Collection methods

Collection methods work on both arrays and objects. When you pass an object, lodash iterates its own enumerable string-keyed properties; when you pass an array or array-like value, lodash iterates by index. Check each method for its exact iteratee signature.

_.countBy(collection, [iteratee=identity])

Available since v0.5.0.

Creates an object composed of keys generated from the results of running each element of collection thru iteratee. The corresponding value of each key is the number of times the key was returned by iteratee. The iteratee is invoked with one argument: (value).

Returns Object — Returns the composed aggregate object.

_.countBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': 1, '6': 2 }

// The `_.property` iteratee shorthand.
_.countBy(['one', 'two', 'three'], 'length');
// => { '3': 2, '5': 1 }

_.every(collection, [predicate=identity])

Available since v0.1.0.

Checks if predicate returns truthy for all elements of collection. Iteration is stopped once predicate returns falsey. The predicate is invoked with three arguments: (value, index|key, collection).

Note: This method returns true for empty collections because everything is true of elements of empty collections.

Returns boolean — Returns true if all elements pass the predicate check, else false.

_.every([true, 1, null, 'yes'], Boolean);
// => false

var users = [
  { 'user': 'barney', 'age': 36, 'active': false },
  { 'user': 'fred',   'age': 40, 'active': false }
];

// The `_.matches` iteratee shorthand.
_.every(users, { 'user': 'barney', 'active': false });
// => false

// The `_.matchesProperty` iteratee shorthand.
_.every(users, ['active', false]);
// => true

// The `_.property` iteratee shorthand.
_.every(users, 'active');
// => false

_.filter(collection, [predicate=identity])

Available since v0.1.0.

Iterates over elements of collection, returning an array of all elements predicate returns truthy for. The predicate is invoked with three arguments: (value, index|key, collection).

Note: Unlike _.remove, this method returns a new array.

Returns Array — Returns the new filtered array.

var users = [
  { 'user': 'barney', 'age': 36, 'active': true },
  { 'user': 'fred',   'age': 40, 'active': false }
];

_.filter(users, function(o) { return !o.active; });
// => objects for ['fred']

// The `_.matches` iteratee shorthand.
_.filter(users, { 'age': 36, 'active': true });
// => objects for ['barney']

// The `_.matchesProperty` iteratee shorthand.
_.filter(users, ['active', false]);
// => objects for ['fred']

// The `_.property` iteratee shorthand.
_.filter(users, 'active');
// => objects for ['barney']

// Combining several predicates using `_.overEvery` or `_.overSome`.
_.filter(users, _.overSome([{ 'age': 36 }, ['age', 40]]));
// => objects for ['fred', 'barney']

_.find(collection, [predicate=identity], [fromIndex=0])

Available since v0.1.0.

Iterates over elements of collection, returning the first element predicate returns truthy for. The predicate is invoked with three arguments: (value, index|key, collection).

Returns any — Returns the matched element, else undefined.

var users = [
  { 'user': 'barney',  'age': 36, 'active': true },
  { 'user': 'fred',    'age': 40, 'active': false },
  { 'user': 'pebbles', 'age': 1,  'active': true }
];

_.find(users, function(o) { return o.age < 40; });
// => object for 'barney'

// The `_.matches` iteratee shorthand.
_.find(users, { 'age': 1, 'active': true });
// => object for 'pebbles'

// The `_.matchesProperty` iteratee shorthand.
_.find(users, ['active', false]);
// => object for 'fred'

// The `_.property` iteratee shorthand.
_.find(users, 'active');
// => object for 'barney'

_.findLast(collection, [predicate=identity], [fromIndex=collection.length-1])

Available since v2.0.0.

This method is like _.find except that it iterates over elements of collection from right to left.

Returns any — Returns the matched element, else undefined.

_.findLast([1, 2, 3, 4], function(n) {
  return n % 2 == 1;
});
// => 3

_.flatMap(collection, [iteratee=identity])

Available since v4.0.0.

Creates a flattened array of values by running each element in collection thru iteratee and flattening the mapped results. The iteratee is invoked with three arguments: (value, index|key, collection).

Returns Array — Returns the new flattened array.

function duplicate(n) {
  return [n, n];
}

_.flatMap([1, 2], duplicate);
// => [1, 1, 2, 2]

_.flatMapDeep(collection, [iteratee=identity])

Available since v4.7.0.

This method is like _.flatMap except that it recursively flattens the mapped results.

Returns Array — Returns the new flattened array.

function duplicate(n) {
  return [[[n, n]]];
}

_.flatMapDeep([1, 2], duplicate);
// => [1, 1, 2, 2]

_.flatMapDepth(collection, [iteratee=identity], [depth=1])

Available since v4.7.0.

This method is like _.flatMap except that it recursively flattens the mapped results up to depth times.

Returns Array — Returns the new flattened array.

function duplicate(n) {
  return [[[n, n]]];
}

_.flatMapDepth([1, 2], duplicate, 2);
// => [[1, 1], [2, 2]]

_.forEach(collection, [iteratee=identity])

Available since v0.1.0. Alias: _.each.

Iterates over elements of collection and invokes iteratee for each element. The iteratee is invoked with three arguments: (value, index|key, collection). Iteratee functions may exit iteration early by explicitly returning false.

Note: As with other "Collections" methods, objects with a "length" property are iterated like arrays. To avoid this behavior use _.forIn or forOwn for object iteration.

Returns any — Returns collection.

_.forEach([1, 2], function(value) {
  console.log(value);
});
// => Logs `1` then `2`.

_.forEach({ 'a': 1, 'b': 2 }, function(value, key) {
  console.log(key);
});
// => Logs 'a' then 'b' (iteration order is not guaranteed).

_.forEachRight(collection, [iteratee=identity])

Available since v2.0.0. Alias: _.eachRight.

This method is like _.forEach except that it iterates over elements of collection from right to left.

Returns any — Returns collection.

_.forEachRight([1, 2], function(value) {
  console.log(value);
});
// => Logs `2` then `1`.

_.groupBy(collection, [iteratee=identity])

Available since v0.1.0.

Creates an object composed of keys generated from the results of running each element of collection thru iteratee. The order of grouped values is determined by the order they occur in collection. The corresponding value of each key is an array of elements responsible for generating the key. The iteratee is invoked with one argument: (value).

Returns Object — Returns the composed aggregate object.

_.groupBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': [4.2], '6': [6.1, 6.3] }

// The `_.property` iteratee shorthand.
_.groupBy(['one', 'two', 'three'], 'length');
// => { '3': ['one', 'two'], '5': ['three'] }

_.includes(collection, value, [fromIndex=0])

Available since v0.1.0.

Checks if value is in collection. If collection is a string, it's checked for a substring of value, otherwise SameValueZero is used for equality comparisons. If fromIndex is negative, it's used as the offset from the end of collection.

Returns boolean — Returns true if value is found, else false.

_.includes([1, 2, 3], 1);
// => true

_.includes([1, 2, 3], 1, 2);
// => false

_.includes({ 'a': 1, 'b': 2 }, 1);
// => true

_.includes('abcd', 'bc');
// => true

_.invokeMap(collection, path, [args])

Available since v4.0.0.

Invokes the method at path of each element in collection, returning an array of the results of each invoked method. Any additional arguments are provided to each invoked method. If path is a function, it's invoked for, and this bound to, each element in collection.

Returns Array — Returns the array of results.

_.invokeMap([[5, 1, 7], [3, 2, 1]], 'sort');
// => [[1, 5, 7], [1, 2, 3]]

_.invokeMap([123, 456], String.prototype.split, '');
// => [['1', '2', '3'], ['4', '5', '6']]

_.keyBy(collection, [iteratee=identity])

Available since v4.0.0.

Creates an object composed of keys generated from the results of running each element of collection thru iteratee. The corresponding value of each key is the last element responsible for generating the key. The iteratee is invoked with one argument: (value).

Returns Object — Returns the composed aggregate object.

var array = [
  { 'dir': 'left', 'code': 97 },
  { 'dir': 'right', 'code': 100 }
];

_.keyBy(array, function(o) {
  return String.fromCharCode(o.code);
});
// => { 'a': { 'dir': 'left', 'code': 97 }, 'd': { 'dir': 'right', 'code': 100 } }

_.keyBy(array, 'dir');
// => { 'left': { 'dir': 'left', 'code': 97 }, 'right': { 'dir': 'right', 'code': 100 } }

_.map(collection, [iteratee=identity])

Available since v0.1.0.

Creates an array of values by running each element in collection thru iteratee. The iteratee is invoked with three arguments: (value, index|key, collection).

Many lodash methods are guarded to work as iteratees for methods like _.every, filter, map, mapValues, reject, and some.

The guarded methods are: ary, chunk, curry, curryRight, drop, dropRight, every, fill, invert, parseInt, random, range, rangeRight, repeat, sampleSize, slice, some, sortBy, split, take, takeRight, template, trim, trimEnd, trimStart, and words

Returns Array — Returns the new mapped array.

function square(n) {
  return n * n;
}

_.map([4, 8], square);
// => [16, 64]

_.map({ 'a': 4, 'b': 8 }, square);
// => [16, 64] (iteration order is not guaranteed)

var users = [
  { 'user': 'barney' },
  { 'user': 'fred' }
];

// The `_.property` iteratee shorthand.
_.map(users, 'user');
// => ['barney', 'fred']

_.orderBy(collection, [iteratees=[identity]], [orders])

Available since v4.0.0.

This method is like _.sortBy except that it allows specifying the sort orders of the iteratees to sort by. If orders is unspecified, all values are sorted in ascending order. Otherwise, specify an order of "desc" for descending or "asc" for ascending sort order of corresponding values.

Returns Array — Returns the new sorted array.

var users = [
  { 'user': 'fred',   'age': 48 },
  { 'user': 'barney', 'age': 34 },
  { 'user': 'fred',   'age': 40 },
  { 'user': 'barney', 'age': 36 }
];

// Sort by `user` in ascending order and by `age` in descending order.
_.orderBy(users, ['user', 'age'], ['asc', 'desc']);
// => objects for [['barney', 36], ['barney', 34], ['fred', 48], ['fred', 40]]

_.partition(collection, [predicate=identity])

Available since v3.0.0.

Creates an array of elements split into two groups, the first of which contains elements predicate returns truthy for, the second of which contains elements predicate returns falsey for. The predicate is invoked with one argument: (value).

Returns Array — Returns the array of grouped elements.

var users = [
  { 'user': 'barney',  'age': 36, 'active': false },
  { 'user': 'fred',    'age': 40, 'active': true },
  { 'user': 'pebbles', 'age': 1,  'active': false }
];

_.partition(users, function(o) { return o.active; });
// => objects for [['fred'], ['barney', 'pebbles']]

// The `_.matches` iteratee shorthand.
_.partition(users, { 'age': 1, 'active': false });
// => objects for [['pebbles'], ['barney', 'fred']]

// The `_.matchesProperty` iteratee shorthand.
_.partition(users, ['active', false]);
// => objects for [['barney', 'pebbles'], ['fred']]

// The `_.property` iteratee shorthand.
_.partition(users, 'active');
// => objects for [['fred'], ['barney', 'pebbles']]

_.reduce(collection, [iteratee=identity], [accumulator])

Available since v0.1.0.

Reduces collection to a value which is the accumulated result of running each element in collection thru iteratee, where each successive invocation is supplied the return value of the previous. If accumulator is not given, the first element of collection is used as the initial value. The iteratee is invoked with four arguments: (accumulator, value, index|key, collection).

Many lodash methods are guarded to work as iteratees for methods like _.reduce, reduceRight, and transform.

The guarded methods are: assign, defaults, defaultsDeep, includes, merge, orderBy, and sortBy

Returns any — Returns the accumulated value.

_.reduce([1, 2], function(sum, n) {
  return sum + n;
}, 0);
// => 3

_.reduce({ 'a': 1, 'b': 2, 'c': 1 }, function(result, value, key) {
  (result[value] || (result[value] = [])).push(key);
  return result;
}, {});
// => { '1': ['a', 'c'], '2': ['b'] } (iteration order is not guaranteed)

_.reduceRight(collection, [iteratee=identity], [accumulator])

Available since v0.1.0.

This method is like _.reduce except that it iterates over elements of collection from right to left.

Returns any — Returns the accumulated value.

var array = [[0, 1], [2, 3], [4, 5]];

_.reduceRight(array, function(flattened, other) {
  return flattened.concat(other);
}, []);
// => [4, 5, 2, 3, 0, 1]

_.reject(collection, [predicate=identity])

Available since v0.1.0.

The opposite of _.filter; this method returns the elements of collection that predicate does not return truthy for.

Returns Array — Returns the new filtered array.

var users = [
  { 'user': 'barney', 'age': 36, 'active': false },
  { 'user': 'fred',   'age': 40, 'active': true }
];

_.reject(users, function(o) { return !o.active; });
// => objects for ['fred']

// The `_.matches` iteratee shorthand.
_.reject(users, { 'age': 40, 'active': true });
// => objects for ['barney']

// The `_.matchesProperty` iteratee shorthand.
_.reject(users, ['active', false]);
// => objects for ['fred']

// The `_.property` iteratee shorthand.
_.reject(users, 'active');
// => objects for ['barney']

_.sample(collection)

Available since v2.0.0.

Gets a random element from collection.

Returns any — Returns the random element.

_.sample([1, 2, 3, 4]);
// => 2

_.sampleSize(collection, [n=1])

Available since v4.0.0.

Gets n random elements at unique keys from collection up to the size of collection.

Returns Array — Returns the random elements.

_.sampleSize([1, 2, 3], 2);
// => [3, 1]

_.sampleSize([1, 2, 3], 4);
// => [2, 3, 1]

_.shuffle(collection)

Available since v0.1.0.

Creates an array of shuffled values, using a version of the Fisher-Yates shuffle.

Returns Array — Returns the new shuffled array.

_.shuffle([1, 2, 3, 4]);
// => [4, 1, 3, 2]

_.size(collection)

Available since v0.1.0.

Gets the size of collection by returning its length for array-like values or the number of own enumerable string keyed properties for objects.

Returns number — Returns the collection size.

_.size([1, 2, 3]);
// => 3

_.size({ 'a': 1, 'b': 2 });
// => 2

_.size('pebbles');
// => 7

_.some(collection, [predicate=identity])

Available since v0.1.0.

Checks if predicate returns truthy for any element of collection. Iteration is stopped once predicate returns truthy. The predicate is invoked with three arguments: (value, index|key, collection).

Returns boolean — Returns true if any element passes the predicate check, else false.

_.some([null, 0, 'yes', false], Boolean);
// => true

var users = [
  { 'user': 'barney', 'active': true },
  { 'user': 'fred',   'active': false }
];

// The `_.matches` iteratee shorthand.
_.some(users, { 'user': 'barney', 'active': false });
// => false

// The `_.matchesProperty` iteratee shorthand.
_.some(users, ['active', false]);
// => true

// The `_.property` iteratee shorthand.
_.some(users, 'active');
// => true

_.sortBy(collection, [iteratees=[identity]])

Available since v0.1.0.

Creates an array of elements, sorted in ascending order by the results of running each element in a collection thru each iteratee. This method performs a stable sort, that is, it preserves the original sort order of equal elements. The iteratees are invoked with one argument: (value).

Returns Array — Returns the new sorted array.

var users = [
  { 'user': 'fred',   'age': 48 },
  { 'user': 'barney', 'age': 36 },
  { 'user': 'fred',   'age': 30 },
  { 'user': 'barney', 'age': 34 }
];

_.sortBy(users, [function(o) { return o.user; }]);
// => objects for [['barney', 36], ['barney', 34], ['fred', 48], ['fred', 30]]

_.sortBy(users, ['user', 'age']);
// => objects for [['barney', 34], ['barney', 36], ['fred', 30], ['fred', 48]]