WeakerMap

A WeakerMap is an enumerable Map with weak values rather than keys.

This class implements a pattern similar to the 'WeakMap', however its values are weakly referenced, rather than the keys. This allows for enumeration, clearing and arbitrarily valued keys, with the limitation that the values must be objects. Note that elements may not be garbage collected immediately upon leaving a given scope, however this should not have an impact on memory, since the memory will not be freed until the garbage collector runs, with or without the WeakerMap. See MDN's notes on WeakRefs for more info.

Install

npm install weakermap

import { WeakerMap } from 'weakermap';

Methods

WeakerMap.construct(entries)

Create a new WeakerMap object, optionally prepopulated by entries, similar to the standard Map constructor.

Parameters

• entries - An iterable list of [key, value] pairs to add to the map.

Returns

A newly constructed WeakerMap object.

const values = [
    ['x', {a: 1}],
    ['y', {b: 2}],
    ['z', {c: 3}],
];

const wm = new WeakerMap(values);

You can call Object.entries() to get such a list, given an object:

const wm = new WeakerMap(Object.entries({
    x: {a: 1},
    y: {b: 2},
    z: {c: 3},
}));

[Symbol.iterator]()

Traverse the entries.

Parameters

none

Returns

A new Iterator that traverses the WeakerMap.

const wm = new WeakerMap(Object.entries({
    x: {a: 1},
    y: {b: 2},
    z: {c: 3},
}));

for(const [key, value] of wm)
{
    console.log({key, value});
}
// { key: 'x', value: { a: 1 } }
// { key: 'y', value: { b: 2 } }
// { key: 'z', value: { c: 3 } }

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));
const ar = [...wm];
console.log(ar);
// [
//    [ 'x', { a: 1 } ],
//    [ 'y', { b: 2 } ],
//    [ 'z', { c: 3 } ]
// ]

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

[...wm].map( ( [key, value] ) => console.log( {key, value} ));

// { key: 'x', value: { a: 1 } }
// { key: 'y', value: { b: 2 } }
// { key: 'z', value: { c: 3 } }

WeakerMap.clear()

Clear all entries.

Parameters

none

Returns

none

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

wm.clear();

console.log(wm.size) // 0

WeakerMap.delete(key)

Delete a key.

Parameters

• key - The key to delete from the map.

Returns

none

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

wm.delete('y');

WeakerMap.entries()

Traverse all entries.

Parameters

none

Returns

A new Iterator that traverses the WeakerMap.

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

for(const [key, value] of wm.entries())
{
    console.log({key, value});
}
// { key: 'x', value: { a: 1 } }
// { key: 'y', value: { b: 2 } }
// { key: 'z', value: { c: 3 } }

WeakerMap.forEach(callback)

Traverse all entries with a callback.

Parameters

• callback - a function to run on each entry. Parameters are (value, key, map).

Returns

none

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

wm.forEach((value, key, map) => {
    console.log({value, key});
});
// { value: { a: 1 }, key: 'x' }
// { value: { b: 2 }, key: 'y' }
// { value: { c: 3 }, key: 'z' }

WeakerMap.get(key)

Get the value for a key.

Parameters

• key - The key to look up in the map.

Returns

The object found at key, or undefined if not found.

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

console.log( wm.get('x') ); // {a:1}

WeakerMap.has(key)

Check for the presence of a key.

Parameters

• key - The key to look up in the map.

Returns

true if the object at key is present, or false if not found.

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

console.log( wm.has('x') ); // true
console.log( wm.has('g') ); // false

WeakerMap.keys()

Traverse all keys.

Parameters

none

Returns

A new Iterator that traverses the keys of the WeakerMap.

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

for(const key of wm.keys())
{
    console.log(key);
}
// x
// y
// z

WeakerMap.set(key, value)

Set a key.

Parameters

• key - The key to set in the map.
• value - The value to set in the map.

Returns

none

const wm = new WeakerMap;

wm.set('x', {a:1});

WeakerMap.values()

Traverse all values.

Parameters

none

Returns

A new Iterator that traverses the values of the WeakerMap.

const wm = new WeakerMap(Object.entries({x: {a: 1}, y: {b: 2}, z: {c: 3}}));

for(const value of wm.values())
{
    console.log(value);
}
// {a:1}
// {b:2}
// {c:3}

Example

A WeakerMap will only hold onto its values as long as they aren't garbage collected. Once that happens, they will be removed without any further intervention from the programmer.

⚠️ NOTE ⚠️: The following example makes use of global.gc() to force garbage collection to run regardless of existing heuristics. This requires node to be run with the --expose-gc flag. This is not necessary except to demonstrate the behavior in a short script, where the garbage collector would not normally run until the program exits.

import { WeakerMap } from 'weakermap';

const wm = new WeakerMap;
const retain = []; // This will hold refs in-scope and prevent garbage collection.

{
    let a = {a:1}, b = {b:2}, c = {c:3};

    [['a', a], ['b', b], ['c', c]].forEach(e => wm.set(...e));

    retain.push(b, c);
};

const printRemaining = () => {
    retain;       // keep some refs in this-scope as well.
    global.gc();  // force the garbage collector
    console.log(wm.values());
};

printRemaining();
// The garbage collector hasn't run yet,
// so we still have all three refs
// [ { a: 1 }, { b: 2 }, { c: 3 } ]

setTimeout(printRemaining, 500);
// Once we swap to a new 'job', it can run
// and we only have two objects now
// [ { b: 2 }, { c: 3 } ]

🍻 Licensed under the Apache License, Version 2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

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