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)

9. Event-Driven Programming and Built-In Modules

Activity 13: Building an Event-Driven Module


Perform the following steps to complete this activity:

  1. Import the events module:
    const EventEmitter = require('events');
  2. Create the SmokeDetector class that extends EventEmitter and set batteryLevel to 10:
    class SmokeDetector extends EventEmitter {
        constructor() {
            this.batteryLevel = 10;

    In our constructor, because we are extending the EventEmitter class and we are assigning a custom property, batteryLevel, we will need to call super inside the constructor and set batteryLevel to 10.

  3. Create a test method inside the SmokeDetector class that will test the battery level and emit a low battery message in the event that the battery is low:
    test() {
            if (this...