A DefaultWeakMap is a WeakMap that will use the given factory to automatically create values for non-existing keys on demand. It’s similar to DefaultMap but the keys are stored as weak references and can be garbage collected. Keys must be objects, and cannot be primitive data types (string, number, bigint, boolean, undefined, and symbol).

var DefaultWeakMap = require('mnemonist/default-weak-map');

Use case

Sometimes, it might be tedious to check whether a key exists in a weak map before initializing the attached value and then proceed to operate on it.

This is the exact problem DefaultWeakMap tries to address.

Let’s say we want to have keys pointing to arrays of values:

const group1 = { room: '1' }
const group2 = { room: '2' }
const group3 = { room: '3' }

const personsToGroup = {
  John: group1,
  Martha: group2,
  Philip: group1,
  Lenny: group3

// We want:
// WeakMap {
//   group1 => ['John', 'Philip'],
//   group2 => ['Martha'],
//   group3 => ['Lenny']
// }

// Using a WeakMap
const map = new WeakMap();

for (const [person, group] of Object.entries(personsToGroup)) {
  // This is tedious & leads to more lookups if written carelessly
  if (!map.has(group))
    map.set(group, []);


// Using a DefaultWeakMap
const map = new DefaultWeakMap(() => []);

for (const [person, group] of Object.entries(personsToGroup)) {

But there are plenty of other use cases. Why not use the DefaultWeakMap with more complex containers?

// Maps K => <V, number>
const map = new DefaultWeakMap(() => new MultiSet());

// Let's be creative :)
const map = new DefaultWeakMap(() => new DefaultMap(() => []));


The DefaultWeakMap takes a factory function as single argument.

const map = new DefaultWeakMap(() => []);

const unknown = {};

// The factory takes the key being accessed
const map = new DefaultWeakMap((key) => `${JSON.stringify(key)}`);

Members & Methods

The DefaultWeakMap has the exact same interface as JavaScript’s WeakMap. Except for the following:


Same as #.get except that it won’t create a value using the provided factory if key is not found.

const map = new DefaultWeakMap(() => []);
const one = {};

>>> undefined