❗️Что будет на выходе:

const weakMap = new WeakMap();
const objs = [{}, {}, {}];

objs.forEach((obj, index) => weakMap.set(obj, index + 1));

const result = objs.filter(obj => weakMap.has(obj)).map(obj => weakMap.get(obj) * 2);

console.log(result);

Ответ: [2, 4, 6]

JavaScript test | #JavaScript