Aurelio De Rosa - Practical ES6
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Copyright 2018 SitePoint Pty. Ltd.
- Cover Design: Alex Walker
All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical articles or reviews.
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Rather than indicating every occurrence of a trademarked name as such, this book uses the names only in an editorial fashion and to the benefit of the trademark owner with no intention of infringement of the trademark.
Published by SitePoint Pty. Ltd.
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SitePoint specializes in publishing fun, practical, and easy-to-understand content for web professionals. Visit http://www.sitepoint.com/ to access our blogs, books, newsletters, articles, and community forums. Youll find a stack of information on JavaScript, PHP, design, and more.
Theres no doubt that the JavaScript ecosystem changes fast. Not only are new tools and frameworks introduced and developed at a rapid rate, the language itself has undergone big changes with the introduction of ES2015 (aka ES6). Understandably, many articles have been written complaining about how difficult it is to learn modern JavaScript development these days. We're aiming to minimize that confusion with this set of books on modern JavaScript.
This book provides an introduction to many of the powerful new JavaScript language features that were introduced in ECMAScript 2015, as well as features introduced in ECMAScript 2016 and 2017. It also takes a look at the features planned for ECMAScript 2018 in this rapidly evolving language.
This book is for all front-end developers who wish to improve their JavaScript skills. Youll need to be familiar with HTML and CSS and have a reasonable level of understanding of JavaScript in order to follow the discussion.
Code in this book is displayed using a fixed-width font, like so:
A Perfect Summer's DayIt was a lovely day for a walk in the park.The birds were singing and the kids were all back at school.
Where existing code is required for context, rather than repeat all of it, will be displayed:
function animate() { new_variable = "Hello";}Some lines of code should be entered on one line, but weve had to wrap them because of page constraints. An indicates a line break that exists for formatting purposes only, and should be ignored:
URL.open("http://www.sitepoint.com/responsive-web-design-real-user-testing/?responsive1");Youll notice that weve used certain layout styles throughout this book to signify different types of information. Look out for the following items.
Tips provide helpful little pointers.
Notes are useful asides that are relatedbut not criticalto the topic at hand. Think of them as extra tidbits of information.
... pay attention to these important points.
Warnings highlight any gotchas that are likely to trip you up along the way.
In this tutorial, I'll introduce let and const, two new keywords added to JavaScript with the arrival of ES6. They enhance JavaScript by providing a way to define block-scope variables and constants.
letUp to ES5, JavaScript had only two types of scope, function scope and global scope. This caused a lot of frustration and unexpected behaviors for developers coming from other languages such as C, C++ or Java. JavaScript lacked block scope, meaning that a variable is only accessible within the block in which its defined. A block is everything inside an opening and closing curly bracket. Let's take a look at the following example:
function foo() { var par = 1; if (par >= 0) { var bar = 2; console.log(par); // prints 1 console.log(bar); // prints 2 } console.log(par); // prints 1 console.log(bar); // prints 2}foo();After running this code, you'll see the following output in the console:
1212What most developers coming from the languages mentioned above would expect, is that outside the if block you can't access the bar variable. For example, running the equivalent code in C results in the error 'bar' undeclared at line ... which refers to the use of bar outside the if.
This situation changed in ES6 with the availability of block scope. The ECMA organization members knew that they could not change the behavior of the keyword var, as that would break backward compatibility. So they decided to introduce a new keyword called let. The latter can be used to define variables limiting their scope to the block in which they are declared. In addition, unlike var, variables declared using let aren't hoisted. If you reference a variable in a block before the let declaration for that variable is encountered, this results in a ReferenceError. But what does this mean in practice? Is it only good for newbies? Not at all!
To explain you why you'll love let, consider the following code taken from my article 5 More JavaScript Interview Exercises:
var nodes = document.getElementsByTagName('button');for (var i = 0; i < nodes.length; i++) { nodes[i].addEventListener('click', function() { console.log('You clicked element #' + i); });}Here you can recognize a well-known issue that comes from variable declaration, their scope, and event handlers. If you don't know what I'm talking about, go check the article I mentioned and than come back.
Thanks to ES6, we can easily solve this issue by declaring the i variable in the for loop using let:
var nodes = document.getElementsByTagName('button');for (let i = 0; i < nodes.length; i++) { nodes[i].addEventListener('click', function() { console.log('You clicked element #' + i); });}The let statement is supported in Node and all modern browsers. There are, however, a couple of gotchas in Internet Explorer 11 which you can read about in the ES6 compatibility table.
A live demo that shows the difference between var and let is available at JSBin.
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