Book Image

Learn ECMAScript - Second Edition

By : MEHUL MOHAN, Narayan Prusty
Book Image

Learn ECMAScript - Second Edition

By: MEHUL MOHAN, Narayan Prusty

Overview of this book

Learn ECMAScript explores implementation of the latest ECMAScript features to add to your developer toolbox, helping you to progress to an advanced level. Learn to add 1 to a variable andsafely access shared memory data within multiple threads to avoid race conditions. You’ll start the book by building on your existing knowledge of JavaScript, covering performing arithmetic operations, using arrow functions and dealing with closures. Next, you will grasp the most commonly used ECMAScript skills such as reflection, proxies, and classes. Furthermore, you’ll learn modularizing the JS code base, implementing JS on the web and how the modern HTML5 + JS APIs provide power to developers on the web. Finally, you will learn the deeper parts of the language, which include making JavaScript multithreaded with dedicated and shared web workers, memory management, shared memory, and atomics. It doesn’t end here; this book is 100% compatible with ES.Next. By the end of this book, you'll have fully mastered all the features of ECMAScript!
Table of Contents (18 chapters)
Title Page

The rest parameter

The rest parameter is also represented by the ... token. The last parameter of a function with ... is called a rest parameter. The rest parameter is an array type and contains the rest of the parameters of a function when the number of arguments exceeds the number of named parameters. The rest parameter is used to capture a variable number of function arguments from within a function. The arguments object can also be used to access all arguments passed. The argument object is not strictly an array, but it provides some interfaces that are similar to an array. The following example code shows how to use the arguments object to retrieve the extra arguments:

function myFunction(a, b) {
const args =, myFunction.length);
myFunction(1, 2, 3, 4, 5); //Output "3, 4, 5"

This can be done in a much easier and cleaner way, by using the rest parameter. The following example demonstrates to use the rest parameter:

function myFunction(a, b, ...args) {
    console.log(args); //Output "3, 4, 5"
myFunction(1, 2, 3, 4, 5);

The arguments object is not an array object. Therefore, to do array operations on the arguments object, you need to convert it to an array. The rest parameter is easy to work with.


What is the ... token called? The ... token is called the spread operator or rest parameter, depending on where and how it's used.