let obj = { name: 'Sarah', age: 25 };
let weakMap = new WeakMap();
let map = new Map();

weakMap.set(obj, 'weak reference');
map.set(obj, 'strong reference');

console.log(weakMap.has(obj));
console.log(map.has(obj));

obj = null;

console.log(weakMap.has(null));
console.log(map.has(null));
console.log(map.size);