over, lensIndex, lensProp.
const xLens = R.lens(R.prop('x'), R.assoc('x'));
R.view(xLens, {x: 1, y: 2}); //=> 1
R.set(xLens, 4, {x: 1, y: 2}); //=> {x: 4, y: 2}
R.over(xLens, R.negate, {x: 1, y: 2}); //=> {x: -1, y: 2} Number → Lens s a
Lens s a = Functor f => (a → f a) → s → f sn Returns a lens whose focus is the specified index.
const headLens = R.lensIndex(0); R.view(headLens, ['a', 'b', 'c']); //=> 'a' R.set(headLens, 'x', ['a', 'b', 'c']); //=> ['x', 'b', 'c'] R.over(headLens, R.toUpper, ['a', 'b', 'c']); //=> ['A', 'b', 'c']
[Idx] → Lens s a
Idx = String | IntLens s a = Functor f => (a → f a) → s → f spath The path to use.
Returns a lens whose focus is the specified path.
const xHeadYLens = R.lensPath(['x', 0, 'y']);
R.view(xHeadYLens, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> 2
R.set(xHeadYLens, 1, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> {x: [{y: 1, z: 3}, {y: 4, z: 5}]}
R.over(xHeadYLens, R.negate, {x: [{y: 2, z: 3}, {y: 4, z: 5}]});
//=> {x: [{y: -2, z: 3}, {y: 4, z: 5}]} String → Lens s a
Lens s a = Functor f => (a → f a) → s → f sk Returns a lens whose focus is the specified property.
const xLens = R.lensProp('x');
R.view(xLens, {x: 1, y: 2}); //=> 1
R.set(xLens, 4, {x: 1, y: 2}); //=> {x: 4, y: 2}
R.over(xLens, R.negate, {x: 1, y: 2}); //=> {x: -1, y: 2} (*… → *) → ([*]… → [*])
fn The function to lift into higher context
"lifts" a function of arity > 1 so that it may "map over" a list, Function or other object that satisfies the FantasyLand Apply spec.
See also liftN.
const madd3 = R.lift((a, b, c) => a + b + c); madd3([1,2,3], [1,2,3], [1]); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7] const madd5 = R.lift((a, b, c, d, e) => a + b + c + d + e); madd5([1,2], [3], [4, 5], [6], [7, 8]); //=> [21, 22, 22, 23, 22, 23, 23, 24]
Number → (*… → *) → ([*]… → [*])
fn The function to lift into higher context
"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.
const madd3 = R.liftN(3, (...args) => R.sum(args)); madd3([1,2,3], [1,2,3], [1]); //=> [3, 4, 5, 4, 5, 6, 5, 6, 7]
Ord a => a → a → Boolean
a b Returns true if the first argument is less than the second; false otherwise.
See also gt.
R.lt(2, 1); //=> false
R.lt(2, 2); //=> false
R.lt(2, 3); //=> true
R.lt('a', 'z'); //=> true
R.lt('z', 'a'); //=> false Ord a => a → a → Boolean
a b Returns true if the first argument is less than or equal to the second; false otherwise.
See also gte.
R.lte(2, 1); //=> false
R.lte(2, 2); //=> true
R.lte(2, 3); //=> true
R.lte('a', 'z'); //=> true
R.lte('z', 'a'); //=> false Functor f => (a → b) → f a → f b
fn The function to be called on every element of the input list.
list The list to be iterated over.
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.
const double = x => x * 2;
R.map(double, [1, 2, 3]); //=> [2, 4, 6]
R.map(double, {x: 1, y: 2, z: 3}); //=> {x: 2, y: 4, z: 6} ((acc, x) → (acc, y)) → acc → [x] → (acc, [y])
fn The function to be called on every element of the input list.
acc The accumulator value.
list The list to iterate over.
The mapAccum function behaves like a combination of map and reduce; it applies a function to each element of a list, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new list.
The iterator function receives two arguments, acc and value, and should return a tuple [acc, value].
See also scan, addIndex, mapAccumRight.
const digits = ['1', '2', '3', '4']; const appender = (a, b) => [a + b, a + b]; R.mapAccum(appender, 0, digits); //=> ['01234', ['01', '012', '0123', '01234']]
((acc, x) → (acc, y)) → acc → [x] → (acc, [y])
fn The function to be called on every element of the input list.
acc The accumulator value.
list The list to iterate over.
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].
const digits = ['1', '2', '3', '4']; const appender = (a, b) => [b + a, b + a]; R.mapAccumRight(appender, 5, digits); //=> ['12345', ['12345', '2345', '345', '45']]
((*, String, Object) → *) → Object → Object
fn obj An Object-specific version of map. The function is applied to three arguments: (value, key, obj). If only the value is significant, use map instead.
See also map.
const xyz = { x: 1, y: 2, z: 3 };
const prependKeyAndDouble = (num, key, obj) => key + (num * 2);
R.mapObjIndexed(prependKeyAndDouble, xyz); //=> { x: 'x2', y: 'y4', z: 'z6' } RegExp → String → [String | Undefined]
rx A regular expression.
str The string to match against
Tests a regular expression against a String. Note that this function will return an empty array when there are no matches. This differs from String.prototype.match which returns null when there are no matches.
See also test.
R.match(/([a-z]a)/g, 'bananas'); //=> ['ba', 'na', 'na'] R.match(/a/, 'b'); //=> [] R.match(/a/, null); //=> TypeError: null does not have a method named "match"
Number → Number → Number
m The dividend.
p the modulus.
mathMod behaves like the modulo operator should mathematically, unlike the % operator (and by extension, R.modulo). So while -17 % 5 is -2, mathMod(-17, 5) is 3. mathMod requires Integer arguments, and returns NaN when the modulus is zero or negative.
See also modulo.
R.mathMod(-17, 5); //=> 3 R.mathMod(17, 5); //=> 2 R.mathMod(17, -5); //=> NaN R.mathMod(17, 0); //=> NaN R.mathMod(17.2, 5); //=> NaN R.mathMod(17, 5.3); //=> NaN const clock = R.mathMod(R.__, 12); clock(15); //=> 3 clock(24); //=> 0 const seventeenMod = R.mathMod(17); seventeenMod(3); //=> 2 seventeenMod(4); //=> 1 seventeenMod(10); //=> 7
Ord a => a → a → a
a b Returns the larger of its two arguments.
R.max(789, 123); //=> 789
R.max('a', 'b'); //=> 'b' Ord b => (a → b) → a → a → a
f a b Takes a function and two values, and returns whichever value produces the larger result when passed to the provided function.
// square :: Number -> Number const square = n => n * n; R.maxBy(square, -3, 2); //=> -3 R.reduce(R.maxBy(square), 0, [3, -5, 4, 1, -2]); //=> -5 R.reduce(R.maxBy(square), 0, []); //=> 0
[Number] → Number
list Returns the mean of the given list of numbers.
See also median.
R.mean([2, 7, 9]); //=> 6 R.mean([]); //=> NaN
[Number] → Number
list Returns the median of the given list of numbers.
See also mean.
R.median([2, 9, 7]); //=> 7 R.median([7, 2, 10, 9]); //=> 8 R.median([]); //=> NaN
(*… → String) → (*… → a) → (*… → a)
fn The function to generate the cache key.
fn The function to memoize.
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.
let count = 0;
const factorial = R.memoizeWith(R.identity, n => {
count += 1;
return R.product(R.range(1, n + 1));
});
factorial(5); //=> 120
factorial(5); //=> 120
factorial(5); //=> 120
count; //=> 1 {k: v} → {k: v} → {k: v}l r Create a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects, the value from the second object will be used.
See also mergeRight, mergeDeepRight, mergeWith, mergeWithKey.
R.merge({ 'name': 'fred', 'age': 10 }, { 'age': 40 });
//=> { 'name': 'fred', 'age': 40 }
const withDefaults = R.merge({x: 0, y: 0});
withDefaults({y: 2}); //=> {x: 0, y: 2} [{k: v}] → {k: v} list An array of objects
Merges a list of objects together into one object.
See also reduce.
R.mergeAll([{foo:1},{bar:2},{baz:3}]); //=> {foo:1,bar:2,baz:3}
R.mergeAll([{foo:1},{foo:2},{bar:2}]); //=> {foo:2,bar:2} {a} → {a} → {a} lObj rObj Creates a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects:
See also merge, mergeDeepRight, mergeDeepWith, mergeDeepWithKey.
R.mergeDeepLeft({ name: 'fred', age: 10, contact: { email: '[email protected]' }},
{ age: 40, contact: { email: '[email protected]' }});
//=> { name: 'fred', age: 10, contact: { email: '[email protected]' }} {a} → {a} → {a} lObj rObj Creates a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects:
See also merge, mergeDeepLeft, mergeDeepWith, mergeDeepWithKey.
R.mergeDeepRight({ name: 'fred', age: 10, contact: { email: '[email protected]' }},
{ age: 40, contact: { email: '[email protected]' }});
//=> { name: 'fred', age: 40, contact: { email: '[email protected]' }} ((a, a) → a) → {a} → {a} → {a} fn lObj rObj Creates a new object with the own properties of the two provided objects. If a key exists in both objects:
See also mergeWith, mergeDeepWithKey.
R.mergeDeepWith(R.concat,
{ a: true, c: { values: [10, 20] }},
{ b: true, c: { values: [15, 35] }});
//=> { a: true, b: true, c: { values: [10, 20, 15, 35] }} ((String, a, a) → a) → {a} → {a} → {a} fn lObj rObj Creates a new object with the own properties of the two provided objects. If a key exists in both objects:
See also mergeWithKey, mergeDeepWith.
let concatValues = (k, l, r) => k == 'values' ? R.concat(l, r) : r
R.mergeDeepWithKey(concatValues,
{ a: true, c: { thing: 'foo', values: [10, 20] }},
{ b: true, c: { thing: 'bar', values: [15, 35] }});
//=> { a: true, b: true, c: { thing: 'bar', values: [10, 20, 15, 35] }} {k: v} → {k: v} → {k: v} l r Create a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects, the value from the first object will be used.
See also mergeRight, mergeDeepLeft, mergeWith, mergeWithKey.
R.mergeLeft({ 'age': 40 }, { 'name': 'fred', 'age': 10 });
//=> { 'name': 'fred', 'age': 40 }
const resetToDefault = R.mergeLeft({x: 0});
resetToDefault({x: 5, y: 2}); //=> {x: 0, y: 2} {k: v} → {k: v} → {k: v} l r Create a new object with the own properties of the first object merged with the own properties of the second object. If a key exists in both objects, the value from the second object will be used.
See also mergeLeft, mergeDeepRight, mergeWith, mergeWithKey.
R.mergeRight({ 'name': 'fred', 'age': 10 }, { 'age': 40 });
//=> { 'name': 'fred', 'age': 40 }
const withDefaults = R.mergeRight({x: 0, y: 0});
withDefaults({y: 2}); //=> {x: 0, y: 2} ((a, a) → a) → {a} → {a} → {a} fn l r Creates a new object with the own properties of the two provided objects. If a key exists in both objects, the provided function is applied to the values associated with the key in each object, with the result being used as the value associated with the key in the returned object.
See also mergeDeepWith, merge, mergeWithKey.
R.mergeWith(R.concat,
{ a: true, values: [10, 20] },
{ b: true, values: [15, 35] });
//=> { a: true, b: true, values: [10, 20, 15, 35] } ((String, a, a) → a) → {a} → {a} → {a} fn l r Creates a new object with the own properties of the two provided objects. If a key exists in both objects, the provided function is applied to the key and the values associated with the key in each object, with the result being used as the value associated with the key in the returned object.
See also mergeDeepWithKey, merge, mergeWith.
let concatValues = (k, l, r) => k == 'values' ? R.concat(l, r) : r
R.mergeWithKey(concatValues,
{ a: true, thing: 'foo', values: [10, 20] },
{ b: true, thing: 'bar', values: [15, 35] });
//=> { a: true, b: true, thing: 'bar', values: [10, 20, 15, 35] } Ord a => a → a → a
a b Returns the smaller of its two arguments.
R.min(789, 123); //=> 123
R.min('a', 'b'); //=> 'a' Ord b => (a → b) → a → a → a
f a b Takes a function and two values, and returns whichever value produces the smaller result when passed to the provided function.
// square :: Number -> Number const square = n => n * n; R.minBy(square, -3, 2); //=> 2 R.reduce(R.minBy(square), Infinity, [3, -5, 4, 1, -2]); //=> 1 R.reduce(R.minBy(square), Infinity, []); //=> Infinity
Number → Number → Number
a The value to the divide.
b The pseudo-modulus
Divides the first parameter by the second and returns the remainder. Note that this function preserves the JavaScript-style behavior for modulo. For mathematical modulo see mathMod.
See also mathMod.
R.modulo(17, 3); //=> 2 // JS behavior: R.modulo(-17, 3); //=> -2 R.modulo(17, -3); //=> 2 const isOdd = R.modulo(R.__, 2); isOdd(42); //=> 0 isOdd(21); //=> 1
Number → Number → [a] → [a]
from The source index
to The destination index
list The list which will serve to realise the move
Move an item, at index from, to index to, in a list of elements. A new list will be created containing the new elements order.
R.move(0, 2, ['a', 'b', 'c', 'd', 'e', 'f']); //=> ['b', 'c', 'a', 'd', 'e', 'f'] R.move(-1, 0, ['a', 'b', 'c', 'd', 'e', 'f']); //=> ['f', 'a', 'b', 'c', 'd', 'e'] list rotation
Number → Number → Number
a The first value.
b The second value.
Multiplies two numbers. Equivalent to a * b but curried.
See also divide.
const double = R.multiply(2); const triple = R.multiply(3); double(3); //=> 6 triple(4); //=> 12 R.multiply(2, 5); //=> 10
Number → (* → a) → (* → a)
n The desired arity of the new function.
fn The function to wrap.
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.
const takesTwoArgs = (a, b) => [a, b]; takesTwoArgs.length; //=> 2 takesTwoArgs(1, 2); //=> [1, 2] const takesOneArg = R.nAry(1, takesTwoArgs); takesOneArg.length; //=> 1 // Only `n` arguments are passed to the wrapped function takesOneArg(1, 2); //=> [1, undefined]
Number → Number
n Negates its argument.
R.negate(42); //=> -42
(a → Boolean) → [a] → Boolean
fn The predicate function.
list The array to consider.
Returns true if no elements of the list match the predicate, false otherwise.
Dispatches to the all method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
const isEven = n => n % 2 === 0; const isOdd = n => n % 2 === 1; R.none(isEven, [1, 3, 5, 7, 9, 11]); //=> true R.none(isOdd, [1, 3, 5, 7, 8, 11]); //=> false
* → Boolean
a any value
A function that returns the ! of its argument. It will return true when passed false-y value, and false when passed a truth-y one.
See also complement.
R.not(true); //=> false R.not(false); //=> true R.not(0); //=> true R.not(1); //=> false
Number → [a] → a | Undefined
Number → String → Stringoffset list Returns the nth element of the given list or string. If n is negative the element at index length + n is returned.
const list = ['foo', 'bar', 'baz', 'quux']; R.nth(1, list); //=> 'bar' R.nth(-1, list); //=> 'quux' R.nth(-99, list); //=> undefined R.nth(2, 'abc'); //=> 'c' R.nth(3, 'abc'); //=> ''
Number → *… → *
n Returns a function which returns its nth argument.
R.nthArg(1)('a', 'b', 'c'); //=> 'b'
R.nthArg(-1)('a', 'b', 'c'); //=> 'c' (b → c) → (a → b) → a → c
f g o is a curried composition function that returns a unary function. Like compose, o performs right-to-left function composition. Unlike compose, the rightmost function passed to o will be invoked with only one argument. Also, unlike compose, o is limited to accepting only 2 unary functions. The name o was chosen because of its similarity to the mathematical composition operator ∘.
const classyGreeting = name => "The name's " + name.last + ", " + name.first + " " + name.last
const yellGreeting = R.o(R.toUpper, classyGreeting);
yellGreeting({first: 'James', last: 'Bond'}); //=> "THE NAME'S BOND, JAMES BOND"
R.o(R.multiply(10), R.add(10))(-4) //=> 60 String → a → {String:a} key val Creates an object containing a single key:value pair.
See also pair.
const matchPhrases = R.compose(
R.objOf('must'),
R.map(R.objOf('match_phrase'))
);
matchPhrases(['foo', 'bar', 'baz']); //=> {must: [{match_phrase: 'foo'}, {match_phrase: 'bar'}, {match_phrase: 'baz'}]} a → [a]
x any value
Returns a singleton array containing the value provided.
Note this of is different from the ES6 of; See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/of
R.of(null); //=> [null] R.of([42]); //=> [[42]]
[String] → {String: *} → {String: *} names an array of String property names to omit from the new object
obj The object to copy from
Returns a partial copy of an object omitting the keys specified.
See also pick.
R.omit(['a', 'd'], {a: 1, b: 2, c: 3, d: 4}); //=> {b: 2, c: 3} (a… → b) → (a… → b)
fn The function to wrap in a call-only-once wrapper.
Accepts a function fn and returns a function that guards invocation of fn such that fn can only ever be called once, no matter how many times the returned function is invoked. The first value calculated is returned in subsequent invocations.
const addOneOnce = R.once(x => x + 1); addOneOnce(10); //=> 11 addOneOnce(addOneOnce(50)); //=> 11
a → b → a | b
a b Returns true if one or both of its arguments are true. Returns false if both arguments are false.
See also either.
R.or(true, true); //=> true R.or(true, false); //=> true R.or(false, true); //=> true R.or(false, false); //=> false
(e → b) → (Promise e a) → (Promise e b)
(e → (Promise f b)) → (Promise e a) → (Promise f b)onFailure The function to apply. Can return a value or a promise of a value.
p Returns the result of applying the onFailure function to the value inside a failed promise. This is useful for handling rejected promises inside function compositions.
See also then.
var failedFetch = (id) => Promise.reject('bad ID');
var useDefault = () => ({ firstName: 'Bob', lastName: 'Loblaw' })
//recoverFromFailure :: String -> Promise ({firstName, lastName})
var recoverFromFailure = R.pipe(
failedFetch,
R.otherwise(useDefault),
R.then(R.pick(['firstName', 'lastName'])),
);
recoverFromFailure(12345).then(console.log) Lens s a → (a → a) → s → s
Lens s a = Functor f => (a → f a) → s → f slens v x Returns the result of "setting" the portion of the given data structure focused by the given lens to the result of applying the given function to the focused value.
See also prop, lensIndex, lensProp.
const headLens = R.lensIndex(0); R.over(headLens, R.toUpper, ['foo', 'bar', 'baz']); //=> ['FOO', 'bar', 'baz']
a → b → (a,b)
fst snd Takes two arguments, fst and snd, and returns [fst, snd].
R.pair('foo', 'bar'); //=> ['foo', 'bar'] ((a, b, c, …, n) → x) → [a, b, c, …] → ((d, e, f, …, n) → x)
f args Takes a function f and a list of arguments, and returns a function g. When applied, g returns the result of applying f to the arguments provided initially followed by the arguments provided to g.
See also partialRight, curry.
const multiply2 = (a, b) => a * b;
const double = R.partial(multiply2, [2]);
double(2); //=> 4
const greet = (salutation, title, firstName, lastName) =>
salutation + ', ' + title + ' ' + firstName + ' ' + lastName + '!';
const sayHello = R.partial(greet, ['Hello']);
const sayHelloToMs = R.partial(sayHello, ['Ms.']);
sayHelloToMs('Jane', 'Jones'); //=> 'Hello, Ms. Jane Jones!' ((a, b, c, …, n) → x) → [d, e, f, …, n] → ((a, b, c, …) → x)
f args Takes a function f and a list of arguments, and returns a function g. When applied, g returns the result of applying f to the arguments provided to g followed by the arguments provided initially.
See also partial.
const greet = (salutation, title, firstName, lastName) =>
salutation + ', ' + title + ' ' + firstName + ' ' + lastName + '!';
const greetMsJaneJones = R.partialRight(greet, ['Ms.', 'Jane', 'Jones']);
greetMsJaneJones('Hello'); //=> 'Hello, Ms. Jane Jones!' Filterable f => (a → Boolean) → f a → [f a, f a]
pred A predicate to determine which side the element belongs to.
filterable the list (or other filterable) to partition.
Takes a predicate and a list or other Filterable object and returns the pair of filterable objects of the same type of elements which do and do not satisfy, the predicate, respectively. Filterable objects include plain objects or any object that has a filter method such as Array.
R.partition(R.includes('s'), ['sss', 'ttt', 'foo', 'bars']);
// => [ [ 'sss', 'bars' ], [ 'ttt', 'foo' ] ]
R.partition(R.includes('s'), { a: 'sss', b: 'ttt', foo: 'bars' });
// => [ { a: 'sss', foo: 'bars' }, { b: 'ttt' } ] [Idx] → {a} → a | Undefined Idx = String | Intpath The path to use.
obj The object to retrieve the nested property from.
Retrieve the value at a given path.
See also prop.
R.path(['a', 'b'], {a: {b: 2}}); //=> 2
R.path(['a', 'b'], {c: {b: 2}}); //=> undefined [Idx] → a → {a} → Boolean Idx = String | Intpath The path of the nested property to use
val The value to compare the nested property with
obj The object to check the nested property in
Determines whether a nested path on an object has a specific value, in R.equals terms. Most likely used to filter a list.
const user1 = { address: { zipCode: 90210 } };
const user2 = { address: { zipCode: 55555 } };
const user3 = { name: 'Bob' };
const users = [ user1, user2, user3 ];
const isFamous = R.pathEq(['address', 'zipCode'], 90210);
R.filter(isFamous, users); //=> [ user1 ] a → [Idx] → {a} → a Idx = String | Intd The default value.
p The path to use.
obj The object to retrieve the nested property from.
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" (a → Boolean) → [Idx] → {a} → Boolean Idx = String | Intpred propPath obj Returns true if the specified object property at given path satisfies the given predicate; false otherwise.
See also propSatisfies, path.
R.pathSatisfies(y => y > 0, ['x', 'y'], {x: {y: 2}}); //=> true [k] → {k: v} → {k: v} names an array of String property names to copy onto a new object
obj The object to copy from
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} [k] → {k: v} → {k: v} names an array of String property names to copy onto a new object
obj The object to copy from
Similar to pick except that this one includes a key: undefined pair for properties that don't exist.
See also pick.
R.pickAll(['a', 'd'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1, d: 4}
R.pickAll(['a', 'e', 'f'], {a: 1, b: 2, c: 3, d: 4}); //=> {a: 1, e: undefined, f: undefined} ((v, k) → Boolean) → {k: v} → {k: v} pred A predicate to determine whether or not a key should be included on the output object.
obj The object to copy from
Returns a partial copy of an object containing only the keys that satisfy the supplied predicate.
const isUpperCase = (val, key) => key.toUpperCase() === key;
R.pickBy(isUpperCase, {a: 1, b: 2, A: 3, B: 4}); //=> {A: 3, B: 4} (((a, b, …, n) → o), (o → p), …, (x → y), (y → z)) → ((a, b, …, n) → z)
functions 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.
See also compose.
const f = R.pipe(Math.pow, R.negate, R.inc); f(3, 4); // -(3^4) + 1
Chain m => ((a → m b), (b → m c), …, (y → m z)) → (a → m z)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(f, R.chain(g), R.chain(h)).
See also composeK.
// parseJson :: String -> Maybe *
// get :: String -> Object -> Maybe *
// getStateCode :: Maybe String -> Maybe String
const getStateCode = R.pipeK(
parseJson,
get('user'),
get('address'),
get('state'),
R.compose(Maybe.of, R.toUpper)
);
getStateCode('{"user":{"address":{"state":"ny"}}}');
//=> Just('NY')
getStateCode('[Invalid JSON]');
//=> Nothing() ((a → Promise b), (b → Promise c), …, (y → Promise z)) → (a → Promise z)functions 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.
See also composeP.
// followersForUser :: String -> Promise [User] const followersForUser = R.pipeP(db.getUserById, db.getFollowers);
((* → *), [((a, b, …, n) → o), (o → p), …, (x → y), (y → z)]) → ((a, b, …, n) → z)
functions Performs left-to-right function composition using transforming function. The leftmost function may have any arity; the remaining functions must be unary.
Note: The result of pipeWith is not automatically curried.
See also composeWith, pipe.
const pipeWhileNotNil = R.pipeWith((f, res) => R.isNil(res) ? res : f(res)); const f = pipeWhileNotNil([Math.pow, R.negate, R.inc]) f(3, 4); // -(3^4) + 1
Functor f => k → f {k: v} → f v key The key name to pluck off of each object.
f The array or functor to consider.
Returns a new list by plucking the same named property off all objects in the list supplied.
pluck will work on any functor in addition to arrays, as it is equivalent to R.map(R.prop(k), f).
See also props.
var getAges = R.pluck('age');
getAges([{name: 'fred', age: 29}, {name: 'wilma', age: 27}]); //=> [29, 27]
R.pluck(0, [[1, 2], [3, 4]]); //=> [1, 3]
R.pluck('val', {a: {val: 3}, b: {val: 5}}); //=> {a: 3, b: 5} a → [a] → [a]
el The item to add to the head of the output list.
list The array to add to the tail of the output list.
Returns a new list with the given element at the front, followed by the contents of the list.
See also append.
R.prepend('fee', ['fi', 'fo', 'fum']); //=> ['fee', 'fi', 'fo', 'fum'] [Number] → Number
list An array of numbers
Multiplies together all the elements of a list.
See also reduce.
R.product([2,4,6,8,100,1]); //=> 38400
[k] → [{k: v}] → [{k: v}] props The property names to project
objs The objects to query
Reasonable analog to SQL select statement.
const abby = {name: 'Abby', age: 7, hair: 'blond', grade: 2};
const fred = {name: 'Fred', age: 12, hair: 'brown', grade: 7};
const kids = [abby, fred];
R.project(['name', 'grade'], kids); //=> [{name: 'Abby', grade: 2}, {name: 'Fred', grade: 7}] s → {s: a} → a | Undefined p The property name
obj The object to query
Returns a function that when supplied an object returns the indicated property of that object, if it exists.
See also path.
R.prop('x', {x: 100}); //=> 100
R.prop('x', {}); //=> undefined
R.compose(R.inc, R.prop('x'))({ x: 3 }) //=> 4 String → a → Object → Boolean
name val obj Returns true if the specified object property is equal, in R.equals terms, to the given value; false otherwise. You can test multiple properties with R.whereEq.
See also whereEq, propSatisfies, equals.
const abby = {name: 'Abby', age: 7, hair: 'blond'};
const fred = {name: 'Fred', age: 12, hair: 'brown'};
const rusty = {name: 'Rusty', age: 10, hair: 'brown'};
const alois = {name: 'Alois', age: 15, disposition: 'surly'};
const kids = [abby, fred, rusty, alois];
const hasBrownHair = R.propEq('hair', 'brown');
R.filter(hasBrownHair, kids); //=> [fred, rusty] Type → String → Object → Boolean
type name obj Returns true if the specified object property is of the given type; false otherwise.
See also is, propSatisfies.
R.propIs(Number, 'x', {x: 1, y: 2}); //=> true
R.propIs(Number, 'x', {x: 'foo'}); //=> false
R.propIs(Number, 'x', {}); //=> false a → String → Object → a
val The default value.
p The name of the property to return.
obj The object to query.
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.
const alice = {
name: 'ALICE',
age: 101
};
const favorite = R.prop('favoriteLibrary');
const favoriteWithDefault = R.propOr('Ramda', 'favoriteLibrary');
favorite(alice); //=> undefined
favoriteWithDefault(alice); //=> 'Ramda' [k] → {k: v} → [v] ps The property names to fetch
obj The object to query
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]
const fullName = R.compose(R.join(' '), R.props(['first', 'last']));
fullName({last: 'Bullet-Tooth', age: 33, first: 'Tony'}); //=> 'Tony Bullet-Tooth' (a → Boolean) → String → {String: a} → Boolean pred name obj Returns true if the specified object property satisfies the given predicate; false otherwise. You can test multiple properties with R.where.
See also where, propEq, propIs.
R.propSatisfies(x => x > 0, 'x', {x: 1, y: 2}); //=> true Number → Number → [Number]
from The first number in the list.
to One more than the last number in the list.
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]
((a, b) → a) → a → [b] → a
fn The iterator function. Receives two values, the accumulator and the current element from the array.
acc The accumulator value.
list The list to iterate over.
Returns a single item by iterating through the list, successively calling the iterator function and passing it an accumulator value and the current value from the array, and then passing the result to the next call.
The iterator function receives two values: (acc, value). It may use R.reduced to shortcut the iteration.
The arguments' order of reduceRight's iterator function is (value, acc).
Note: R.reduce does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.reduce method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce#Description
Dispatches to the reduce method of the third argument, if present. When doing so, it is up to the user to handle the R.reduced shortcuting, as this is not implemented by reduce.
See also reduced, addIndex, reduceRight.
R.reduce(R.subtract, 0, [1, 2, 3, 4]) // => ((((0 - 1) - 2) - 3) - 4) = -10 // - -10 // / \ / \ // - 4 -6 4 // / \ / \ // - 3 ==> -3 3 // / \ / \ // - 2 -1 2 // / \ / \ // 0 1 0 1
((a, b) → a) → a → (b → String) → [b] → {String: a} valueFn The function that reduces the elements of each group to a single value. Receives two values, accumulator for a particular group and the current element.
acc The (initial) accumulator value for each group.
keyFn The function that maps the list's element into a key.
list The array to group.
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.
const groupNames = (acc, {name}) => acc.concat(name)
const toGrade = ({score}) =>
score < 65 ? 'F' :
score < 70 ? 'D' :
score < 80 ? 'C' :
score < 90 ? 'B' : 'A'
var students = [
{name: 'Abby', score: 83},
{name: 'Bart', score: 62},
{name: 'Curt', score: 88},
{name: 'Dora', score: 92},
]
reduceBy(groupNames, [], toGrade, students)
//=> {"A": ["Dora"], "B": ["Abby", "Curt"], "F": ["Bart"]} a → *
x The final value of the reduce.
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 only available to the below functions:
See also reduce, reduceWhile, transduce.
R.reduce( (acc, item) => item > 3 ? R.reduced(acc) : acc.concat(item), [], [1, 2, 3, 4, 5]) // [1, 2, 3]
((a, b) → b) → b → [a] → b
fn The iterator function. Receives two values, the current element from the array and the accumulator.
acc The accumulator value.
list The list to iterate over.
Returns a single item by iterating through the list, successively calling the iterator function and passing it an accumulator value and the current value from the array, and then passing the result to the next call.
Similar to reduce, except moves through the input list from the right to the left.
The iterator function receives two values: (value, acc), while the arguments' order of reduce's iterator function is (acc, value).
Note: R.reduceRight does not skip deleted or unassigned indices (sparse arrays), unlike the native Array.prototype.reduceRight method. For more details on this behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight#Description
R.reduceRight(R.subtract, 0, [1, 2, 3, 4]) // => (1 - (2 - (3 - (4 - 0)))) = -2 // - -2 // / \ / \ // 1 - 1 3 // / \ / \ // 2 - ==> 2 -1 // / \ / \ // 3 - 3 4 // / \ / \ // 4 0 4 0
((a, b) → Boolean) → ((a, b) → a) → a → [b] → a
pred The predicate. It is passed the accumulator and the current element.
fn The iterator function. Receives two values, the accumulator and the current element.
a The accumulator value.
list The list to iterate over.
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.
const isOdd = (acc, x) => x % 2 === 1; const xs = [1, 3, 5, 60, 777, 800]; R.reduceWhile(isOdd, R.add, 0, xs); //=> 9 const ys = [2, 4, 6] R.reduceWhile(isOdd, R.add, 111, ys); //=> 111
Filterable f => (a → Boolean) → f a → f a
pred filterable The complement of filter.
Acts as a transducer if a transformer is given in list position. Filterable objects include plain objects or any object that has a filter method such as Array.
See also filter, transduce, addIndex.
const isOdd = (n) => n % 2 === 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} Number → Number → [a] → [a]
start The position to start removing elements
count The number of elements to remove
list The list to remove from
Removes the sub-list of list starting at index start and containing count elements. Note that this is not destructive: it returns a copy of the list with the changes. No lists have been harmed in the application of this function.
See also without.
R.remove(2, 3, [1,2,3,4,5,6,7,8]); //=> [1,2,6,7,8]
a → n → [a]
value The value to repeat.
n The desired size of the output list.
Returns a fixed list of size n containing a specified identical value.
See also times.
R.repeat('hi', 5); //=> ['hi', 'hi', 'hi', 'hi', 'hi']
const obj = {};
const repeatedObjs = R.repeat(obj, 5); //=> [{}, {}, {}, {}, {}]
repeatedObjs[0] === repeatedObjs[1]; //=> true RegExp|String → String → String → String
pattern A regular expression or a substring to match.
replacement The string to replace the matches with.
str The String to do the search and replacement in.
Replace a substring or regex match in a string with a replacement.
The first two parameters correspond to the parameters of the String.prototype.replace() function, so the second parameter can also be a function.
R.replace('foo', 'bar', 'foo foo foo'); //=> 'bar foo foo'
R.replace(/foo/, 'bar', 'foo foo foo'); //=> 'bar foo foo'
// Use the "g" (global) flag to replace all occurrences:
R.replace(/foo/g, 'bar', 'foo foo foo'); //=> 'bar bar bar' [a] → [a]
String → Stringlist Returns a new list or string with the elements or characters in reverse order.
R.reverse([1, 2, 3]); //=> [3, 2, 1]
R.reverse([1, 2]); //=> [2, 1]
R.reverse([1]); //=> [1]
R.reverse([]); //=> []
R.reverse('abc'); //=> 'cba'
R.reverse('ab'); //=> 'ba'
R.reverse('a'); //=> 'a'
R.reverse(''); //=> '' ((a, b) → a) → a → [b] → [a]
fn The iterator function. Receives two values, the accumulator and the current element from the array
acc The accumulator value.
list The list to iterate over.
Scan is similar to reduce, but returns a list of successively reduced values from the left
const numbers = [1, 2, 3, 4]; const factorials = R.scan(R.multiply, 1, numbers); //=> [1, 1, 2, 6, 24]
(Applicative f, Traversable t) => (a → f a) → t (f a) → f (t a)
of traversable Transforms a Traversable of Applicative into an Applicative of Traversable.
Dispatches to the sequence method of the second argument, if present.
See also traverse.
R.sequence(Maybe.of, [Just(1), Just(2), Just(3)]); //=> Just([1, 2, 3]) R.sequence(Maybe.of, [Just(1), Just(2), Nothing()]); //=> Nothing() R.sequence(R.of, Just([1, 2, 3])); //=> [Just(1), Just(2), Just(3)] R.sequence(R.of, Nothing()); //=> [Nothing()]
Lens s a → a → s → s
Lens s a = Functor f => (a → f a) → s → f slens v x Returns the result of "setting" the portion of the given data structure focused by the given lens to the given value.
See also prop, lensIndex, lensProp.
const xLens = R.lensProp('x');
R.set(xLens, 4, {x: 1, y: 2}); //=> {x: 4, y: 2}
R.set(xLens, 8, {x: 1, y: 2}); //=> {x: 8, y: 2} Number → Number → [a] → [a]
Number → Number → String → StringfromIndex The start index (inclusive).
toIndex The end index (exclusive).
list 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'
((a, a) → Number) → [a] → [a]
comparator A sorting function :: a -> b -> Int
list The list to sort
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.
const diff = function(a, b) { return a - b; };
R.sort(diff, [4,2,7,5]); //=> [2, 4, 5, 7] Ord b => (a → b) → [a] → [a]
fn list The list to sort.
Sorts the list according to the supplied function.
const sortByFirstItem = R.sortBy(R.prop(0));
const pairs = [[-1, 1], [-2, 2], [-3, 3]];
sortByFirstItem(pairs); //=> [[-3, 3], [-2, 2], [-1, 1]]
const sortByNameCaseInsensitive = R.sortBy(R.compose(R.toLower, R.prop('name')));
const alice = {
name: 'ALICE',
age: 101
};
const bob = {
name: 'Bob',
age: -10
};
const clara = {
name: 'clara',
age: 314.159
};
const people = [clara, bob, alice];
sortByNameCaseInsensitive(people); //=> [alice, bob, clara] [(a, a) → Number] → [a] → [a]
functions A list of comparator functions.
list The list to sort.
Sorts a list according to a list of comparators.
const alice = {
name: 'alice',
age: 40
};
const bob = {
name: 'bob',
age: 30
};
const clara = {
name: 'clara',
age: 40
};
const people = [clara, bob, alice];
const ageNameSort = R.sortWith([
R.descend(R.prop('age')),
R.ascend(R.prop('name'))
]);
ageNameSort(people); //=> [alice, clara, bob] (String | RegExp) → String → [String]
sep The pattern.
str The string to separate into an array.
Splits a string into an array of strings based on the given separator.
See also join.
const pathComponents = R.split('/');
R.tail(pathComponents('/usr/local/bin/node')); //=> ['usr', 'local', 'bin', 'node']
R.split('.', 'a.b.c.xyz.d'); //=> ['a', 'b', 'c', 'xyz', 'd'] Number → [a] → [[a], [a]]
Number → String → [String, String]index The index where the array/string is split.
array The array/string to be split.
Splits a given list or string at a given index.
R.splitAt(1, [1, 2, 3]); //=> [[1], [2, 3]] R.splitAt(5, 'hello world'); //=> ['hello', ' world'] R.splitAt(-1, 'foobar'); //=> ['fooba', 'r']
Number → [a] → [[a]]
Number → String → [String]n list Splits a collection into slices of the specified length.
R.splitEvery(3, [1, 2, 3, 4, 5, 6, 7]); //=> [[1, 2, 3], [4, 5, 6], [7]] R.splitEvery(3, 'foobarbaz'); //=> ['foo', 'bar', 'baz']
(a → Boolean) → [a] → [[a], [a]]
pred The predicate that determines where the array is split.
list The array to be split.
Takes a list and a predicate and returns a pair of lists with the following properties:
R.splitWhen(R.equals(2), [1, 2, 3, 1, 2, 3]); //=> [[1], [2, 3, 1, 2, 3]]
[a] → [a] → Boolean
String → String → Booleanprefix list Checks if a list starts with the provided sublist.
Similarly, checks if a string starts with the provided substring.
See also endsWith.
R.startsWith('a', 'abc') //=> true
R.startsWith('b', 'abc') //=> false
R.startsWith(['a'], ['a', 'b', 'c']) //=> true
R.startsWith(['b'], ['a', 'b', 'c']) //=> false Number → Number → Number
a The first value.
b The second value.
Subtracts its second argument from its first argument.
See also add.
R.subtract(10, 8); //=> 2 const minus5 = R.subtract(R.__, 5); minus5(17); //=> 12 const complementaryAngle = R.subtract(90); complementaryAngle(30); //=> 60 complementaryAngle(72); //=> 18
[Number] → Number
list An array of numbers
Adds together all the elements of a list.
See also reduce.
R.sum([2,4,6,8,100,1]); //=> 121
[*] → [*] → [*]
list1 The first list.
list2 The second list.
Finds the set (i.e. no duplicates) of all elements contained in the first or second list, but not both.
See also symmetricDifferenceWith, difference, differenceWith.
R.symmetricDifference([1,2,3,4], [7,6,5,4,3]); //=> [1,2,7,6,5] R.symmetricDifference([7,6,5,4,3], [1,2,3,4]); //=> [7,6,5,1,2]
((a, a) → Boolean) → [a] → [a] → [a]
pred A predicate used to test whether two items are equal.
list1 The first list.
list2 The second list.
Finds the set (i.e. no duplicates) of all elements contained in the first or second list, but not both. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.
See also symmetricDifference, difference, differenceWith.
const eqA = R.eqBy(R.prop('a'));
const l1 = [{a: 1}, {a: 2}, {a: 3}, {a: 4}];
const l2 = [{a: 3}, {a: 4}, {a: 5}, {a: 6}];
R.symmetricDifferenceWith(eqA, l1, l2); //=> [{a: 1}, {a: 2}, {a: 5}, {a: 6}] * → Boolean
A function that always returns true. Any passed in parameters are ignored.
See also F.
R.T(); //=> true
[a] → [a]
String → Stringlist 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]); //=> [2]
R.tail([1]); //=> []
R.tail([]); //=> []
R.tail('abc'); //=> 'bc'
R.tail('ab'); //=> 'b'
R.tail('a'); //=> ''
R.tail(''); //=> '' Number → [a] → [a]
Number → String → Stringn list Returns the first n elements of the given list, string, or transducer/transformer (or object with a take method).
Dispatches to the take method of the second argument, if present.
See also drop.
R.take(1, ['foo', 'bar', 'baz']); //=> ['foo'] R.take(2, ['foo', 'bar', 'baz']); //=> ['foo', 'bar'] R.take(3, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz'] R.take(4, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz'] R.take(3, 'ramda'); //=> 'ram' const personnel = [ 'Dave Brubeck', 'Paul Desmond', 'Eugene Wright', 'Joe Morello', 'Gerry Mulligan', 'Bob Bates', 'Joe Dodge', 'Ron Crotty' ]; const takeFive = R.take(5); takeFive(personnel); //=> ['Dave Brubeck', 'Paul Desmond', 'Eugene Wright', 'Joe Morello', 'Gerry Mulligan']
Number → [a] → [a]
Number → String → Stringn The number of elements to return.
xs The collection to consider.
Returns a new list containing the last n elements of the given list. If n > list.length, returns a list of list.length elements.
See also dropLast.
R.takeLast(1, ['foo', 'bar', 'baz']); //=> ['baz'] R.takeLast(2, ['foo', 'bar', 'baz']); //=> ['bar', 'baz'] R.takeLast(3, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz'] R.takeLast(4, ['foo', 'bar', 'baz']); //=> ['foo', 'bar', 'baz'] R.takeLast(3, 'ramda'); //=> 'mda'
(a → Boolean) → [a] → [a]
(a → Boolean) → String → Stringfn The function called per iteration.
xs The collection to iterate over.
Returns a new list containing the last n elements of a given list, passing each value to the supplied predicate function, and terminating when the predicate function returns false. Excludes the element that caused the predicate function to fail. The predicate function is passed one argument: (value).
See also dropLastWhile, addIndex.
const isNotOne = x => x !== 1; R.takeLastWhile(isNotOne, [1, 2, 3, 4]); //=> [2, 3, 4] R.takeLastWhile(x => x !== 'R' , 'Ramda'); //=> 'amda'
(a → Boolean) → [a] → [a]
(a → Boolean) → String → Stringfn The function called per iteration.
xs The collection to iterate over.
Returns a new list containing the first n elements of a given list, passing each value to the supplied predicate function, and terminating when the predicate function returns false. Excludes the element that caused the predicate function to fail. The predicate function is passed one argument: (value).
Dispatches to the takeWhile method of the second argument, if present.
Acts as a transducer if a transformer is given in list position.
See also dropWhile, transduce, addIndex.
const isNotFour = x => x !== 4; R.takeWhile(isNotFour, [1, 2, 3, 4, 3, 2, 1]); //=> [1, 2, 3] R.takeWhile(x => x !== 'd' , 'Ramda'); //=> 'Ram'
(a → *) → a → a
fn The function to call with x. The return value of fn will be thrown away.
x Runs the given function with the supplied object, then returns the object.
Acts as a transducer if a transformer is given as second parameter.
const sayX = x => console.log('x is ' + x);
R.tap(sayX, 100); //=> 100
// logs 'x is 100' RegExp → String → Boolean
pattern str Determines whether a given string matches a given regular expression.
See also match.
R.test(/^x/, 'xyz'); //=> true R.test(/^y/, 'xyz'); //=> false
(a → b) → (Promise e a) → (Promise e b)
(a → (Promise e b)) → (Promise e a) → (Promise e b)onSuccess The function to apply. Can return a value or a promise of a value.
p Returns the result of applying the onSuccess function to the value inside a successfully resolved promise. This is useful for working with promises inside function compositions.
See also otherwise.
var makeQuery = (email) => ({ query: { email }});
//getMemberName :: String -> Promise ({firstName, lastName})
var getMemberName = R.pipe(
makeQuery,
fetchMember,
R.then(R.pick(['firstName', 'lastName']))
); ((a, b, …, j) → k) → (a, b, …, j) → (() → k)
fn A function to wrap in a thunk
Creates a thunk out of a function. A thunk delays a calculation until its result is needed, providing lazy evaluation of arguments.
See also partial, partialRight.
R.thunkify(R.identity)(42)(); //=> 42 R.thunkify((a, b) => a + b)(25, 17)(); //=> 42
(Number → a) → Number → [a]
fn The function to invoke. Passed one argument, the current value of n.
n A value between 0 and n - 1. Increments after each function call.
Calls an input function n times, returning an array containing the results of those function calls.
fn is passed one argument: The current value of n, which begins at 0 and is gradually incremented to n - 1.
See also repeat.
R.times(R.identity, 5); //=> [0, 1, 2, 3, 4]
String → String
str The string to lower case.
The lower case version of a string.
See also toUpper.
R.toLower('XYZ'); //=> 'xyz' {String: *} → [[String,*]] obj The object to extract from
Converts an object into an array of key, value arrays. Only the object's own properties are used. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.
See also fromPairs.
R.toPairs({a: 1, b: 2, c: 3}); //=> [['a', 1], ['b', 2], ['c', 3]] {String: *} → [[String,*]] obj The object to extract from
Converts an object into an array of key, value arrays. The object's own properties and prototype properties are used. Note that the order of the output array is not guaranteed to be consistent across different JS platforms.
const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.toPairsIn(f); //=> [['x','X'], ['y','Y']] * → String
val 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")' String → String
str The string to upper case.
The upper case version of a string.
See also toLower.
R.toUpper('abc'); //=> 'ABC' (c → c) → ((a, b) → a) → a → [b] → a
xf The transducer function. Receives a transformer and returns a transformer.
fn The iterator function. Receives two values, the accumulator and the current element from the array. Wrapped as transformer, if necessary, and used to initialize the transducer
acc The initial accumulator value.
list The list to iterate over.
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.
See also reduce, reduced, into.
const numbers = [1, 2, 3, 4]; const transducer = R.compose(R.map(R.add(1)), R.take(2)); R.transduce(transducer, R.flip(R.append), [], numbers); //=> [2, 3] const isOdd = (x) => x % 2 === 1; const firstOddTransducer = R.compose(R.filter(isOdd), R.take(1)); R.transduce(firstOddTransducer, R.flip(R.append), [], R.range(0, 100)); //=> [1]
[[a]] → [[a]]
list 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], [20], [], [30, 31, 32]]) //=> [[10, 20, 30], [11, 31], [32]]
(Applicative f, Traversable t) => (a → f a) → (a → f b) → t a → f (t b)
of f traversable Maps an Applicative-returning function over a Traversable, then uses sequence to transform the resulting Traversable of Applicative into an Applicative of Traversable.
Dispatches to the traverse method of the third argument, if present.
See also sequence.
// Returns `Maybe.Nothing` if the given divisor is `0` const safeDiv = n => d => d === 0 ? Maybe.Nothing() : Maybe.Just(n / d) R.traverse(Maybe.of, safeDiv(10), [2, 4, 5]); //=> Maybe.Just([5, 2.5, 2]) R.traverse(Maybe.of, safeDiv(10), [2, 0, 5]); //=> Maybe.Nothing
String → String
str The string to trim.
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'] (…x → a) → ((e, …x) → a) → (…x → a)
tryer The function that may throw.
catcher The function that will be evaluated if tryer throws.
tryCatch takes two functions, a tryer and a catcher. The returned function evaluates the tryer; if it does not throw, it simply returns the result. If the tryer does throw, the returned function evaluates the catcher function and returns its result. Note that for effective composition with this function, both the tryer and catcher functions must return the same type of results.
R.tryCatch(R.prop('x'), R.F)({x: true}); //=> true
R.tryCatch(() => { throw 'foo'}, R.always('catched'))('bar') // => 'catched'
R.tryCatch(R.times(R.identity), R.always([]))('s') // => []
`` (* → {*}) → String val The value to test
Gives a single-word string description of the (native) type of a value, returning such answers as 'Object', 'Number', 'Array', or 'Null'. Does not attempt to distinguish user Object types any further, reporting them all as 'Object'.
R.type({}); //=> "Object"
R.type(1); //=> "Number"
R.type(false); //=> "Boolean"
R.type('s'); //=> "String"
R.type(null); //=> "Null"
R.type([]); //=> "Array"
R.type(/[A-z]/); //=> "RegExp"
R.type(() => {}); //=> "Function"
R.type(undefined); //=> "Undefined" ([*…] → a) → (*… → a)
fn Takes a function fn, which takes a single array argument, and returns a function which:
fn as an array; andIn other words, R.unapply derives a variadic function from a function which takes an array. R.unapply is the inverse of R.apply.
See also apply.
R.unapply(JSON.stringify)(1, 2, 3); //=> '[1,2,3]'
(* → b) → (a → b)
fn The function to wrap.
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.
const takesTwoArgs = function(a, b) {
return [a, b];
};
takesTwoArgs.length; //=> 2
takesTwoArgs(1, 2); //=> [1, 2]
const takesOneArg = R.unary(takesTwoArgs);
takesOneArg.length; //=> 1
// Only 1 argument is passed to the wrapped function
takesOneArg(1, 2); //=> [1, undefined] Number → (a → b) → (a → c)
length The arity for the returned function.
fn The function to uncurry.
Returns a function of arity n from a (manually) curried function.
See also curry.
const addFour = a => b => c => d => a + b + c + d; const uncurriedAddFour = R.uncurryN(4, addFour); uncurriedAddFour(1, 2, 3, 4); //=> 10
(a → [b]) → * → [b]
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.
Builds a list from a seed value. Accepts an iterator function, which returns either false to stop iteration or an array of length 2 containing the value to add to the resulting list and the seed to be used in the next call to the iterator function.
The iterator function receives one argument: (seed).
const f = n => n > 50 ? false : [-n, n + 10]; R.unfold(f, 10); //=> [-10, -20, -30, -40, -50]
[*] → [*] → [*]
as The first list.
bs The second list.
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]
((a, a) → Boolean) → [*] → [*] → [*]
pred A predicate used to test whether two items are equal.
list1 The first list.
list2 The second list.
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.
See also union.
const l1 = [{a: 1}, {a: 2}];
const l2 = [{a: 1}, {a: 4}];
R.unionWith(R.eqBy(R.prop('a')), l1, l2); //=> [{a: 1}, {a: 2}, {a: 4}] [a] → [a]
list The array to consider.
Returns a new list containing only one copy of each element in the original list. R.equals is used to determine equality.
R.uniq([1, 1, 2, 1]); //=> [1, 2] R.uniq([1, '1']); //=> [1, '1'] R.uniq([[42], [42]]); //=> [[42]]
(a → b) → [a] → [a]
fn A function used to produce a value to use during comparisons.
list The array to consider.
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]
((a, a) → Boolean) → [a] → [a]
pred A predicate used to test whether two items are equal.
list The array to consider.
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.
const strEq = R.eqBy(String);
R.uniqWith(strEq)([1, '1', 2, 1]); //=> [1, 2]
R.uniqWith(strEq)([{}, {}]); //=> [{}]
R.uniqWith(strEq)([1, '1', 1]); //=> [1]
R.uniqWith(strEq)(['1', 1, 1]); //=> ['1'] (a → Boolean) → (a → a) → a → a
pred A predicate function
whenFalseFn A function to invoke when the pred evaluates to a falsy value.
x An object to test with the pred function and pass to whenFalseFn if necessary.
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.
let safeInc = R.unless(R.isNil, R.inc); safeInc(null); //=> null safeInc(1); //=> 2
Chain c => c (c a) → c a
list Shorthand for R.chain(R.identity), which removes one level of nesting from any Chain.
R.unnest([1, [2], [[3]]]); //=> [1, 2, [3]] R.unnest([[1, 2], [3, 4], [5, 6]]); //=> [1, 2, 3, 4, 5, 6]
(a → Boolean) → (a → a) → a → a
pred A predicate function
fn The iterator function
init Initial value
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
Number → a → [a] → [a]
idx The index to update.
x The value to exist at the given index of the returned array.
list The source array-like object to be updated.
Returns a new copy of the array with the element at the provided index replaced with the given value.
See also adjust.
R.update(1, '_', ['a', 'b', 'c']); //=> ['a', '_', 'c'] R.update(-1, '_', ['a', 'b', 'c']); //=> ['a', 'b', '_']
((x1, x2, …) → z) → [(a → x1), (b → x2), …] → (a → b → … → z)
fn The function to wrap.
transformers A list of transformer functions
Accepts a function fn and a list of transformer functions and returns a new curried function. When the new function is invoked, it calls the function fn with parameters consisting of the result of calling each supplied handler on successive arguments to the new function.
If more arguments are passed to the returned function than transformer functions, those arguments are passed directly to fn as additional parameters. If you expect additional arguments that don't need to be transformed, although you can ignore them, it's best to pass an identity function so that the new function reports the correct arity.
See also converge.
R.useWith(Math.pow, [R.identity, R.identity])(3, 4); //=> 81 R.useWith(Math.pow, [R.identity, R.identity])(3)(4); //=> 81 R.useWith(Math.pow, [R.dec, R.inc])(3, 4); //=> 32 R.useWith(Math.pow, [R.dec, R.inc])(3)(4); //=> 32
{k: v} → [v] obj The object to extract values from
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] {k: v} → [v] obj The object to extract values from
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.
const F = function() { this.x = 'X'; };
F.prototype.y = 'Y';
const f = new F();
R.valuesIn(f); //=> ['X', 'Y'] Lens s a → s → a
Lens s a = Functor f => (a → f a) → s → f slens x Returns a "view" of the given data structure, determined by the given lens. The lens's focus determines which portion of the data structure is visible.
See also prop, lensIndex, lensProp.
const xLens = R.lensProp('x');
R.view(xLens, {x: 1, y: 2}); //=> 1
R.view(xLens, {x: 4, y: 2}); //=> 4 (a → Boolean) → (a → a) → a → a
pred A predicate function
whenTrueFn A function to invoke when the condition evaluates to a truthy value.
x An object to test with the pred function and pass to whenTrueFn if necessary.
Tests the final argument by passing it to the given predicate function. If the predicate is satisfied, the function will return the result of calling the whenTrueFn function with the same argument. If the predicate is not satisfied, the argument is returned as is.
See also ifElse, unless, cond.
// truncate :: String -> String
const truncate = R.when(
R.propSatisfies(R.gt(R.__, 10), 'length'),
R.pipe(R.take(10), R.append('…'), R.join(''))
);
truncate('12345'); //=> '12345'
truncate('0123456789ABC'); //=> '0123456789…' {String: (* → Boolean)} → {String: *} → Boolean spec testObj Takes a spec object and a test object; returns true if the test satisfies the spec. Each of the spec's own properties must be a predicate function. Each predicate is applied to the value of the corresponding property of the test object. where returns true if all the predicates return true, false otherwise.
where is well suited to declaratively expressing constraints for other functions such as filter and find.
See also propSatisfies, whereEq.
// pred :: Object -> Boolean
const pred = R.where({
a: R.equals('foo'),
b: R.complement(R.equals('bar')),
x: R.gt(R.__, 10),
y: R.lt(R.__, 20)
});
pred({a: 'foo', b: 'xxx', x: 11, y: 19}); //=> true
pred({a: 'xxx', b: 'xxx', x: 11, y: 19}); //=> false
pred({a: 'foo', b: 'bar', x: 11, y: 19}); //=> false
pred({a: 'foo', b: 'xxx', x: 10, y: 19}); //=> false
pred({a: 'foo', b: 'xxx', x: 11, y: 20}); //=> false {String: *} → {String: *} → Boolean spec testObj 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
const pred = R.whereEq({a: 1, b: 2});
pred({a: 1}); //=> false
pred({a: 1, b: 2}); //=> true
pred({a: 1, b: 2, c: 3}); //=> true
pred({a: 1, b: 1}); //=> false [a] → [a] → [a]
list1 The values to be removed from list2.
list2 The array to remove values from.
Returns a new list without values in the first argument. R.equals is used to determine equality.
Acts as a transducer if a transformer is given in list position.
See also transduce, difference, remove.
R.without([1, 2], [1, 2, 1, 3, 4]); //=> [3, 4]
[a] → [b] → [[a,b]]
as The first list.
bs The second list.
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']]
[a] → [b] → [[a,b]]
list1 The first array to consider.
list2 The second array to consider.
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']]
[String] → [*] → {String: *} keys The array that will be properties on the output object.
values The list of values on the output object.
Creates a new object out of a list of keys and a list of values. Key/value pairing is truncated to the length of the shorter of the two lists. Note: zipObj is equivalent to pipe(zip, fromPairs).
R.zipObj(['a', 'b', 'c'], [1, 2, 3]); //=> {a: 1, b: 2, c: 3} ((a, b) → c) → [a] → [b] → [c]
fn The function used to combine the two elements into one value.
list1 The first array to consider.
list2 The second array to consider.
Creates a new list out of the two supplied by applying the function to each equally-positioned pair in the lists. The returned list is truncated to the length of the shorter of the two input lists.
const f = (x, y) => {
// ...
};
R.zipWith(f, [1, 2, 3], ['a', 'b', 'c']);
//=> [f(1, 'a'), f(2, 'b'), f(3, 'c')]
© 2013–2016 Scott Sauyet and Michael Hurley
Licensed under the MIT License.
https://ramdajs.com/0.26.1/docs/