text: let weakmap = new WeakMap();
  
  (function(){
    let o = {n: 1};
    weakmap.set(o, "A");
  })()
  
  //here 'o' key is garbage collected
  
  let s = {m: 1};
  weakmap.set(s, "B");
  
  console.log(weakmap.get(s));
  
  console.log(...weakmap); //exception thrown
  
  weakmap.delete(s);
  weakmap.clear(); //Exception, no such function
  
  let weakmap_1 = new WeakMap([[{}, 2], [{}, 5]]);   //this works
  console.log(weakmap_1.size); //undefined