Harmony, JavaScript for Web apps

Object to process in sequence the elements it contains.

Iterator:

var lang = { 0 : 'JavaScript', 1: 'php' };
var it = Iterator(lang);
var n = it.next();
document.write(n);

This provides a form of concurrency to JavaScript. Every time you call the function, you go to the next yield and get the associated value, even if yield is in a loop.
In the example below it displays successively one, two, three, 1, 2 and false because i must be less than 3.

Generator and reserved word yield:

function simpleGenerator(){
  yield "one";
  yield "two";
  yield "three";
  for (var i = 0; i < 3; i++)
    yield i;
}
var g = simpleGenerator();
document.write(g.next());  // displays 'one'

Let is an alternative to var for restricting visibility to the body of a block, rather than to a function.

Let :

var testlet = 10;
if(true) {
  let testlet = 20;
  if(testlet == 20)
    alert(20);
}
alert(testlet); // displays 10.

This allows you to make grouped assignments. Including from the return of a function.

[a , b ] = [x , y];

var ada = 1;
var adb = 2;
var adx = 3;
var ady = 4;
[ada , adb] =  [adx, ady];  // ada now is 3 and adb 4

This new type of array can not be confused with an associative array. The numeric keys are always keys, not indices.
In the example, 1 is part of the list.

Set/add()/has()/size :

var testset = new Set();
testset.add(1);
testset.add("hello");
document.write(testset.has(1));  // return true.

This new type of associative array (name with a capital letter unlike the method) has a size attribute unlike conventional arrays which are actually dynamic objects. The set and get methods modify its content.

Map/set()/get()/size :

var testmap = new Map();
testmap.set("a", "one");
testmap.set("b", "two");
document.write(testmap.get("b")); // displays two.

New String method that removes extra front and trailing spaces . Support IE 9.

String.trim() :

var tests = " Hello ";
document.write(tests.trim()); // displays "Hello".

True classes with inheritance.

Class :

var tclass = class {
  constructor() { }
  tmet() {
    return "ok"; 
  }
}
document.write(tclass.tmet()); // should display ok.

An object to wait for the end and the result of a preliminary process before to continue a processing, asynchronously.

Promise :

var tprom = new Promise(function(a, b) {
  a(true);
});

function success(res) { 
	document.getElementById("tpromid").innerHTML="Success";
}

tprom.then(promtrue, function(res) {});