2704. JavaScript - ES6
ES6

ECMAScript 6 is also known as ES6 and ECMAScript 2015.

1. Constants

const TAX = 0.06;

2. Block Scoped Variables and Functions

var vs let

function varTest() {
  var x = 31;
  if (true) {
    var x = 71;  // same variable!
    console.log(x);  // 71
  }
  console.log(x);  // 71
}
 
function letTest() {
  let x = 31;
  if (true) {
    let x = 71;  // different variable
    console.log(x);  // 71
  }
  console.log(x);  // 31
}

3. Arrow Functions

// ES5
var multiply = function(x, y) {
    return x * y;
}
 
// ES6
var multiply = (x, y) => { return x * y };

4. Default Parameter Values

function multiply(a = 10, b = 20){
  return a * b;
}
console.log(multiply(1,2));  // print 2
console.log(multiply(1));    // print 20
console.log(multiply());     // print 200

5. Rest Parameters

function multiply(...a){
  var result = 1;
  for (let arg in a){
    result = result * a[arg];
  }
  return result;
}
console.log(multiply(5,6));   // print 30
console.log(multiply(5,6,2)); // print 60

6. String Interpolation

The interpolation only works with the new quote character ` used for template literals. It doesn’t work with strings enclosed in the usual quotes " and '.

var person = {name: "julie", city: "atlanta"};
console.log(person.name);
// works
console.log(`${person.name} lives in ${person.city}`);
// doesnt work
console.log("${person.name} lives in ${person.city}");
console.log('${person.name} lives in ${person.city}');

output

produces this output:
julie
julie lives in atlanta
${person.name} lives in ${person.city}
${person.name} lives in ${person.city}

7. Modules

7.1 Named Exports

Module js file mymath.js

export const sqrt = Math.sqrt;
 
export function square(x) {
    return x * x;
}
 
export function diag(x, y) {
    return sqrt(square(x) + square(y));
}

Usage

import { square, diag } from 'mymath';
 
console.log(square(11)); // print 121
console.log(diag(4, 3)); // print 5

7.2 Default Exports

Module js file mymath.js

export default function square(x) {
    return x * x;
}

usage, the name sq doesn’t match the function in the module. Using default exports allows you to use “nicknames”.

import sq from 'mymath';
sq();

8. Reference