How to use WeakMap WeakSet in javascript.

Javascript GC will collect object and release space which not used

let obj = { name: "foo" };
obj = null;
// GC will release obj space in memory

But this case will not release space

let obj = { name: "foo" };
let maps = new Map();
maps.set(obj, "obj-value");
obj = null; // set obj to null want to release sapce
maps.get(obj) // 'obj-value'
// maps => Map(1) {{…} => "obj-value"}

Emmm.. we just set null to obj but not release space in memory, we can fetch the value use maps.get(obj) why?

If you want to release obj maybe you can use WeakMap or WeakSet

let weakMap = new WeakMap();
let obj = { name: "foo" };
weakMap.set(obj, "obj-value");
console.log(weakMap.get(obj)) //=> "obj-value"
obj = null
console.log(weakMap.get(obj)) //=> undefined 

I think WeakMap more useful when you need store object key to map , aslo WeakSet too.

Hope it can help you :)