Collection

The Collection is a set of (key, value) entries which can be iterated, and is the base class for all collections in immutable, allowing them to make use of all the Collection methods (such as map and filter).

type Collection<K, V> extends ValueObject

Discussion

Note: A collection is always iterated in the same order, however that order may not always be well defined, as is the case for the Map and Set.

Collection is the abstract base class for concrete data structures. It cannot be constructed directly.

Implementations should extend one of the subclasses, Collection.Keyed, Collection.Indexed, or Collection.Set.

Construction

Collection()

Collection<I>(collection: I): I
Collection<T>(collection: Iterable<T> | ArrayLike<T>): Collection.Indexed<T>
Collection<V>(obj: {[key: string]: V}): Collection.Keyed<string, V>
Collection<K, V>(): Collection<K, V>

Value equality

equals()

True if this and the other Collection have value equality, as defined by Immutable.is().

equals(other: unknown): boolean

Overrides

ValueObject#equals()

Discussion

Note: This is equivalent to Immutable.is(this, other), but provided to allow for chained expressions.

hashCode()

Computes and returns the hashed identity for this Collection.

hashCode(): number

Overrides

ValueObject#hashCode()

Discussion

The hashCode of a Collection is used to determine potential equality, and is used when adding this to a Set or as a key in a Map, enabling lookup via a different instance.

const a = List([ 1, 2, 3 ]);
const b = List([ 1, 2, 3 ]);
assert.notStrictEqual(a, b); // different instances
const set = Set([ a ]);
assert.equal(set.has(b), true);run it

If two values have the same hashCode, they are not guaranteed to be equal. If two values have different hashCodes, they must not be equal.

Reading values

get()

get<NSV>(key: K, notSetValue: NSV): V | NSV
get(key: K): V | undefined

has()

True if a key exists within this Collection, using Immutable.is to determine equality

has(key: K): boolean

includes()

True if a value exists within this Collection, using Immutable.is to determine equality

includes(value: V): boolean

alias

contains()

first()

first<NSV>(notSetValue: NSV): V | NSV
first(): V | undefined

last()

last<NSV>(notSetValue: NSV): V | NSV
last(): V | undefined

Reading deep values

getIn()

Returns the value found by following a path of keys or indices through nested Collections.

getIn(searchKeyPath: Iterable<unknown>, notSetValue?: unknown): unknown

Discussion

const { Map, List } = require('immutable') const deepData = Map({ x: List([ Map({ y: 123 }) ]) }); deepData.getIn(['x', 0, 'y']) // 123run it

Plain JavaScript Object or Arrays may be nested within an Immutable.js Collection, and getIn() can access those values as well:

const { Map, List } = require('immutable') const deepData = Map({ x: [ { y: 123 } ] }); deepData.getIn(['x', 0, 'y']) // 123run it

hasIn()

True if the result of following a path of keys or indices through nested Collections results in a set value.

hasIn(searchKeyPath: Iterable<unknown>): boolean

Persistent changes

update()

This can be very useful as a way to "chain" a normal function into a sequence of methods. RxJS calls this "let" and lodash calls it "thru".

update<R>(updater: (value: this) => R): R

Discussion

For example, to sum a Seq after mapping and filtering:

const { Seq } = require('immutable')

function sum(collection) {
  return collection.reduce((sum, x) => sum + x, 0)
}

Seq([ 1, 2, 3 ]) .map(x => x + 1) .filter(x => x % 2 === 0) .update(sum) // 6run it

Conversion to JavaScript types

toJS()

Deeply converts this Collection to equivalent native JavaScript Array or Object.

toJS(): Array<DeepCopy<V>> | {[key: string]: DeepCopy<V>}

Discussion

Collection.Indexed, and Collection.Set become Array, while Collection.Keyed become Object, converting keys to Strings.

toJSON()

Shallowly converts this Collection to equivalent native JavaScript Array or Object.

toJSON(): Array<V> | {[key: string]: V}

Discussion

Collection.Indexed, and Collection.Set become Array, while Collection.Keyed become Object, converting keys to Strings.

toArray()

Shallowly converts this collection to an Array.

toArray(): Array<V> | Array<[K, V]>

Discussion

Collection.Indexed, and Collection.Set produce an Array of values. Collection.Keyed produce an Array of [key, value] tuples.

toObject()

Shallowly converts this Collection to an Object.

toObject(): {[key: string]: V}

Discussion

Converts keys to Strings.

Conversion to Collections

toMap()

Converts this Collection to a Map, Throws if keys are not hashable.

toMap(): Map<K, V>

Discussion

Note: This is equivalent to Map(this.toKeyedSeq()), but provided for convenience and to allow for chained expressions.

toOrderedMap()

Converts this Collection to a Map, maintaining the order of iteration.

toOrderedMap(): OrderedMap<K, V>

Discussion

Note: This is equivalent to OrderedMap(this.toKeyedSeq()), but provided for convenience and to allow for chained expressions.

toSet()

Converts this Collection to a Set, discarding keys. Throws if values are not hashable.

toSet(): Set<V>

Discussion

Note: This is equivalent to Set(this), but provided to allow for chained expressions.

toOrderedSet()

Converts this Collection to a Set, maintaining the order of iteration and discarding keys.

toOrderedSet(): OrderedSet<V>

Discussion

Note: This is equivalent to OrderedSet(this.valueSeq()), but provided for convenience and to allow for chained expressions.

toList()

Converts this Collection to a List, discarding keys.

toList(): List<V>

Discussion

This is similar to List(collection), but provided to allow for chained expressions. However, when called on Map or other keyed collections, collection.toList() discards the keys and creates a list of only the values, whereas List(collection) creates a list of entry tuples.

const { Map, List } = require('immutable')
var myMap = Map({ a: 'Apple', b: 'Banana' })
List(myMap) // List [ [ "a", "Apple" ], [ "b", "Banana" ] ]
myMap.toList() // List [ "Apple", "Banana" ]run it

toStack()

Converts this Collection to a Stack, discarding keys. Throws if values are not hashable.

toStack(): Stack<V>

Discussion

Note: This is equivalent to Stack(this), but provided to allow for chained expressions.

Conversion to Seq

toSeq()

Converts this Collection to a Seq of the same kind (indexed, keyed, or set).

toSeq(): Seq<K, V>

toKeyedSeq()

Returns a Seq.Keyed from this Collection where indices are treated as keys.

toKeyedSeq(): Seq.Keyed<K, V>

Discussion

This is useful if you want to operate on an Collection.Indexed and preserve the [index, value] pairs.

The returned Seq will have identical iteration order as this Collection.

const { Seq } = require('immutable')
const indexedSeq = Seq([ 'A', 'B', 'C' ])
// Seq [ "A", "B", "C" ]
indexedSeq.filter(v => v === 'B')
// Seq [ "B" ]
const keyedSeq = indexedSeq.toKeyedSeq()
// Seq { 0: "A", 1: "B", 2: "C" }
keyedSeq.filter(v => v === 'B')
// Seq { 1: "B" }run it

toIndexedSeq()

Returns an Seq.Indexed of the values of this Collection, discarding keys.

toIndexedSeq(): Seq.Indexed<V>

toSetSeq()

Returns a Seq.Set of the values of this Collection, discarding keys.

toSetSeq(): Seq.Set<V>

Iterators

keys()

An iterator of this Collection's keys.

keys(): IterableIterator<K>

Discussion

Note: this will return an ES6 iterator which does not support Immutable.js sequence algorithms. Use keySeq instead, if this is what you want.

values()

An iterator of this Collection's values.

values(): IterableIterator<V>

Discussion

Note: this will return an ES6 iterator which does not support Immutable.js sequence algorithms. Use valueSeq instead, if this is what you want.

entries()

An iterator of this Collection's entries as [ key, value ] tuples.

entries(): IterableIterator<[K, V]>

Discussion

Note: this will return an ES6 iterator which does not support Immutable.js sequence algorithms. Use entrySeq instead, if this is what you want.

[Symbol.iterator]()

[Symbol.iterator](): IterableIterator<unknown>

Collections (Seq)

keySeq()

Returns a new Seq.Indexed of the keys of this Collection, discarding values.

keySeq(): Seq.Indexed<K>

valueSeq()

Returns an Seq.Indexed of the values of this Collection, discarding keys.

valueSeq(): Seq.Indexed<V>

entrySeq()

Returns a new Seq.Indexed of [key, value] tuples.

entrySeq(): Seq.Indexed<[K, V]>

Sequence algorithms

map()

Returns a new Collection of the same type with values passed through a mapper function.

map<M>(mapper: (value: V, key: K, iter: this) => M,
context?: unknown
): Collection<K, M>

Discussion

const { Collection } = require('immutable')
Collection({ a: 1, b: 2 }).map(x => 10 * x)
// Seq { "a": 10, "b": 20 }run it

Note: map() always returns a new instance, even if it produced the same value at every step.

filter()

filter<F>(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): Collection<K, F>
filter(predicate: (value: V, key: K, iter: this) => unknown,
context?: unknown
): this

filterNot()

Returns a new Collection of the same type with only the entries for which the predicate function returns false.

filterNot(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): this

Discussion

const { Map } = require('immutable') Map({ a: 1, b: 2, c: 3, d: 4}).filterNot(x => x % 2 === 0) // Map { "a": 1, "c": 3 }run it

Note: filterNot() always returns a new instance, even if it results in not filtering out any values.

partition()

partition<F, C>(predicate: (this: C, value: V, key: K, iter: this) => boolean,
context?: C
): [Collection<K, V>, Collection<K, F>]
partition<C>(predicate: (this: C, value: V, key: K, iter: this) => unknown,
context?: C
): [this, this]

reverse()

Returns a new Collection of the same type in reverse order.

reverse(): this

sort()

Returns a new Collection of the same type which includes the same entries, stably sorted by using a comparator.

sort(comparator?: Comparator<V>): this

Discussion

If a comparator is not provided, a default comparator uses < and >.

comparator(valueA, valueB):

Returns 0 if the elements should not be swapped.
Returns -1 (or any negative number) if valueA comes before valueB
Returns 1 (or any positive number) if valueA comes after valueB
Alternatively, can return a value of the PairSorting enum type
Is pure, i.e. it must always return the same value for the same pair of values.

When sorting collections which have no defined order, their ordered equivalents will be returned. e.g. map.sort() returns OrderedMap.

const { Map } = require('immutable')
Map({ "c": 3, "a": 1, "b": 2 }).sort((a, b) => {
  if (a < b) { return -1; }
  if (a > b) { return 1; }
  if (a === b) { return 0; }
});
// OrderedMap { "a": 1, "b": 2, "c": 3 }run it

Note: sort() Always returns a new instance, even if the original was already sorted.

Note: This is always an eager operation.

sortBy()

Like sort, but also accepts a comparatorValueMapper which allows for sorting by more sophisticated means:

sortBy<C>(comparatorValueMapper: (value: V, key: K, iter: this) => C,
comparator?: Comparator<C>
): this

Discussion

const { Map } = require('immutable') const beattles = Map({ John: { name: "Lennon" }, Paul: { name: "McCartney" }, George: { name: "Harrison" }, Ringo: { name: "Starr" }, }); beattles.sortBy(member => member.name);run it

Note: sortBy() Always returns a new instance, even if the original was already sorted.

Note: This is always an eager operation.

groupBy()

Returns a Map of Collection, grouped by the return value of the grouper function.

groupBy<G>(grouper: (value: V, key: K, iter: this) => G,
context?: unknown
): Map<G, this>

Discussion

Note: This is always an eager operation.

const { List, Map } = require('immutable')
const listOfMaps = List([
  Map({ v: 0 }),
  Map({ v: 1 }),
  Map({ v: 1 }),
  Map({ v: 0 }),
  Map({ v: 2 })
])
const groupsOfMaps = listOfMaps.groupBy(x => x.get('v'))
// Map {
//   0: List [ Map{ "v": 0 }, Map { "v": 0 } ],
//   1: List [ Map{ "v": 1 }, Map { "v": 1 } ],
//   2: List [ Map{ "v": 2 } ],
// }run it

Side effects

forEach()

The sideEffect is executed for every entry in the Collection.

forEach(sideEffect: (value: V, key: K, iter: this) => unknown,
context?: unknown
): number

Discussion

Unlike Array#forEach, if any call of sideEffect returns false, the iteration will stop. Returns the number of entries iterated (including the last iteration which returned false).

Creating subsets

slice()

Returns a new Collection of the same type representing a portion of this Collection from start up to but not including end.

slice(begin?: number, end?: number): this

Discussion

If begin is negative, it is offset from the end of the Collection. e.g. slice(-2) returns a Collection of the last two entries. If it is not provided the new Collection will begin at the beginning of this Collection.

If end is negative, it is offset from the end of the Collection. e.g. slice(0, -1) returns a Collection of everything but the last entry. If it is not provided, the new Collection will continue through the end of this Collection.

If the requested slice is equivalent to the current Collection, then it will return itself.

rest()

Returns a new Collection of the same type containing all entries except the first.

rest(): this

butLast()

Returns a new Collection of the same type containing all entries except the last.

butLast(): this

skip()

Returns a new Collection of the same type which excludes the first amount entries from this Collection.

skip(amount: number): this

skipLast()

Returns a new Collection of the same type which excludes the last amount entries from this Collection.

skipLast(amount: number): this

skipWhile()

Returns a new Collection of the same type which includes entries starting from when predicate first returns false.

skipWhile(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): this

Discussion

const { List } = require('immutable') List([ 'dog', 'frog', 'cat', 'hat', 'god' ]) .skipWhile(x => x.match(/g/)) // List [ "cat", "hat", "god" ]run it

skipUntil()

Returns a new Collection of the same type which includes entries starting from when predicate first returns true.

skipUntil(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): this

Discussion

const { List } = require('immutable') List([ 'dog', 'frog', 'cat', 'hat', 'god' ]) .skipUntil(x => x.match(/hat/)) // List [ "hat", "god" ]run it

take()

Returns a new Collection of the same type which includes the first amount entries from this Collection.

take(amount: number): this

takeLast()

Returns a new Collection of the same type which includes the last amount entries from this Collection.

takeLast(amount: number): this

takeWhile()

Returns a new Collection of the same type which includes entries from this Collection as long as the predicate returns true.

takeWhile(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): this

Discussion

const { List } = require('immutable') List([ 'dog', 'frog', 'cat', 'hat', 'god' ]) .takeWhile(x => x.match(/o/)) // List [ "dog", "frog" ]run it

takeUntil()

Returns a new Collection of the same type which includes entries from this Collection as long as the predicate returns false.

takeUntil(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): this

Discussion

const { List } = require('immutable') List([ 'dog', 'frog', 'cat', 'hat', 'god' ]) .takeUntil(x => x.match(/at/)) // List [ "dog", "frog" ]run it

Combination

concat()

Returns a new Collection of the same type with other values and collection-like concatenated to this one.

concat(...valuesOrCollections: Array<unknown>): Collection<unknown, unknown>

Discussion

For Seqs, all entries will be present in the resulting Seq, even if they have the same key.

flatten()

flatten(depth?: number): Collection<unknown, unknown>
flatten(shallow?: boolean): Collection<unknown, unknown>

flatMap()

Flat-maps the Collection, returning a Collection of the same type.

flatMap<M>(mapper: (value: V, key: K, iter: this) => Iterable<M>,
context?: unknown
): Collection<K, M>
flatMap<KM, VM>(mapper: (value: V, key: K, iter: this) => Iterable<[KM, VM]>,
context?: unknown
): Collection<KM, VM>

Discussion

Similar to collection.map(...).flatten(true). Used for Dictionaries only.

Reducing a value

reduce()

reduce<R>(reducer: (reduction: R, value: V, key: K, iter: this) => R,
initialReduction: R,
context?: unknown
): R
reduce<R>(reducer: (reduction: V | R, value: V, key: K, iter: this) => R): R

reduceRight()

reduceRight<R>(reducer: (reduction: R, value: V, key: K, iter: this) => R,
initialReduction: R,
context?: unknown
): R
reduceRight<R>(reducer: (reduction: V | R, value: V, key: K, iter: this) => R
): R

every()

True if predicate returns true for all entries in the Collection.

every(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): boolean

some()

True if predicate returns true for any entry in the Collection.

some(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): boolean

join()

Joins values together as a string, inserting a separator between each. The default separator is ",".

join(separator?: string): string

isEmpty()

Returns true if this Collection includes no values.

isEmpty(): boolean

Discussion

For some lazy Seq, isEmpty might need to iterate to determine emptiness. At most one iteration will occur.

count()

count(): number
count(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): number

countBy()

Returns a Seq.Keyed of counts, grouped by the return value of the grouper function.

countBy<G>(grouper: (value: V, key: K, iter: this) => G,
context?: unknown
): Map<G, number>

Discussion

Note: This is not a lazy operation.

Search for value

find()

Returns the first value for which the predicate returns true.

find(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown,
notSetValue?: V
): V | undefined

findLast()

Returns the last value for which the predicate returns true.

findLast(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown,
notSetValue?: V
): V | undefined

Discussion

Note: predicate will be called for each entry in reverse.

findEntry()

Returns the first [key, value] entry for which the predicate returns true.

findEntry(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown,
notSetValue?: V
): [K, V] | undefined

findLastEntry()

Returns the last [key, value] entry for which the predicate returns true.

findLastEntry(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown,
notSetValue?: V
): [K, V] | undefined

Discussion

Note: predicate will be called for each entry in reverse.

findKey()

Returns the key for which the predicate returns true.

findKey(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): K | undefined

findLastKey()

Returns the last key for which the predicate returns true.

findLastKey(predicate: (value: V, key: K, iter: this) => boolean,
context?: unknown
): K | undefined

Discussion

Note: predicate will be called for each entry in reverse.

keyOf()

Returns the key associated with the search value, or undefined.

keyOf(searchValue: V): K | undefined

lastKeyOf()

Returns the last key associated with the search value, or undefined.

lastKeyOf(searchValue: V): K | undefined

max()

Returns the maximum value in this collection. If any values are comparatively equivalent, the first one found will be returned.

max(comparator?: Comparator<V>): V | undefined

Discussion

The comparator is used in the same way as Collection#sort. If it is not provided, the default comparator is >.

When two values are considered equivalent, the first encountered will be returned. Otherwise, max will operate independent of the order of input as long as the comparator is commutative. The default comparator > is commutative only when types do not differ.

If comparator returns 0 and either value is NaN, undefined, or null, that value will be returned.

maxBy()

Like max, but also accepts a comparatorValueMapper which allows for comparing by more sophisticated means:

maxBy<C>(comparatorValueMapper: (value: V, key: K, iter: this) => C,
comparator?: Comparator<C>
): V | undefined

Discussion

const { List, } = require('immutable'); const l = List([ { name: 'Bob', avgHit: 1 }, { name: 'Max', avgHit: 3 }, { name: 'Lili', avgHit: 2 } , ]); l.maxBy(i => i.avgHit); // will output { name: 'Max', avgHit: 3 }run it

min()

Returns the minimum value in this collection. If any values are comparatively equivalent, the first one found will be returned.

min(comparator?: Comparator<V>): V | undefined

Discussion

The comparator is used in the same way as Collection#sort. If it is not provided, the default comparator is <.

When two values are considered equivalent, the first encountered will be returned. Otherwise, min will operate independent of the order of input as long as the comparator is commutative. The default comparator < is commutative only when types do not differ.

If comparator returns 0 and either value is NaN, undefined, or null, that value will be returned.

minBy()

Like min, but also accepts a comparatorValueMapper which allows for comparing by more sophisticated means:

minBy<C>(comparatorValueMapper: (value: V, key: K, iter: this) => C,
comparator?: Comparator<C>
): V | undefined

Discussion

const { List, } = require('immutable'); const l = List([ { name: 'Bob', avgHit: 1 }, { name: 'Max', avgHit: 3 }, { name: 'Lili', avgHit: 2 } , ]); l.minBy(i => i.avgHit); // will output { name: 'Bob', avgHit: 1 }run it

Comparison

isSubset()

True if iter includes every value in this Collection.

isSubset(iter: Iterable<V>): boolean

isSuperset()

True if this Collection includes every value in iter.

isSuperset(iter: Iterable<V>): boolean

This documentation is generated from immutable.d.ts. Pull requests and Issues welcome.