var sayings = new Map();
sayings.set("dog", "woof");
sayings.set("cat", "meow").set("elephant", "toot");
//  .set  Map,  set    

sayings.set("dog", "-"); //    

sayings.size; // 3
sayings.get("fox"); // undefined
sayings.has("bird"); // false
sayings.delete("dog");

for (var [key, value] of sayings) {
  console.log(key + " goes " + value);
}
// "cat goes meow"
// "elephant goes toot"

const privates = new WeakMap();

export class Public() {

  constructor() {
    const me = {
    //    
    };
    // 'me'     'this' !!!
    privates.set(this, me);
  }

  method () {
    const me = privates.get(this);
    //  -     'me'...
  }
}

var mySet = new Set();
mySet.add(1);
mySet.add("some text");
mySet.add("foo");

mySet.has(1); // true
mySet.delete("foo");
mySet.size; // 2

for (let item of mySet) console.log(item);
// 1
// "some text"

Array.from(mySet);
[...mySet2];

mySet2 = new Set([1, 2, 3, 4]);