JavaScript WeakMap keep referencing gc'ed objects

Question

I am experiencing with JavaScript weakmaps, after trying this code in google chrome developer console, running with --js-flags="--expose-gc", I don't understand why the weakmap keep having a reference to a.b if a is gc'ed.

var a = {listener: function(){ console.log('A') }}
a.b = {listener: function(){ console.log('B') }}

var map = new WeakMap()

map.set(a.b, [])
map.set(a, [a.b.listener])

console.log(map) // has both a and a.b

gc()
console.log(map) // still have both a and a.b

a = undefined
gc()
console.log(map) // only have a.b: why does still have a reference to a.b? Should'nt be erased?

Answer 1

In your example code, you're not releasing your a variable. It's a top level var that never goes out of scope and never gets explicitly de-referenced, so it stays in the WeakMap. WeakMap/WeakSet releases objects once there are no more references to it in your code. In your example, if you console.log(a) after one of your gc() calls, you'd still expect a to be alive, right?

So here's a working example showing WeakSet in action and how it'll delete an entry once all references to it are gone: https://embed.plnkr.co/cDqi5lFDEbvmjl5S19Wr/

const wset = new WeakSet();

// top level static var, should show up in `console.log(wset)` after a run
let arr = [1];
wset.add(arr);

function test() {
  let obj = {a:1}; //stack var, should get GCed
  wset.add(obj);
}

test();

//if we wanted to get rid of `arr` in `wset`, we could explicitly de-reference it
//arr = null;

// when run with devtools console open, `wset` always holds onto `obj`
// when devtools are closed and then opened after, `wset` has the `arr` entry,
// but not the `obj` entry, as expected
console.log(wset);

Note that having Chrome dev tools opened prevents some objects from getting garbage collected, which makes seeing this in action more difficult than expected :)