You Don't Know JS: ES6 & Beyond

Chapter 2. Syntax

If you’ve been writing JS for any length of time, odds are the syntax is pretty familiar to you. There are certainly many quirks, but overall it’s a fairly reasonable and straightforward syntax that draws many similarities from other languages.

However, ES6 adds quite a few new syntactic forms that take some getting used to. In this chapter, we’ll tour through them to find out what’s in store.

Tip

At the time of this writing, some of the features discussed in this book have been implemented in various browsers (Firefox, Chrome, etc.), but some have only been partially implemented and many others have not been implemented at all. Your experience may be mixed trying these examples directly. If so, try them out with transpilers, as most of these features are covered by those tools.

ES6Fiddle is a great, easy-to-use playground for trying out ES6, as is the online REPL for the Babel transpiler.

Block-Scoped Declarations

You’re probably aware that the fundamental unit of variable scoping in JavaScript has always been the function. If you needed to create a block of scope, the most prevalent way to do so other than a regular function declaration was the immediately invoked function expression (IIFE). For example:

var a = 2;

(function IIFE(){
    var a = 3;
    console.log( a );   // 3
})();

console.log( a );       // 2

let Declarations

However, we can now create declarations that are bound to any block, called (unsurprisingly) block scoping. This means all we need is a ...