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A MapSet is a class partially implementing the standard JavaScript Set and Map classes, allowing insertion of arbitrary objects which can be indexed using a key: (item: V) => K function supplied to the constructor, as opposed to the standard Set which only supports object reference equality for objects.


$ npm install mapset


The class behaves like a Set for insertion: add(value: V) and iteration of values, and as a Map for key-based operations like get(key: K), has(key: K) and delete(key: K).

This enables adding unique objects in the same way as for a standard Set, but where the uniqueness is defined by the key function, while at the same time allowing Map operations to operate on the collection using keys.

To simplify typing, two interfaces defining the parts of Set and Map supported by MapSet are also defined, so that

  • Set<V> implements PartialSet<V>
  • Map<K, V> implements PartialMap<K, V>
  • MapSet<K, V> implements PartialSet<V> and PartialMap<K, V>

This means that at an interface may refer to a PartialSet or a PartialMap and be implemented either by the standard Set and Map classes, or by MapSet.

Differences from Set and Map:

Properties missing from Set:

  • keys() - replaced by same method in Map returning IterableIterator<K> instead of IterableIterator<V>
  • entries() - replaced by same method in Map returning IterableIterator<[K, V]> instead of IterableIterator<[V, V]>
  • delete(value: V) - replaced by same method in Map taking a key instead of a value
  • has(value: V) - replaced by same method in Map taking a key instead of a value
  • forEach(callbackfn: (value: V, value2: V, map: Set<V>) => void, thisArg?: any) - replaced by same method in Map calling the callback with (value: V, key: K, map: this)

Properties missing from Map:

  • [Symbol.iterator]() - replaced by same method in Set returning IterableIterator<V> instead of IterableIterator<[K, V]>
  • set(key: K, value: V) - replaced by add method in Set automatically calculating the key from the value using the key function, to prevent inconsistent items in the MapSet

Example usage

interface Foo {
  id: string;
  name?: string;
  age?: number;

const foos = new MapSet((item: Foo) => item.id); // MapSet<string, Foo>

foos.add({id: '42', name: 'John Doe'});
foos.add({id: '42', name: 'Jane Doe'}); // replaces John
foos.add({id: '79', name: 'Foo Bar'});

for (const foo of foos) {

foos.size; // 2
foos.has('79'); // true
foos.get('42'); // {id: '42', name: 'Jane Doe'}
foos.get('7'); // undefined
foos.delete('79'); // true
foos.has('79'); // false




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npm i @henhal/mapset

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