const sayings = new Map();
sayings.set("dog", "woof");
sayings.set("cat", "meow");
sayings.set("elephant", "toot");
sayings.size; // 3
sayings.get("dog"); // woof
sayings.get("fox"); // undefined
sayings.has("bird"); // false
sayings.delete("dog");
sayings.has("dog"); // false

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

sayings.clear();
sayings.size; // 0

const privates = new WeakMap();

export default function Public() {
  const me = {
    // 
  };
  privates.set(this, me);
}

Public.prototype.method = function () {
  const me = privates.get(this);
  // `me` 
  // 
};

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

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

for (const item of mySet) {
  console.log(item);
}
// 1
// "some text"

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

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