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Digging Into the Modern JavaScript

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Digging Into the Modern JavaScript

In this post, I'm going to explain some of the features of the current and upcoming JavaScript generation. I'm going to focus specifically in the ES6 syntax, leaving React for another post.

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As in the case of many JS developers nowadays, we are in the process of migrating our existent products to new technologies like ES6/Babel, Angular2, React, and Webpack.

In this amazing journey of trying to find the best tools for our daily jobs (and not new sexy things that just reinvent the wheel), a member of the dev team shared with us a tweet of @ericdfields about Modern JavaScript:

WTF tweet

After that WTF moment, I realized that now is a good time to start writing some posts in a way to ease the path to this "new modern world" for the ones with no clue of that syntax (like me a few weeks ago). In this post, I'm going to explain some of the features of the current and upcoming JavaScript generation. I'm going to focus specifically in the ES6 syntax of the tweet above, leaving the React stuff for another post. With that being said, let's get started:

ES6/Babel Introduction

ECMAScript 2015, ECMAScript Harmony, or simply ES6, is the sixth edition of the ECMAScript specification. While its predecesor (ES5) is widely adopted by browser and other JS engines, the implementation of the ES6 spec is still in progress. Instead of waiting, devs can use tools like Babel to traduce our hand-made ES6 (and even ES7!) code into its equivalent in ES5. This transpiler helps us to adopt the latest tools in our code without worrying about if they are supported by browsers or servers; as long as they supports ES5 (and most of them do) our code will work. There are a lot of transpilers out there, and you can find a more complete list here.

Now that we know what Babel is and what it does, let's dig into some of the features of ES6 used in the code snippet.

ES6 Features


ES6 introduces language support for classes, via the class and constructor keywords. While in ES5 you had to use something like this:

var Car = (function () {
  function Car(model) {
    this.model = model;
    this.currentSpeed = 0;

  Car.prototype.printCurrentSpeed = function printCurrentSpeed() {
    console.log(this.model + ' is going ' + this.currentSpeed + ' km/h');

  return Car;

In ES6, you can simply write the lines below to produce the same result:

class Car {
  constructor(model) {
    this.model = model;
    this.currentSpeed = 0;

    console.log(this.model + ' is going ' + this.currentSpeed + ' km/h');

You also have other improvements like class inheritance (via the extends keyword), static members (via the static keyword) and getters/setters (via the get and set keywords). You get the point.

Arrow Functions

A simple way to create closures with the addition of auto context handling (i.e., you don't have to worry about saving the this). Currently it's typical to see something like this in the code (notice that we are adding a new member to the Car class):

Car.prototype.accelerate = function accelerate() {
  var self = this;
  setInterval(function () {
  }, 1000);

Now with ES6 you can implement the same using this code:

  setInterval(() => {
  }, 1000);

And even this, which makes our code more readable:

  setInterval(() => this.currentSpeed++, 1000);


This is a huge improvement for client-side apps: the ability to organize our code as independent modules and explicitly decide what to expose between them (just like Node!). And it's provided natively in the browser, meaning that we don't have to use custom libraries for this, like RequireJS as we do today.

There are several combinations to import/export things (variables, functions, objects, etc.), let's analyze the basic ones first:

// module1.js
export function add(a, b){ return a + b; };
export default (a, b) => a * b; 
export var customValue = 2;
export var otherValue = 3;

// main.js
import add from './module1.js';
import multiply from './module1.js';
import {customValue, otherValue} from './module1.js';
import {customValue: a, otherValue: b} from './module1.js';

multiply(customValue, otherValue); // 2 * 3 = 6
add(a, b); // 2 + 3 = 5

In detail:

  • import add from './module1.js' will import the add function declared in the module.1js file to the 'main.js' file (notice that file and module are now equivalents).
  • import multiply from './module1.js' will import the function declared as default export of the 'module1.js' file to the variable multiply of the 'main.js' file. Note that there can be only one default export per module, but many named exports (like the one above), which makes sense.
  • import {customValue, otherValue} from './module1.js' will import the customValue and otherValue variables to the 'main.js' file. This syntax is named Destructuring Assignment and is another feature provided by ES6, we'll get to that in the next section.
  • import {customValue: a, otherValue: b} from './module1.js' will import customValue and otherValue to the 'main.js' file, but this time by assigning their values to the variables named a and b, respectively.

One last thing. Alternatively, you can use a wildcard to import the whole module namespace into another file/module, using a single line. For instance, import * as module1 from './module1.js' will give you access to the whole module via module1 (e.g. module1.customValue).

Note: Babel translates the import lines to something similar to the example below:

var $ = require('lib/jquery');

This is because browsers currently do not support import/export functionality. Hence, Babel falls back to CommonJS module syntax because it's natively supported in node applications. But, to make your client-side code understand this, you should use an external module loader library that supports CommonJS module syntax, like Webpack or JSPM.

Destructuring Assignment

ES6 introduces a new syntax to easily extract data from arrays or objects that is similar to the syntax used for constructing arrays of objects. For instance:

// ES6
var item = { id: 1, name: 'bike', qty: 10 };
var { id, name, qty } = item;
// ES5 equivalent
var item = { id: 1, name: 'bike', qty: 10 };
var id = item.id;
var name = item.name;
var qty = item.qty;

In the case of arrays:

// ES6
var items = [ 1, 2, 3 ];
var [ a, , c] = items;
// ES5 equivalent
var items = [1, 2, 3];
var a = items[0];
var c = items[2];

You can do a lot of other things, like "deep matching" (extract values from nested properties), swapping variables in one line ([a, b] = [b, a]), separate the values of an array returned by a function into several variables (in just one line), etc.

Spread Operator

This operator (...iterableObj) allows an expression to be expanded in places where multiple arguments/elements are expected, removing the need of (for instance) concatenating arrays, splitting strings, or pushing items to an array before doing a primary task.

Let's see some examples:

var elements = ['a', 1, false];
var other = [1, 2, ...params] // [1, 2, 'a', 1, false]
doSomething(1, 2, ...params); // doSomething.apply(null, [1, 2, 'a', 1, false])

var hello = 'Hello!';
var chars = [...hello] // ['H', 'e', 'l', 'l', 'o', '!']

var items = [1, 2, 3];
var otherItems = [4, 5, 6];
items.push(...otherItems); // [1, 2, 3, 4, 5, 6]

Weird, but much simpler, right?

Variable declaration

ES6 adds two new keywords for declaring variables: let and const, which could in time replace the var keyword. The difference is scoping: while var is scoped to the nearest function block (or global), let is scoped to the nearest enclosing block (for instance, a for loop or an if statement), improving the encapsulation of our code. Notice that let can be used the same way we use var, just by moving it to the nearest function block (probably just one level up).

function doSomething() {
  var a = 1;
  if (a === 1) {
    let b = 2;
    console.log(b); // Outputs 2
  console.log(a); // Outputs 1
  console.log(b); // Throws ReferenceError

console.log(b); // Throws ReferenceError

Finally, const is used for any variable where the reference should never be changed, and mantains the same scoping rules as let. Of course there are differences depending on the value: if we use const in a primitive we are not allowing a change of the value, while if we use it in an array we are just allowing a change in the reference, but we can add or remove elements inside the array.

Wait, What About the "@"?

Remember this line in the tweet @connect(state ==> ({ .... })? Well, the "@" syntax is used for ES7 Decorators and it's used to add behavior to a given object, independently of other instances of the same class. You could think that it's too soon to use ES7, but since Babel supports it, some libraries like Redux are already using it. You cand find a very good explanation of decorators here.

And, I think that's pretty much it for now. Hope you enjoyed reading—happy coding!

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es6 ,javascript ,ecma script ,react

Published at DZone with permission of Mariano Vazquez, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.


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