githubmirror/immutable-js

website/docs/Collection.Keyed.mdx
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Julien Deniau
Require to import
27 май 2025, 01:33
27 май 2025, 01:33b72fc3b
import Repl from '@/repl/Repl.tsx';
import CodeLink from '@/mdx-components/CodeLink.tsx';

# Collection.Keyed

Keyed Collections have discrete keys tied to each value.

<Signature code="type Collection.Keyed<K, V> extends Collection<K, V>" />

When iterating `Collection.Keyed`, each iteration will yield a [K, V] tuple, in other words, `Collection#entries` is the default iterator for Keyed Collections.

## Construction

<MemberLabel label="Collection.Keyed()" />

Creates a Collection.Keyed

Similar to `Collection()`, however it expects collection-likes of [K, V]
tuples if not constructed from a Collection.Keyed or JS Object.

Note: `Collection.Keyed` is a conversion function and not a class, and
does not use the `new` keyword during construction.

## Conversion to JavaScript types

<MemberLabel label="toJS()" />

Deeply converts this Keyed collection to equivalent native JavaScript Object.

<Signature code={`toJS(): { [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(): { [key in PropertyKey]: V };`} />
Converts keys to Strings.

<MemberLabel label="toArray()" />

Shallowly converts this collection to an Array of [key, value] pairs.

<Signature code={`toArray(): 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()" />

Returns Seq.Keyed.

<MemberLabel label="toKeyedSeq()" />

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

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

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.

```js
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');
```

<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>;`} />

## Sequence functions

<MemberLabel label="flip()" />

Returns a new Collection.Keyed of the same type where the keys and values have been flipped.

<Signature code="flip(): Collection.Keyed<V, K>;" />

```js
import { Map } from 'immutable';
Map({ a: 'z', b: 'y' }).flip();
// Map { "z": "a", "y": "b" }
```

<MemberLabel label="concat()" />

Returns a new Collection with other collections concatenated to this one.

<Signature
  code={`concat<KC, VC>(...collections: Array<Iterable<[KC, VC]>>): Collection.Keyed<K | KC, V | VC>;`}
/>
<Signature
  code={`concat<C>(...collections: Array<{ [key: string]: C }>): Collection.Keyed<K | string, V | C>;`}
/>

<MemberLabel label="map()" />

Returns a new Collection.Keyed with values passed through a `mapper` function.

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

```js
import { Collection } from 'immutable';
Collection.Keyed({ a: 1, b: 2 }).map((x) => 10 * x);
// Seq { "a": 10, "b": 20 }
```

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

<MemberLabel label="mapKeys()" />

Returns a new Collection.Keyed of the same type with keys passed through a `mapper` function.

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

```js
import { Map } from 'immutable';
Map({ a: 1, b: 2 }).mapKeys((x) => x.toUpperCase());
// Map { "A": 1, "B": 2 }
```

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

<MemberLabel label="mapEntries()" />

Returns a new Collection.Keyed of the same type 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): Collection.Keyed<KM, VM>;`}
/>

```js
import { Map } from 'immutable';
Map({ a: 1, b: 2 }).mapEntries(([k, v]) => [k.toUpperCase(), v * 2]);
// Map { "A": 2, "B": 4 }
```

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 Collection, returning a Collection of the same type.

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

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

<MemberLabel label="filter()" />

Returns a new Collection with only the values for which the `predicate` function returns true.

<Signature
  code={`filter<F extends V>(predicate: (value: V, key: K, iter: this) => value is F, context?: unknown): Collection.Keyed<K, F>;`}
/>
<Signature
  code={`filter(predicate: (value: V, key: K, iter: this) => unknown, context?: unknown): this;`}
/>

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

<MemberLabel label="partition()" />

<Signature
  code={`partition<F extends V, C>(predicate: (this: C, value: V, key: K, iter: this) => value is F, context?: C): [Collection.Keyed<K, V>, Collection.Keyed<K, F>];`}
/>
<Signature
  code={`partition<C>(predicate: (this: C, value: V, key: K, iter: this) => unknown, context?: C): [this, this];`}
/>

Returns a new keyed Collection with the values for which the `predicate` function returns false and another for which it returns true.

<MemberLabel label="[Symbol.iterator]()" />

<Signature code={`[Symbol.iterator](): IterableIterator<[K, V]>;`} />

Yields [key, value] pairs.

## Value equality

<MemberLabel label="equals()" />

Returns 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.

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

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.

```js
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);
```

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, or notSetValue if the Collection does not contain this key.

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

Note: it is possible a key may be associated with an `undefined` value, so if `notSetValue` is not provided and this method returns `undefined`, that does not guarantee the key was not found.

<MemberLabel label="has()" />

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

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

<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;`} />

<MemberLabel label="first()" />

In case the `Collection` is not empty returns the first element of the `Collection`. In case the `Collection` is empty returns the optional default value if provided, if no default value is provided returns undefined.

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

<MemberLabel label="last()" />

In case the `Collection` is not empty returns the last element of the `Collection`. In case the `Collection` is empty returns the optional default value if provided, if no default value is provided returns undefined.

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

## 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: List([Map({ y: 123 })]) });
deepData.getIn(['x', 0, 'y']);`}
/>

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', 0, 'y']);`}
/>

<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;`} />

## Persistent changes

<MemberLabel label="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".

<Signature code={`update<R>(updater: (value: this) => R): R;`} />

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

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

## 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()` ignores the keys and creates a list of just the values, whereas `List(collection)` creates a list of entry tuples.

<Repl
  defaultValue={`const myMap = Map({ a: 'Apple', b: 'Banana' });
List(myMap);`}
/>

<Repl
  defaultValue={`const myMap = Map({ a: 'Apple', b: 'Banana' });
myMap.toList();`}
/>

<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 you want an Immutable.js Seq.

<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 you want an Immutable.js Seq.

<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 you want an Immutable.js Seq.

## 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]>;`} />

## Sequence algorithms

<MemberLabel label="filterNot()" />

Returns a new Collection of the same type 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;`}
/>

```js
import { Map } from 'immutable';
Map({ a: 1, b: 2, c: 3, d: 4 }).filterNot((x) => x % 2 === 0);
// Map { "a": 1, "c": 3 }
```

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

<MemberLabel label="reverse()" />

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

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

<MemberLabel label="sort()" />

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

<Signature code={`sort(comparator?: Comparator<V>): this;`} />

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.

```js
import { Map } from '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 }
```

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): this;`}
/>

<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 }),
]);
listOfMaps.groupBy((x) => x.get('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): this;`} />

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(): this;`} />

<MemberLabel label="butLast()" />

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

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

<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): this;`} />

<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): this;`} />

<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): this;`}
/>

<Repl
  defaultValue={`List([ 'dog', 'frog', 'cat', 'hat', 'god' ])
  .skipWhile(x => x.match(/g/))
// List [ "cat", "hat", "god" ]
`}
/>

<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): this;`}
/>

<Repl
  defaultValue={`List([ 'dog', 'frog', 'cat', 'hat', 'god' ])
  .skipUntil(x => x.match(/hat/))
// List [ "hat", "god" ]
`}
/>

<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): this;`} />

<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): this;`} />

<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): this;`}
/>

```js
import { List } from 'immutable';
List(['dog', 'frog', 'cat', 'hat', 'god']).takeWhile((x) => x.match(/o/));
// List [ "dog", "frog" ]
```

<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): this;`}
/>

```js
import { List } from 'immutable';
List(['dog', 'frog', 'cat', 'hat', 'god']).takeUntil((x) => x.match(/at/));
// List [ "dog", "frog" ]
```

## Combination

<MemberLabel label="flatten()" />

Flattens nested Collections.

<Signature
  code={`flatten(depth?: number): Collection<unknown, unknown>;
flatten(shallow?: boolean): Collection<unknown, unknown>;`}
/>

Will deeply flatten the Collection by default, returning a Collection of the same type, but a `depth` can be provided in the form of a number or boolean (where true means to shallowly flatten one level). A depth of 0 (or shallow: false) will deeply flatten.

Flattens only other Collections, not Arrays or Objects.

Note: `flatten(true)` operates on `Collection<unknown, Collection<K, V>>` and returns `Collection<K, V>`.

## 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.

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

<Signature
  code={`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;`}
/>

<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;
reduceRight<R>(reducer: (reduction: V | R, value: V, key: K, iter: this) => R): R;`}
/>

<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.

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

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

<MemberLabel label="count()" />

Returns the size of this Collection.

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.

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

<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` returns true.

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

<MemberLabel label="findLast()" />

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

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

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

<MemberLabel label="findEntry()" />

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

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

<MemberLabel label="findLastEntry()" />

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

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

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

<MemberLabel label="findKey()" />

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

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

<MemberLabel label="findLastKey()" />

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

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

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

<MemberLabel label="keyOf()" />

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

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

<MemberLabel label="lastKeyOf()" />

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

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

<MemberLabel label="max()" />

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

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.

<Signature code={`max(comparator?: Comparator<V>): V | undefined;`} />

<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?: Comparator<C>): V | undefined;`}
/>

```js
import { List } from '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
```

<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?: Comparator<V>): V | undefined;`} />

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.

<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?: Comparator<C>): V | undefined;`}
/>

```js
import { List } from '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 }
```

## 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;`} />