JS: WeakMap

By Xah Lee. Date: . Last updated: .

what is WeakMap

WeakMap is similar to the Map Object, with 3 exceptions:

  1. Keys can only be Object Type, or unregistered Symbol type (meaning, symbols user created, not builtin symbols.).
  2. If a object in the key is not used elsewehere in your program, or no longer used, that key entry is deleted automatically.
  3. Weakmap cannot be iterated over, nor get its keys, nor looped over, nor get its item count, nor clear all.

All methods for WeakMap

they are similar to methods in JS: Map.prototype 

// demo of methods for WeakMap

const wm = new WeakMap();

const xkey = { "hi": 3 };

// add
wm.set(xkey, 1);

// get
console.assert(wm.get(xkey) === 1);

// has
console.assert(wm.has(xkey) === true);

// delete
wm.delete(xkey);

console.assert(wm.has(xkey) === false);

demo of the auto remove keys

// demo of the auto key removal
const wp = new WeakMap();

let xobj = { "hi": 1 };

wp.set(xobj, 1);
console.assert(wp.get(xobj) === 1);

// reset the object
xobj = null;

// the wp no longer has the key

why WeakMap

// this shows the situation that WeakMap was invented

// create a map
const xmap = new Map();

// create a array of objects, e.g. [ { k: 0 }, { k: 1 }, { k: 2 }, { k: 3 } ]
let xar = Array.from((Array(4)).keys(), (x) => ({ "k": x }));

// add them all to map
xar.forEach((x) => {
 xmap.set(x, 1);
});

console.log(xmap.size);
// 4

// delete our array
xar = null;

// the map now contain objects that wont go away, and there is no reference handle to them except iterating this map
console.log(xmap.size);
// 4

JavaScript, map and set