Основы ES6

const obj = { foo: 1 }; 

obj.bar = 2;

function mystery(...params) {

  return params; 

}

let a = mystery(1,23,4);

(function() {

 let f = this ? class g { } : class h { };

 return [ typeof f, typeof h ];

 })();

[...[...'...']].length

typeof (new (class F extends (String, Array) { })).substring

(function() { 

if (false) {

 let f = { g() => 1 };

 }

 return typeof f;

 })()

let some_text = "some string"; 

if(some_text.indexOf('str') !=-1){

 return true;

 }

class Slave { // ... };

 const slave = Slave();