javascript ES6 ES2015

const

const myVar = 'value';
myVar = 'value2'; // error

let

var myVar = 'value';

if( myVar ){

    let myVar = 'value2';

    console.log(myVar);  // 'value2'

}

console.log(myVar);  // 'value'

String Template

let myHtml = `<div>${myVar}</div>`; // ` -> back tick character

let myTemplate = `

    <div>

        <span>${var1}</span>

        <span>${var2}</span>

    </div>

`;

Default values

function myFunction(par='some value'){

     // code

}

var defaultValue = 'value';

function MyFunction2(par=defaultValue){

    // code

}

Arrow function

let add5 = par1 => par1+5;

let sum = (par1, par2) => par1 + par2;

let myFunc = (par1, par2) => {

    if(par1<=0){

        par1 = 1;

   }

   return par1*par2;

}

let myObj = {

    myMeth: function(){   

         // code

   }

   myMeth2: function(){

         setTimeout(function(){

              this.myMeth(); // ERROR! function not preserve scope

        }, 1000);

   }

   myMeth2: function(){

        setTimeout( ()=>{

            this.myMeth();  // scope is myObj

        },1000);

   }

}

Destructuring

let obj = {k1:v1, k2: v2};

let {k1, k2} = obj;

console.log(k1, k2);

let myFunc = ({k1}) => console.log(k1);

let arr = ['first', 'second', 'third'];

let [firstItem] = arr;

let [,, thirdItem] = arr;

Object Literal Ehancement

let var1 = value1;

let var2 = value2;//rest is array without first item

let obj = {var1, var2}; //obj= {var1:value1, var2:value2}

let myFunc = function(){  

     return this.var1;

};

let obj2 =  {var1, var2, myFunc};

let obj3 = {

    prop1,

    prop2,

    myMeth(){

        //code

   }

}

Spread Operator

let arr1 = [v1, v2, v3];

let arr2 = [v4, v5, v6];

let arr3 = [...arr1, ...arr2]; //arr3 = [v1, v2, v3, v4, v5, v6];

let [last] = arr.reverse(); //array altered

let [last] = [...arr].reverse(); 

let [first, ...rest] = arr; //rest is array without first item

let obj = {k1:v1, k2:v2};

let prop = value;

let obj2 = {...obj, prop};

Promise

const getUsers = count => new Promise(

    (resolves, rejects) => {

         const url = `http://myapi/user/?limit:${count}`;

         const req = new XMLHttpRequest/(;

         req.open('GET', url);

         req.onload = () =>  req,status == 200 ? 

                            resolves(JSON.parse(req.response)).results :

                            reject(Error(req.statusText));

         req.onerror = (err) => reject(err);

         req.send();

   }

);

getUsers(10).then(

         users=>console.log(users), 

         err => console.error(err)

     );

getUsers(10)

    .then(users=>console.log(users))

    .catch(err=>console.log(err));

Class

class MyClass {

    constructor(par1, par2){

        // code

    }

    myMeth(){

        // code

    }

}

class MyClass2 extends MyClass{

    constructor(par1, par2, par3){

        super(par1, par2);

        // code

    }

    myMeth2(){

        // code

    }

}

Module

Module -> file

file1.js

export const myFunc = (par1) => par1+5;

export const myFunc2 = (par2) => par2^2;

file2,js

export const default myVar = 5;

file3.js

import {myFunc, myFunct2} from "./file1";

import {myVar} from "./file2";

alias

import {myFunc as f1, myFunct2 as f2} from "./file1";

all

import * as f from "./file1"

NodeJS (Common js)

file1.js

module.exports = {myFunc, myFunct2};

file3.js

const {myFunc, MyFunc2} = require("./file1");