Book Image

Professional JavaScript

By : Hugo Di Francesco, Siyuan Gao, Vinicius Isola, Philip Kirkbride
Book Image

Professional JavaScript

By: Hugo Di Francesco, Siyuan Gao, Vinicius Isola, Philip Kirkbride

Overview of this book

In depth knowledge of JavaScript makes it easier to learn a variety of other frameworks, including React, Angular, and related tools and libraries. This book is designed to help you cover the core JavaScript concepts you need to build modern applications. You'll start by learning how to represent an HTML document in the Document Object Model (DOM). Then, you'll combine your knowledge of the DOM and Node.js to create a web scraper for practical situations. As you read through further lessons, you'll create a Node.js-based RESTful API using the Express library for Node.js. You'll also understand how modular designs can be used for better reusability and collaboration with multiple developers on a single project. Later lessons will guide you through building unit tests, which ensure that the core functionality of your program is not affected over time. The book will also demonstrate how constructors, async/await, and events can load your applications quickly and efficiently. Finally, you'll gain useful insights into functional programming concepts such as immutability, pure functions, and higher-order functions. By the end of this book, you'll have the skills you need to tackle any real-world JavaScript development problem using a modern JavaScript approach, both for the client and server sides.
Table of Contents (12 chapters)

7. Advanced JavaScript

Activity 8: Creating a User Tracker


  1. Open the Activity08.js file and define logUser. It will add the user to the userList argument. Make sure no duplicates are added:
    function logUser(userList, user) {
    if(!userList.includes(user)) {

    Here, we used an includes method to check whether the user already exists. If they don't, they will be added to our list.

  2. Define userLeft. It will remove the user from the userList argument. If the user doesn't exist, it will do nothing:
    function userLeft(userList, user) {
    const userIndex = userList.indexOf(user);
    if (userIndex >= 0) {
        userList.splice(userIndex, 1);

    Here, we are using indexOf to get the current index of the user we want to remove. If the item doesn't exist, indexOf will return –1, so we are only using splice to remove the item if it exists.

  3. Define numUsers, which returns the number of users currently inside the list...