Collection

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

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

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

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

Construction

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.

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

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

equals(other): boolean;

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

Computes and returns the hashed identity for this Collection.

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.

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. If two values have different hashCodes, they must not be equal.

Reading values

Returns the value associated with the provided key, or notSetValue if the Collection does not contain this key.

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

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.

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

has(key: K): boolean;

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

includes(value: V): boolean;

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.

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

first(): V | undefined;

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.

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

last(): V | undefined;

Reading deep values

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

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

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

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

hasIn(searchKeyPath: Iterable<unknown>): boolean;

Persistent changes

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

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

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

Conversion to JavaScript types

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

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.

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

toJSON(): Array<V> | { [key in PropertyKey]: V };

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

Shallowly converts this collection to an Array.

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.

Shallowly converts this Collection to an Object.

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

Converts keys to Strings.

Conversion to Collections

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

toMap(): Map<K, V>;

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

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

toOrderedMap(): OrderedMap<K, V>;

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

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

toSet(): Set<V>;

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

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

toOrderedSet(): OrderedSet<V>;

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

Converts this Collection to a List, discarding keys.

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.

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

toStack(): Stack<V>;

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

Conversion to Seq

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

toSeq(): Seq<K, V>;

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

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.

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

toIndexedSeq(): Seq.Indexed<V>;

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

toSetSeq(): Seq.Set<V>;

Iterators

An iterator of this Collection's keys.

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.

An iterator of this Collection's values.

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.

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

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.

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

Collections (Seq)

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

keySeq(): Seq.Indexed<K>;

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

valueSeq(): Seq.Indexed<V>;

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

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

Sequence algorithms

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

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

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

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

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

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.

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

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

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

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

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>];

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

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

reverse(): this;

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

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.

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

Note: This is always an eager operation.

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

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

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

Note: This is always an eager operation.

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

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

Note: This is always an eager operation.

Side effects

The sideEffect is executed for every entry in the Collection.

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

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

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

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.

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

rest(): this;

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

butLast(): this;

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

skip(amount: number): this;

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

skipLast(amount: number): this;

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

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

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

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

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

take(amount: number): this;

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

takeLast(amount: number): this;

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

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

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

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

Combination

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

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

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

Flattens nested Collections.

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 others Collection, not Arrays or Objects.

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

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

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

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

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

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;

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

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;

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

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

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

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

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

join(separator?: string): string;

Returns true if this Collection includes no values.

isEmpty(): boolean;

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

Returns the size of this Collection.

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.

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

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

Note: This is not a lazy operation.

Search for value

Returns the first value for which the predicate returns true.

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

Returns the last value for which the predicate returns true.

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

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

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

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

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

Returns the key for which the predicate returns true.

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

Returns the last key for which the predicate returns true.

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

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

keyOf(searchValue: V): K | undefined;

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

lastKeyOf(searchValue: V): K | undefined;

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

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

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.

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

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

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

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

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.

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

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

Comparison

True if iter includes every value in this Collection.

isSubset(iter: Iterable<V>): boolean;

True if this Collection includes every value in iter.

isSuperset(iter: Iterable<V>): boolean;