website/docs/Collection.mdx - main - githubmirror/immutable-js

website/docs/Collection.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';

# 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 <CodeLink to="map" /> and <CodeLink to="filter" />).

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 <CodeLink to="../Map" /> and <CodeLink to="../Set" />.

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

Implementations should extend one of the subclasses, <CodeLink to="../Collection.Keyed" />, <CodeLink to="../Collection.Indexed" />, or <CodeLink to="../Collection.Set" />.

## Construction

<MemberLabel label="Collection()" />

Creates a Collection.

The type of Collection created is based on the input.

- If an `Collection`, that same `Collection`.
- If an Array-like, an `Collection.Indexed`.
- If an Object with an Iterator defined, an `Collection.Indexed`.
- If an Object, an `Collection.Keyed`.

This methods forces the conversion of Objects and Strings to Collections.
If you want to ensure that a Collection of one item is returned, use
`Seq.of`.

Note: An Iterator itself will be treated as an object, becoming a `Seq.Keyed`,
which is usually not what you want. You should turn your Iterator Object into
an iterable object by defining a `Symbol.iterator` (or `@@iterator`) method which
returns `this`.

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

<Signature
  code={`
function Collection<T>(collection: Iterable<T> | ArrayLike<T>): Collection.Indexed<T>;
function Collection<V>(obj: {  [key: string]: V; }): Collection.Keyed<string, V>;
function Collection(): Collection<K, 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.

<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 JavaScript types

<MemberLabel label="toJS()" />

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

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

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

<MemberLabel label="toJSON()" />

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

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

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

<MemberLabel label="toArray()" />

Shallowly converts this collection to an Array.

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

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

<MemberLabel label="toObject()" />

Shallowly converts this Collection to an Object.

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

Converts keys to Strings.

## 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={`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.

## 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.

<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.

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

## 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.

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

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

## 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="map()" />

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

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

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

<MemberLabel label="filter()" />

Returns a new Collection of the same type with only the entries 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<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="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;`}
/>

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

<MemberLabel label="partition()" />

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

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

<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.

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

<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?: Comparator<C>): 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;`}
/>

<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;`}
/>

<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;`}
/>

<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;`}
/>

## Combination

<MemberLabel label="concat()" />

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

<Signature
  code={`concat(...valuesOrCollections: Array<unknown>): Collection<unknown, unknown>;`}
/>

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

<MemberLabel label="flatten()" />

Flattens nested Collections.

<Signature code={`flatten(depth?: number): Collection<unknown, unknown>;`} />
<Signature code={`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 others Collection, not Arrays or Objects.

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

<MemberLabel label="flatMap()" />

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

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

<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;`}
/>
<Signature
  code={`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;`}
/>
<Signature
  code={`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.

<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;`} />
<Signature
  code={`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` 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.

<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.

<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.

<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.

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

<Repl
  defaultValue={`const l = List([
  { name: 'Bob', avgHit: 1 },
  { name: 'Max', avgHit: 3 },
  { name: 'Lili', avgHit: 2 },
]);
l.maxBy((i) => i.avgHit);`}
/>

<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;`}
/>

<Repl
  defaultValue={`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;`} />