JavaScript ES6/ES2015 feature

 <script src="https://google.github.io/traceur-compiler/bin/traceur.js"></script>
<script src="https://google.github.io/traceur-compiler/bin/BrowserSystem.js"></script>
<script src="https://google.github.io/traceur-compiler/src/bootstrap.js"></script>   

if(true){
  var username = "xy";
  let cntr = 0;
}
document.write(cntr);

for(let i=0;...){
    //use let here
}

const ORG = "xy";
const ORG = "xy";
document.write(ORG);
//get an error: Identifier 'ORG' has already been declared

 var arr = ['ar','xy','hi'];

 //var username = arr[0];
 var [username,org,place]= arr;

 document.write(username);
 //output: ar

 function retData(){
   var arr = ['ar','xy','hi'];
   return arr;
 };

 var [username,org,place]= retData();
 //we also can: var [ , ,place] = retData();
 document.write(username);

 var userObj = {
   fn:"x",
   ln:"y",
   pl:"nj"
 };

 var {
   fn:fname,
   ln:lname,
   pl:place
 } = userObj;

 document.write(fname);
 document.write(lname);
 document.write(place);
 //output: xynj

 //given default value at below
 function abc(username="x",org="m"){

   return 'un='+username+',org='+org;
 }

 var resp = abc();
 //var resp = abc('newname',''newplace);

 document.write(resp);

 //old
 function add(x,y){
   return x+y;
 }
 //new in es6
 var add_new= (x,y)=>x+y;
 //or
 var add_new= (x,y)=>{
   return x+y;
 }
 document.write(add_new(4,5));

 function add(x,y){
   // return "x:"+x+",y:"+y;
   //` here
   return `x:${x}`;
 }

 document.write(add(4,5));

 class user{
   getData(){
     return "xx";
   }
 }

 var user1 = new user();

 console.log(user1.getData());

 class user{
   //constructor
   constructor(username){
       this.uname = username;
   }
      
   getData(){
     return "hi"+this.uname;
   }
 }

 class emp extends user{
   constructor(empCode,org){
     super(org);
     this.emp_code = empCode;
   }
   getEmpCode(){
     return "empcode:<br />"+this.emp_code;
   }
   getData(){
     return super.getData();
   }
 }

 //example before we have constructor in child class, we can use the code committed

 var user1 = new user("xyyy<br />");
 // var emp1 = new emp("emmm<br />");
 var emp2 = new emp("1<br />","emp2");

 document.write(user1.getData());
 // document.write(emp1.getData());
 document.write(emp2.getEmpCode());
 document.write(emp2.getData());

var set1 = new Set(),
  arr = ["1","2","3"],
  obj = {"un":"xy"};
      
set1.add("ar");
set1.add(arr);
set1.add(obj);

set1.has(obj);

set1.forEach(function(value){
  document.write(value+"<br />hi");
});

document.write(set1);
document.write(set1.has(obj));
set1.clear();
document.write(set1.has(obj));
/...true, false

var map1 = new Map();
var obj = {
  "fn":"xy"
};

map1.set("un","ar");
map1.set(obj,true);

map1.forEach(function(value,key){
  document.write("k:"+key+",v:"+value+"<br />");
});

console.log(map1);
document.write(map1.get("un"));
//output:k:un,v:ar-k:[object Object],v:true-ar

var promise1 = new Promise(function(resolve,reject){
  resolve('1');
});
var promise2 = new Promise(function(resolve,reject){
  reject('2');
});
var promise3 = new Promise(function(resolve,reject){
  resolve('3');
});

Promise.all([promise1,promise2,promise3]).then(function(data){
  console.log(data);
},
function(err){
  console.log(err);
});
//output:["1","2","3"](when all resolve)
//output:2, when as above, promise2: reject
//output:1, when Promise.all-->Promise.race

//index.html
<script type = "module" src="script.js"></script>

//script.js
import {abc, user_arr, user} from './mod.js';
document.write(user.fn);
//or use this:
import * as moduleContent from './mod.js';
document.write(moduleContent.user.fn);

//mod.js
export function abc(){
  return 'hello';
}

export var user_arr = ['ar','na'];

export var user = {
  fn:"x",
  ln:"y",
  pl:"nj"
}
//or export everything in one line
export {abc, user_arr, user}

 // import * as moduleContent from './mod.js';

 // document.write(moduleContent.user.fn);


 function abc2(...args){
   console.log("2:"+args);
 }

 function abc1(){
   console.log("1:"+arguments);
 }

 function abc3(...[username,org]){
   console.log("3:"+username+","+org);
 }

 function abc4(para1,phone,...[username,org]){
   console.log("4:"+username+","+phone);
 }

 abc1("1","2");
 abc2("1","2");
 abc3("1","2","3");
 abc4("ar","ma","pi","99");

 var arr = ["010","020"];
 var newa = ["1",...arr,"2"];
 console.log(newa);