(async function() {
  return await 10;
})().then(console.log);

function foo() {}
foo.prototype.bar = 42;
const obj = new foo();
foo.prototype = { bar: 100 };
console.log(obj.bar);

const obj = {
  0: "zero",
  1: "one",
  length: 2
};
console.log(Array.from(obj));

const map = new Map();
map.set("key", undefined);
console.log(map.has("key"), map.get("key"));

class MyClass {
  constructor() {
    this.a = 10;
  }
}
MyClass.prototype.b = 20;
const obj = new MyClass();
delete obj.b;
console.log(obj.b);