JavaScript Refresher

let and const

var used to be the de facto variable declaration keyword used in JavaScript till ES6. Now, it’s encouraged to use let and const. First, let’s go through what var does.

var test = "Hi";
var test = "Hi"
var no = 5
if (no == 5) {
var test = 0;

Arrow functions

Arrow functions are the norm nowadays in JavaScript. A normal JS function looks like the below snippet.

function myDobuleFun(no){
return no * 2;
const myDoubleFun = (no) => {
return no * 2;
const myDobuleFun = no => no * 2;

Exports and Imports(Modules)

We can write modular code with ES6. The way to do it is through exports and imports. Let’s look at export and import as a few examples.

const test = 10;
export default-test;
import test from './test.js'
import t from './test.js'
export const pi = 3.14;
export const e = 2.718;
import {pi} from './util.js'
import {e} from './util.js'
import {pi as math_pi} from './util.js'
import * as bundled from './util.js'

Spread and Rest Operators

Both are represented by .

let no = [1, 2, 3];
let newNo = [, 4];//[1, 2, 3, 4]
let newNo = [no, 4];//[[1, 2, 3], 4]
const filter = (...args) => {
return args.filter(e1 => e1 === 1);


This is used to extract array elements or object properties and store them in variables.

[a, b] = ['Hi', 'World'];
console.log(a); //Hi
console.log(b); //World
{name} = {name: 'John', age: 20};



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