WeakMap.prototype.get()

const weakmap1 = new WeakMap();
const object1 = {};
const object2 = {};

weakmap1.set(object1, 42);

console.log(weakmap1.get(object1));
// Expected output: 42

console.log(weakmap1.get(object2));
// Expected output: undefined

get(key);

const wm = new WeakMap();
wm.set(window, "foo");

wm.get(window); // "foo" 반환.
wm.get("baz"); // undefined 반환.