githubmirror/immutable-js

website/docs/Map.mdx
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Julien Deniau
Doc as mdx instead of export from .d.ts file (#2104)
24 май 2025, 00:47
24 май 2025, 00:4721de701
import Repl from '@/repl/Repl.tsx';
import CodeLink from '@/mdx-components/CodeLink.tsx';

# Map

Immutable Map is an unordered Collection.Keyed of (key, value) pairs with `O(log32 N)` gets and `O(log32 N)` persistent sets.

<Signature code={`type Map<K, V> extends Collection.Keyed<K, V>`} />

Iteration order of a Map is undefined, however is stable. Multiple iterations of the same Map will iterate in the same order.

Map's keys can be of any type, and use `Immutable.is` to determine key equality. This allows the use of any value (including NaN) as a key.

Because `Immutable.is` returns equality based on value semantics, and Immutable collections are treated as values, any Immutable collection may be used as a key.

<Repl
  imports={['Map', 'List']}
  defaultValue={`Map().set(List([ 1 ]), 'listofone').get(List([ 1 ]))`}
/>

Any JavaScript object may be used as a key, however strict identity is used to evaluate key equality. Two similar looking objects will represent two different keys.

Implemented by a hash-array mapped trie.

## Construction

<MemberLabel label="Map()" />

Create a new Immutable Map.

<Signature
  code={`Map<K, V>(collection?: Iterable<[K, V]>): Map<K, V>
Map<K, V>(obj: { [key: PropertyKey]: V }): Map<PropertyKey, V>
`}
/>

Created with the same key value pairs as the provided Collection.Keyed or JavaScript Object or expects a Collection of [K, V] tuple entries.

Note: `Map` is a factory function and not a class, and does not use the `new` keyword during construction.

<Repl defaultValue={`Map({ key: "value" })`} />

<Repl defaultValue={`Map([ [ "key", "value" ] ])`} />

Keep in mind, when using JS objects to construct Immutable Maps, that JavaScript Object properties are always strings, even if written in a quote-less shorthand, while Immutable Maps accept keys of any type.

```js
let obj = { 1: 'one' };
Object.keys(obj); // [ "1" ]
assert.equal(obj['1'], obj[1]); // "one" === "one"

let map = Map(obj);
assert.notEqual(map.get('1'), map.get(1)); // "one" !== undefined
```

Property access for JavaScript Objects first converts the key to a string, but since Immutable Map keys can be of any type the argument to `get()` is not altered.

## Static Methods

<MemberLabel label="Map.isMap()" />

True if the provided value is a Map.

<Signature
  code={`Map.isMap(maybeMap: unknown): maybeMap is Map<unknown, unknown>`}
/>

## Members

<MemberLabel label="size" />

The number of entries in this Map.

<Signature code={`size: number`} />

## Persistent changes

<MemberLabel label="set()" />

Returns a new Map also containing the new key, value pair. If an equivalent key already exists in this Map, it will be replaced.

<Signature code={`set(key: K, value: V): Map<K, V>`} />

<Repl defaultValue={`const originalMap = Map()
const newerMap = originalMap.set('key', 'value')
const newestMap = newerMap.set('key', 'newer value')

originalMap
// Map {}
newerMap
// Map { "key": "value" }
newestMap
// Map { "key": "newer value" }`} />

Note: `set` can be used in `withMutations`.

<MemberLabel label="delete()" alias="remove()" />

Returns a new Map which excludes this `key`.

<Signature code={`delete(key: K): Map<K, V>`} />

Note: `delete` cannot be safely used in IE8, but is provided to mirror the ES6 collection API.

<Repl
  defaultValue={`const originalMap = Map({
  key: 'value',
  otherKey: 'other value'
})
// Map { "key": "value", "otherKey": "other value" }
originalMap.delete('otherKey')`}
/>

Note: `delete` can be used in `withMutations`.

<MemberLabel label="deleteAll()" alias="removeAll()" />

Returns a new Map which excludes the provided `keys`.

<Signature code={`deleteAll(keys: Iterable<K>): this`} />

<Repl
  defaultValue={`const names = Map({ a: "Aaron", b: "Barry", c: "Connor" })
names.deleteAll([ 'a', 'c' ])`}
/>

Note: `deleteAll` can be used in `withMutations`.

<MemberLabel label="clear()" />

Returns a new Map containing no keys or values.

<Signature code={`clear(): Map<K, V>`} />

<Repl defaultValue={`Map({ key: 'value' }).clear()`} />

Note: `clear` can be used in `withMutations`.

<MemberLabel label="update()" />

Returns a new Map having updated the value at this `key` with the return value of calling `updater` with the existing value.

<Signature
  code={`update(key: K, notSetValue: V, updater: (value: V) => V): Map<K, V>
update(key: K, updater: (value: V | undefined) => V | undefined): Map<K, V>
update<R>(updater: (value: this) => R): R`}
/>

Similar to: `map.set(key, updater(map.get(key)))`.

<Repl
  defaultValue={`const aMap = Map({ key: 'value' })
const newMap = aMap.update('key', value => value + value)`}
/>

This is most commonly used to call methods on collections within a structure of data. For example, in order to `.push()` onto a nested `List`, `update` and `push` can be used together:

<Repl
  defaultValue={`const aMap = Map({ nestedList: List([ 1, 2, 3 ]) })
const newMap = aMap.update('nestedList', list => list.push(4))`}
/>

When a `notSetValue` is provided, it is provided to the `updater` function when the value at the key does not exist in the Map.

<Repl
  defaultValue={`const aMap = Map({ key: 'value' })
const newMap = aMap.update('noKey', 'no value', value => value + value)`}
/>

However, if the `updater` function returns the same value it was called with, then no change will occur. This is still true if `notSetValue` is provided.

<Repl
  defaultValue={`const aMap = Map({ apples: 10 })
const newMap = aMap.update('oranges', 0, val => val)
// Map { "apples": 10 }
assert.strictEqual(newMap, aMap);`}
/>

For code using ES2015 or later, using `notSetValue` is discouraged in favor of function parameter default values. This helps to avoid any potential confusion with identify functions as described above.

The previous example behaves differently when written with default values:

<Repl
  defaultValue={`const aMap = Map({ apples: 10 })
const newMap = aMap.update('oranges', (val = 0) => val)`}
/>

If no key is provided, then the `updater` function return value is returned as well.

<Repl
  defaultValue={`const aMap = Map({ key: 'value' })
const result = aMap.update(aMap => aMap.get('key'))
// "value"`}
/>

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".

For example, to sum the values in a Map:

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

Map({ x: 1, y: 2, z: 3 })
.map(x => x + 1)
.filter(x => x % 2 === 0)
.update(sum)`} />

Note: `update(key)` can be used in `withMutations`.

<MemberLabel label="merge()" alias="concat()" />

Returns a new Map resulting from merging the provided Collections (or JS objects) into this Map. In other words, this takes each entry of each collection and sets it on this Map.

<Signature code={`merge(...collections): Map<K, V>`} />

Note: Values provided to `merge` are shallowly converted before being merged. No nested values are altered.

<Repl
  defaultValue={`const one = Map({ a: 10, b: 20, c: 30 })
const two = Map({ b: 40, a: 50, d: 60 })
one.merge(two) // Map { "a": 50, "b": 40, "c": 30, "d": 60 }
two.merge(one) // Map { "b": 20, "a": 10, "d": 60, "c": 30 }`}
/>

Note: `merge` can be used in `withMutations`.

<MemberLabel label="mergeWith()" />

Like `merge()`, `mergeWith()` returns a new Map resulting from merging the provided Collections (or JS objects) into this Map, but uses the `merger` function for dealing with conflicts.

<Signature
  code={`mergeWith(merger: (oldVal, newVal, key) => unknown, ...collections): Map<K, V>`}
/>

<Repl
  defaultValue={`const one = Map({ a: 10, b: 20, c: 30 })
const two = Map({ b: 40, a: 50, d: 60 })
one.mergeWith((oldVal, newVal) => oldVal / newVal, two)
// { "a": 0.2, "b": 0.5, "c": 30, "d": 60 }
two.mergeWith((oldVal, newVal) => oldVal / newVal, one)
// { "b": 2, "a": 5, "d": 60, "c": 30 }`}
/>

Note: `mergeWith` can be used in `withMutations`.

<MemberLabel label="mergeDeep()" />

Like `merge()`, but when two compatible collections are encountered with the same key, it merges them as well, recursing deeply through the nested data. Two collections are considered to be compatible (and thus will be merged together) if they both fall into one of three categories: keyed (e.g., `Map`s, `Record`s, and objects), indexed (e.g., `List`s and arrays), or set-like (e.g., `Set`s). If they fall into separate categories, `mergeDeep` will replace the existing collection with the collection being merged in. This behavior can be customized by using `mergeDeepWith()`.

<Signature code={`mergeDeep(...collections): Map<K, V>`} />

Note: Indexed and set-like collections are merged using `concat()`/`union()` and therefore do not recurse.

<Repl
  defaultValue={`const one = Map({ a: Map({ x: 10, y: 10 }), b: Map({ x: 20, y: 50 }) })
const two = Map({ a: Map({ x: 2 }), b: Map({ y: 5 }), c: Map({ z: 3 }) })
one.mergeDeep(two)`}
/>

Note: `mergeDeep` can be used in `withMutations`.

<MemberLabel label="mergeDeepWith()" />

Like `mergeDeep()`, but when two non-collections or incompatible collections are encountered at the same key, it uses the `merger` function to determine the resulting value. Collections are considered incompatible if they fall into separate categories between keyed, indexed, and set-like.

<Signature
  code={`mergeDeepWith(merger: (oldVal, newVal, key) => unknown, ...collections): Map<K, V>`}
/>

<Repl
  defaultValue={`const one = Map({ a: Map({ x: 10, y: 10 }), b: Map({ x: 20, y: 50 }) })
const two = Map({ a: Map({ x: 2 }), b: Map({ y: 5 }), c: Map({ z: 3 }) })
one.mergeDeepWith((oldVal, newVal) => oldVal / newVal, two)`}
/>

Note: `mergeDeepWith` can be used in `withMutations`.

## Deep persistent changes

<MemberLabel label="setIn()" />

Returns a new Map having set `value` at this `keyPath`. If any keys in `keyPath` do not exist, a new immutable Map will be created at that key.

<Signature
  code={`setIn(keyPath: Iterable<unknown>, value: unknown): Map<K, V>`}
/>

<Repl defaultValue={`const originalMap = Map({
  subObject: Map({
    subKey: 'subvalue',
    subSubObject: Map({
      subSubKey: 'subSubValue'
    })
  })
})

originalMap.setIn(
['subObject', 'subSubObject', 'subSubKey'],
'ha ha ha!'
)`} />

Plain JavaScript Object or Arrays may be nested within an Immutable.js Collection, and `setIn()` can update those values as well, treating them immutably by creating new copies of those values with the changes applied.

<Repl defaultValue={`const originalMap = Map({
  subObject: {
    subKey: 'subvalue',
    subSubObject: {
      subSubKey: 'subSubValue'
    }
  }
})

originalMap.setIn(['subObject', 'subKey'], 'ha ha!')`} />

If any key in the path exists but cannot be updated (such as a primitive like number or a custom Object like Date), an error will be thrown.

Note: `setIn` can be used in `withMutations`.

<MemberLabel label="deleteIn()" alias="removeIn()" />

Returns a new Map having removed the value at this `keyPath`. If any keys in `keyPath` do not exist, no change will occur.

<Signature code={`deleteIn(keyPath: Iterable<unknown>): Map<K, V>`} />

Note: `deleteIn` can be used in `withMutations`.

<MemberLabel label="updateIn()" />

Returns a new Map having applied the `updater` to the entry found at the keyPath.

<Signature
  code={`updateIn(keyPath: Iterable<unknown>, notSetValue: unknown, updater: (value) => unknown): Map<K, V>`}
/>

This is most commonly used to call methods on collections nested within a structure of data. For example, in order to `.push()` onto a nested `List`, `updateIn` and `push` can be used together:

<Repl
  defaultValue={`const map = Map({ inMap: Map({ inList: List([ 1, 2, 3 ]) }) })
const newMap = map.updateIn(['inMap', 'inList'], list => list.push(4))`}
/>

If any keys in `keyPath` do not exist, new Immutable `Map`s will be created at those keys. If the `keyPath` does not already contain a value, the `updater` function will be called with `notSetValue`, if provided, otherwise `undefined`.

<Repl
  defaultValue={`const map = Map({ a: Map({ b: Map({ c: 10 }) }) })
const newMap = map.updateIn(['a', 'b', 'c'], val => val * 2)`}
/>

If the `updater` function returns the same value it was called with, then no change will occur. This is still true if `notSetValue` is provided.

<Repl
  defaultValue={`const map = Map({ a: Map({ b: Map({ c: 10 }) }) })
const newMap = map.updateIn(['a', 'b', 'x'], 100, val => val)
// Map { "a": Map { "b": Map { "c": 10 } } }
assert.strictEqual(newMap, map)`}
/>

For code using ES2015 or later, using `notSetValue` is discouraged in favor of function parameter default values. This helps to avoid any potential confusion with identify functions as described above.

The previous example behaves differently when written with default values:

<Repl
  defaultValue={`const map = Map({ a: Map({ b: Map({ c: 10 }) }) })
const newMap = map.updateIn(['a', 'b', 'x'], (val = 100) => val)`}
/>

Plain JavaScript Object or Arrays may be nested within an Immutable.js Collection, and updateIn() can update those values as well, treating them immutably by creating new copies of those values with the changes applied.

<Repl
  defaultValue={`const map = Map({ a: { b: { c: 10 } } })
const newMap = map.updateIn(['a', 'b', 'c'], val => val * 2)`}
/>

If any key in the path exists but cannot be updated (such as a primitive like number or a custom Object like Date), an error will be thrown.

Note: `updateIn` can be used in `withMutations`.

<MemberLabel label="mergeIn()" />

A combination of `updateIn` and `merge`, returning a new Map, but performing the merge at a point arrived at by following the keyPath. In other words, these two lines are equivalent:

```js
map.updateIn(['a', 'b', 'c'], (abc) => abc.merge(y));
map.mergeIn(['a', 'b', 'c'], y);
```

<Signature
  code={`mergeIn(keyPath: Iterable<unknown>, ...collections: Array<unknown>): Map<K, V>`}
/>

Note: `mergeIn` can be used in `withMutations`.

<MemberLabel label="mergeDeepIn()" />

A combination of `updateIn` and `mergeDeep`, returning a new Map, but performing the deep merge at a point arrived at by following the keyPath. In other words, these two lines are equivalent:

```js
map.updateIn(['a', 'b', 'c'], (abc) => abc.mergeDeep(y));
map.mergeDeepIn(['a', 'b', 'c'], y);
```

<Signature
  code={`mergeDeepIn(keyPath: Iterable<unknown>, ...collections: Array<unknown>): Map<K, V>`}
/>

Note: `mergeDeepIn` can be used in `withMutations`.

## Transient changes

<MemberLabel label="withMutations()" />

Every time you call one of the above functions, a new immutable Map is created. If a pure function calls a number of these to produce a final return value, then a penalty on performance and memory has been paid by creating all of the intermediate immutable Maps.

If you need to apply a series of mutations to produce a new immutable Map, `withMutations()` creates a temporary mutable copy of the Map which can apply mutations in a highly performant manner. In fact, this is exactly how complex mutations like `merge` are done.

<Signature
  code={`withMutations(mutator: (mutable: this) => unknown): Map<K, V>`}
/>

As an example, this results in the creation of 2, not 4, new Maps:

<Repl
  defaultValue={`const map1 = Map()
const map2 = map1.withMutations(map => {
  map.set('a', 1).set('b', 2).set('c', 3)
})
assert.equal(map1.size, 0)
assert.equal(map2.size, 3)`}
/>

Note: Not all methods can be used on a mutable collection or within `withMutations`! Read the documentation for each method to see if it is safe to use in `withMutations`.

<MemberLabel label="asMutable()" />

Another way to avoid creation of intermediate Immutable maps is to create a mutable copy of this collection. Mutable copies _always_ return `this`, and thus shouldn't be used for equality. Your function should never return a mutable copy of a collection, only use it internally to create a new collection.

<Signature code={`asMutable(): this`} />

If possible, use `withMutations` to work with temporary mutable copies as it provides an easier to use API and considers many common optimizations.

Note: if the collection is already mutable, `asMutable` returns itself.

Note: Not all methods can be used on a mutable collection or within `withMutations`! Read the documentation for each method to see if it allows being used in `withMutations`.

<MemberLabel label="wasAltered()" />

Returns true if this is a mutable copy (see `asMutable()`) and mutative alterations have been applied.

<Signature code={`wasAltered(): boolean`} />

<MemberLabel label="asImmutable()" />

The yin to `asMutable`'s yang. Because it applies to mutable collections, this operation is _mutable_ and may return itself (though may not return itself, i.e. if the result is an empty collection). Once performed, the original mutable copy must no longer be mutated since it may be the immutable result.

<Signature code={`asImmutable(): this`} />

If possible, use `withMutations` to work with temporary mutable copies as it provides an easier to use API and considers many common optimizations.

## Sequence algorithms

<MemberLabel label="map()" />

Returns a new Map with values passed through a `mapper` function.

<Signature
  code={`map<M>(mapper: (value: V, key: K, iter: this) => M, context?: unknown): Map<K, M>`}
/>

<Repl defaultValue={`Map({ a: 1, b: 2 }).map(x => 10 * x)`} />

<MemberLabel label="mapKeys()" />

Returns a new Map with keys passed through a `mapper` function.

<Signature
  code={`mapKeys<M>(mapper: (key: K, value: V, iter: this) => M, context?: unknown): Map<M, V>`}
/>

<Repl defaultValue={`Map({ a: 1, b: 2 }).mapKeys(x => x.toUpperCase())`} />

<MemberLabel label="mapEntries()" />

Returns a new Map with entries ([key, value] tuples) passed through a `mapper` function.

<Signature
  code={`mapEntries<KM, VM>(mapper: (entry: [K, V], index: number, iter: this) => [KM, VM] | undefined, context?: unknown): Map<KM, VM>`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2 })
  .mapEntries(([ k, v ]) => [ k.toUpperCase(), v * 2 ])`}
/>

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

If the mapper function returns `undefined`, then the entry will be filtered.

<MemberLabel label="flatMap()" />

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

Similar to `map(...).flatten(true)`.

<Signature
  code={`flatMap<KM, VM>(mapper: (value: V, key: K, iter: this) => Iterable<[KM, VM]>, context?: unknown): Map<KM, VM>`}
/>

<MemberLabel label="filter()" />

Returns a new Map with only the entries for which the `predicate` function returns true.

<Signature
  code={`filter(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): Map<K, V>`}
/>

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

<MemberLabel label="filterNot()" />

Returns a new Map with only the entries for which the `predicate` function returns false.

<Signature
  code={`filterNot(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): this`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4}).filterNot(x => x % 2 === 0)`}
/>

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

<MemberLabel label="reverse()" />

Returns a new Map with the order of the entries reversed.

<Signature code={`reverse(): this`} />

<MemberLabel label="partition()" />

Returns a new Map with the entries partitioned into two Maps based on the `predicate` function.

<Signature
  code={`partition(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): [Map<K, V>, Map<K, V>]`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4}).partition(x => x % 2 === 0)`}
/>

<MemberLabel label="flip()" />

Returns a new Map with the keys and values flipped.

<Signature code={`flip(): Map<V, K>`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4}).flip()`} />

<MemberLabel label="sort()" />

Returns an OrderedMap of the same type which includes the same entries, stably sorted by using a `comparator`.

<Signature code={`sort(comparator?: Comparator<V>): this & OrderedMap<K, V>`} />

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.

<Repl
  defaultValue={`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; }
});`}
/>

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

Note: This is always an eager operation.

<MemberLabel label="sortBy()" />

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

<Signature
  code={`sortBy<C>(
    comparatorValueMapper: (value: V, key: K, iter: this) => C,
    comparator?: (valueA: C, valueB: C) => number
): OrderedMap<K, V>`}
/>

<Repl
  defaultValue={`const beattles = Map({
    John: { name: "Lennon" },
    Paul: { name: "McCartney" },
    George: { name: "Harrison" },
    Ringo: { name: "Starr" },
});
beattles.sortBy(member => member.name);`}
/>

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

Note: This is always an eager operation.

<MemberLabel label="groupBy()" />

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

<Signature
  code={`groupBy<G>(
    grouper: (value: V, key: K, iter: this) => G,
    context?: unknown
  ): Map<G, this>`}
/>

Note: This is always an eager operation.

<Repl
  defaultValue={`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'))`}
/>

## Conversion to JavaScript types

<MemberLabel label="toJS()" />

Deeply converts this Keyed collection to equivalent native JavaScript Object.

<Signature
  code={`toJS(): Array<DeepCopy<V>> | { [key in PropertyKey]: DeepCopy<V> }`}
/>

Converts keys to Strings.

<MemberLabel label="toJSON()" />

Shallowly converts this Keyed collection to equivalent native JavaScript Object.

<Signature code={`toJSON(): Array<V> | { [key in PropertyKey]: V }`} />

Converts keys to Strings.

<MemberLabel label="toArray()" />

Shallowly converts this collection to an Array.

<Signature code={`toArray(): Array<V> | Array<[K, V]>`} />

<MemberLabel label="toObject()" />

Shallowly converts this Collection to an Object.

<Signature code={`toObject(): { [key: string]: V }`} />

Converts keys to Strings.

## Conversion to Seq

<MemberLabel label="toSeq()" />

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

<Signature code={`toSeq(): Seq<K, V>`} />

<MemberLabel label="toKeyedSeq()" />

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

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

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

<Signature code={`toKeyedSeq(): Seq.Keyed<K, V>`} />

<MemberLabel label="toIndexedSeq()" />

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

<Signature code={`toIndexedSeq(): Seq.Indexed<V>`} />

<MemberLabel label="toSetSeq()" />

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

<Signature code={`toSetSeq(): Seq.Set<V>`} />

## Value equality

<MemberLabel label="equals()" />

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

<Signature code={`equals(other): boolean`} />

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

<MemberLabel label="hashCode()" />

Computes and returns the hashed identity for this Collection.

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.

<Signature code={`hashCode(): number`} />

<Repl
  defaultValue={`const a = Map({ a: 1, b: 2, c: 3 });
const b = Map({ a: 1, b: 2, c: 3 });
assert.notStrictEqual(a, b); // different instances
const set = Set([ a ]);
assert.equal(set.has(b), true);`}
/>

If two values have the same `hashCode`, they are [not guaranteed to be equal][Hash Collision]. If two values have different `hashCode`s, they must not be equal.

[Hash Collision]: https://en.wikipedia.org/wiki/Collision_(computer_science)

## Reading values

<MemberLabel label="get()" />

Returns the value associated with the provided key.

<Signature code={`get(key: K): V | undefined`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3 }).get('b');`} />

<MemberLabel label="has()" />

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

<Signature code={`has(key: K): boolean`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3 }).has('b');`} />

<MemberLabel label="includes()" alias="contains()" />

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

<Signature code={`includes(value: V): boolean`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3 }).includes(2);`} />

<MemberLabel label="first()" />

Returns the first value in this collection.

<Signature code={`first(): V | undefined`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3 }).first();`} />

<MemberLabel label="last()" />

Returns the last value in this collection.

<Signature code={`last(): V | undefined`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3 }).last();`} />

## Reading deep values

<MemberLabel label="getIn()" />

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

<Signature
  code={`getIn(searchKeyPath: Iterable<unknown>, notSetValue?: unknown): unknown`}
/>

<Repl
  defaultValue={`const deepData = Map({ x: Map({ y: 123 }) });
deepData.getIn(['x', 'y']) // 123`}
/>

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

<Repl
  defaultValue={`const deepData = Map({ x: { y: 123 } });
deepData.getIn(['x', 'y']) // 123`}
/>

<MemberLabel label="hasIn()" />

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

<Signature code={`hasIn(searchKeyPath: Iterable<unknown>): boolean`} />

## Conversion to Collections

<MemberLabel label="toMap()" />

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

<Signature code={`toMap(): Map<K, V>`} />

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

<MemberLabel label="toOrderedMap()" />

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

<Signature code={`toOrderedMap(): OrderedMap<K, V>`} />

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

<MemberLabel label="toSet()" />

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

<Signature code={`toSet(): Set<V>`} />

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

<MemberLabel label="toOrderedSet()" />

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

<Signature code={`toOrderedSet(): OrderedSet<V>`} />

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

<MemberLabel label="toList()" />

Converts this Collection to a List, discarding keys.

<Signature code={`toList(): List<V>`} />

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.

<Repl
  defaultValue={`var myMap = Map({ a: 'Apple', b: 'Banana' })
export const listConstructor = List(myMap) // List [ [ "a", "Apple" ], [ "b", "Banana" ] ]
myMap.toList() // List [ "Apple", "Banana" ]`}
/>

<MemberLabel label="toStack()" />

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

<Signature code={`toStack(): Stack<V>`} />

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

## Iterators

<MemberLabel label="keys()" />

An iterator of this `Collection`'s keys.

<Signature code={`keys(): IterableIterator<K>`} />

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

<MemberLabel label="values()" />

An iterator of this `Collection`'s values.

<Signature code={`values(): IterableIterator<V>`} />

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

<MemberLabel label="entries()" />

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

<Signature code={`entries(): IterableIterator<[K, V]>`} />

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

## Collections (Seq)

<MemberLabel label="keySeq()" />

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

<Signature code={`keySeq(): Seq.Indexed<K>`} />

<MemberLabel label="valueSeq()" />

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

<Signature code={`valueSeq(): Seq.Indexed<V>`} />

<MemberLabel label="entrySeq()" />

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

<Signature code={`entrySeq(): Seq.Indexed<[K, V]>`} />

## Side effects

<MemberLabel label="forEach()" />

The `sideEffect` is executed for every entry in the Collection.

<Signature
  code={`forEach(
    sideEffect: (value: V, key: K, iter: this) => unknown,
    context?: unknown
  ): number`}
/>

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

<MemberLabel label="slice()" />

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

<Signature code={`slice(begin?: number, end?: number): Map<K, V>`} />

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.

<MemberLabel label="rest()" />

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

<Signature code={`rest(): Map<K, V>`} />

<MemberLabel label="butLast()" />

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

<Signature code={`butLast(): Map<K, V>`} />

<MemberLabel label="skip()" />

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

<Signature code={`skip(amount: number): Map<K, V>`} />

<MemberLabel label="skipLast()" />

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

<Signature code={`skipLast(amount: number): Map<K, V>`} />

<MemberLabel label="skipWhile()" />

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

<Signature
  code={`skipWhile(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): Map<K, V>`}
/>

<Repl
  defaultValue={`Map({ a: 'dog', b: 'frog', c: 'cat', d: 'hat', e: 'god' })
  .skipWhile(x => x.match(/g/))`}
/>

<MemberLabel label="skipUntil()" />

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

<Signature
  code={`skipUntil(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): Map<K, V>`}
/>

<Repl
  defaultValue={`Map({ a: 'dog', b: 'frog', c: 'cat', d: 'hat', e: 'god' })
  .skipUntil(x => x.match(/hat/))`}
/>

<MemberLabel label="take()" />

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

<Signature code={`take(amount: number): Map<K, V>`} />

<MemberLabel label="takeLast()" />

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

<Signature code={`takeLast(amount: number): Map<K, V>`} />

<MemberLabel label="takeWhile()" />

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

<Signature
  code={`takeWhile(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): Map<K, V>`}
/>

<Repl
  defaultValue={`Map({ a: 'dog', b: 'frog', c: 'cat', d: 'hat', e: 'god' })
  .takeWhile(x => x.match(/o/))`}
/>

<MemberLabel label="takeUntil()" />

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

<Signature
  code={`takeUntil(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): Map<K, V>`}
/>

<Repl
  defaultValue={`Map({ a: 'dog', b: 'frog', c: 'cat', d: 'hat', e: 'god' })
  .takeUntil(x => x.match(/at/))`}
/>

## Combination

<MemberLabel label="flatten()" />

Returns a new flattened Map, optionally only flattening to a particular depth.

<Signature
  code={`flatten(depth?: number): Map<any, any>
flatten(shallow?: boolean): Map<any, any>`}
/>

<Repl
  defaultValue={`Map({ a: Map({ x: 1, y: 2 }), b: Map({ x: 3, y: 4 }) }).flatten()`}
/>

## Reducing a value

<MemberLabel label="reduce()" />

Reduces the Collection to a value by calling the `reducer` for every entry in the Collection and passing along the reduced value.

<Signature
  code={`reduce<R>(
    reducer: (reduction: R, value: V, key: K, iter: this) => R,
    initialReduction?: R,
    context?: unknown
  ): R`}
/>

If `initialReduction` is not provided, the first item in the Collection will be used.

<MemberLabel label="reduceRight()" />

Reduces the Collection in reverse (from the right side).

<Signature
  code={`reduceRight<R>(
    reducer: (reduction: R, value: V, key: K, iter: this) => R,
    initialReduction?: R,
    context?: unknown
  ): R`}
/>

Note: Similar to `this.reverse().reduce()`, and provided for parity with `Array#reduceRight`.

<MemberLabel label="every()" />

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

<Signature
  code={`every(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): boolean`}
/>

<MemberLabel label="some()" />

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

<Signature
  code={`some(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): boolean`}
/>

<MemberLabel label="join()" />

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

<Signature code={`join(separator?: string = ','): string`} />

<MemberLabel label="isEmpty()" />

Returns true if this Collection includes no values.

<Signature code={`isEmpty(): boolean`} />

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

<MemberLabel label="count()" />

Returns the size of this Collection.

<Signature
  code={`count(): number
count(predicate: (value: V, key: K, iter: this) => boolean, context?: unknown): number`}
/>

Regardless of if this Collection can describe its size lazily (some Seqs cannot), this method will always return the correct size. E.g. it evaluates a lazy `Seq` if necessary.

If `predicate` is provided, then this returns the count of entries in the Collection for which the `predicate` returns true.

<MemberLabel label="countBy()" />

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

<Signature
  code={`countBy<G>(
    grouper: (value: V, key: K, iter: this) => G,
    context?: unknown
  ): Map<G, number>`}
/>

Note: This is not a lazy operation.

## Search for value

<MemberLabel label="find()" />

Returns the first value for which the `predicate` function returns true.

<Signature
  code={`find(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): V | undefined`}
/>

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).find(x => x % 2 === 0)`} />

<MemberLabel label="findLast()" />

Returns the last value for which the `predicate` function returns true.

<Signature
  code={`findLast(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): V | undefined`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, b: 2, d: 4 }).findLast(x => x % 2 === 0)`}
/>

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

<MemberLabel label="findEntry()" />

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

<Signature
  code={`findEntry(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): [K, V] | undefined`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).findEntry(x => x % 2 === 0)`}
/>

<MemberLabel label="findLastEntry()" />

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

<Signature
  code={`findLastEntry(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): [K, V] | undefined`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, b: 2, d: 4 }).findLastEntry(x => x % 2 === 0)`}
/>

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

<MemberLabel label="findKey()" />

Returns the first key for which the `predicate` function returns true.

<Signature
  code={`findKey(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): K | undefined`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).findKey(x => x % 2 === 0)`}
/>

<MemberLabel label="findLastKey()" />

Returns the last key for which the `predicate` function returns true.

<Signature
  code={`findLastKey(
    predicate: (value: V, key: K, iter: this) => boolean,
    context?: unknown
  ): K | undefined`}
/>

<Repl
  defaultValue={`Map({ a: 1, b: 2, c: 3, b: 2, d: 4 }).findLastKey(x => x % 2 === 0)`}
/>

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

<MemberLabel label="keyOf()" />

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

<Signature code={`keyOf(searchValue: V): K | undefined`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).keyOf(3)`} />

<MemberLabel label="lastKeyOf()" />

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

<Signature code={`lastKeyOf(searchValue: V): K | undefined`} />

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, b: 2, d: 4 }).lastKeyOf(2)`} />

<MemberLabel label="max()" />

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

<Signature
  code={`max(comparator?: (valueA: V, valueB: V) => number): V | undefined`}
/>

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).max()`} />

The comparator is used in the same way as <CodeLink to="../Collection#sort()">Collection#sort</CodeLink>. 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.

<MemberLabel label="maxBy()" />

Like `max()`, but also accepts a `comparatorValueMapper` which allows for comparing by more sophisticated means.

<Signature
  code={`maxBy<C>(
  comparatorValueMapper: (value: V, key: K, iter: this) => C,
  comparator?: (valueA: C, valueB: C) => number
): V | undefined`}
/>

<Repl
  defaultValue={`Map({
  a: { name: 'Bob', age: 30 },
  b: { name: 'Max', age: 25 },
  c: { name: 'Lili', age: 35 }
}).maxBy(person => person.age)`}
/>

<MemberLabel label="min()" />

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

<Signature
  code={`min(comparator?: (valueA: V, valueB: V) => number): V | undefined`}
/>

<Repl defaultValue={`Map({ a: 1, b: 2, c: 3, d: 4 }).min()`} />

The comparator is used in the same way as <CodeLink to="../Collection#sort()">Collection#sort</CodeLink>. 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.

<MemberLabel label="minBy()" />

Like `min()`, but also accepts a `comparatorValueMapper` which allows for comparing by more sophisticated means.

<Signature
  code={`minBy<C>(
    comparatorValueMapper: (value: V, key: K, iter: this) => C,
    comparator?: (valueA: C, valueB: C) => number
  ): V | undefined`}
/>

<Repl
  defaultValue={`Map({
  a: { name: 'Bob', age: 30 },
  b: { name: 'Max', age: 25 },
  c: { name: 'Lili', age: 35 }
}).minBy(person => person.age)`}
/>

## Comparison

<MemberLabel label="isSubset()" />

True if `iter` includes every value in this Collection.

<Signature code={`isSubset(iter: Iterable<V>): boolean`} />

<MemberLabel label="isSuperset()" />

True if this Collection includes every value in `iter`.

<Signature code={`isSuperset(iter: Iterable<V>): boolean`} />