WeakMap()

new WeakMap()
new WeakMap(iterable)

const wm1 = new WeakMap();
const wm2 = new WeakMap();
const wm3 = new WeakMap();
const o1 = {};
const o2 = () => {};
const o3 = window;

wm1.set(o1, 37);
wm1.set(o2, "azerty");
wm2.set(o1, o2); // 
wm2.set(o3, undefined);
wm2.set(wm1, wm2); //  WeakMap 

wm1.get(o2); // "azerty"
wm2.get(o2); // undefined o2  wm2 
wm2.get(o3); // undefined 

wm1.has(o2); // true
wm2.has(o2); // false
wm2.has(o3); // true ( 'undefined' )

wm3.set(o1, 37);
wm3.get(o1); // 37

wm1.has(o1); // true
wm1.delete(o1);
wm1.has(o1); // false