JavaScript(ES6) WeakMap garbage collection when set an object to null

Question

I've just read that WeakMaps take advantage of garbage collection by working exclusively with objects as keys, and that assigning an object to null is equivalent to delete it:

let planet1 = {name: 'Coruscant', city: 'Galactic City'};
let planet2 = {name: 'Tatooine', city: 'Mos Eisley'};
let planet3 = {name: 'Kashyyyk', city: 'Rwookrrorro'};

const lore = new WeakMap();
lore.set(planet1, true);
lore.set(planet2, true);
lore.set(planet3, true);
console.log(lore); // output: WeakMap {{…} => true, {…} => true, {…} => true}

Then I set the object equal to null:

planet1 = null;
console.log(lore); // output: WeakMap {{…} => true, {…} => true, {…} => true}

Why is the output the same? Wasn't it supposed to be deleted so that the gc could reuse the memory previously occupied later in the app? I would appreciate any clarification. Thanks!

"assigning an object to null is equivalent to delete it" - no, removing the reference from your variable does make the object eligible for garbage collection (when that happens). It does not immediately delete anything. — Bergi
– Bergi, Commented Apr 15, 2018 at 12:55
Also, these things will all get garbage collected as soon as GC runs since WeakMap doesn't retain a reference to the object. You need to store references to the planets somewhere else or the WeakMap will immediately throw them away. — podperson
– podperson, Commented Feb 25, 2021 at 22:43

Tomasz Kula · Accepted Answer · 2018-04-15 11:44:27Z

19

Garbage collection does not run immediately. If you want your example to work you need to force your browser to run garbage collection.

Run chrome with the following flag: google-chrome --js-flags="--expose-gc".

You can now force the garbage collection by calling the global gc() method.

answered Apr 15, 2018 at 11:44

Tomasz Kula

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3 Comments

David Gilbertson Over a year ago

2020 update: there's now a collect garbage icon on the memory and performance tabs that force garbage collection.

Uahnbu Tran Over a year ago

Will the keys be garbage-collected if they are deleted from the map, i.e. map.delete(key) or map.clear()?

Caleb Taylor Over a year ago

For those where even clicking with garbage icon on memory/performance tabs didn't work, make sure to have more than one item in the WeakMap.

Loon911 · Accepted Answer · 2022-05-19 13:46:00Z

I was trying to figure out the same thing, in 2022, but it looks like the browsers don't bother to garbage collect if the WeakMap is small.

But if you do something like this:

let weakmap = new WeakMap()

for (let i = 0; i < 2000; i++) {
    let john = {meow: i}

    weakmap.set(john, i)
    console.log(weakmap)
    john = null

    let div = document.createElement("div")
    div.innerText = i
    document.body.append(div)
    weakmap.set(div, i)
    div.remove()
}

let john2 = {} // Test to see if it garbage collects this (it shouldn't)

weakmap.set(john2, "random")

console.log(weakmap)

Then the browser will garbage collect after a certain size:

As you can see it went from the size to 4001 too 101. It looks like the browser doesn't bother garbage collecting the rest.

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