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Introduction to JavaScript

What is JavaScript?

JavaScript is a programming language initially designed to interact with elements of web pages. In web browsers, JavaScript consists of three main parts:

  • ECMAScript provides the core functionality.
  • The Document Object Model (DOM) provides interfaces for interacting with elements on web pages
  • The Browser Object Model (BOM) provides the browser API for interacting with the web browser.

JavaScript allows you to add interactivity to a web page. Typically, you use JavaScript with HTML and CSS to enhance a web page’s functionality, such as validating forms, creating interactive maps, and displaying animated charts.

When a web page is loaded, i.e., after downloading HTML and CSS, the web browser’s JavaScript engine executes the JavaScript code. The JavaScript code then modifies the HTML and CSS to update the user interface dynamically.

The JavaScript engine is a program that executes JavaScript code. In the beginning, JavaScript engines were implemented as interpreters.

However, modern JavaScript engines are typically implemented as just-in-time compilers that compile JavaScript code to bytecode for improved performance.

Client-side vs. Server-side JavaScript

When JavaScript is used on a web page, it is executed in web browsers. In this case, JavaScript works as a client-side language.

JavaScript can run on both web browsers and servers. A popular JavaScript server-side environment is Node.js. Unlike client-side JavaScript, server-side JavaScript executes on the server that allows you to access databases, file systems, etc.

JavaScript History

In 1995, JavaScript was created by a Netscape developer named Brendan Eich. First, its name was Mocha. And then, its name was changed to LiveScript.

Netscape decided to change LiveScript to JavaScript to leverage Java’s fame, which was popular. The decision was made just before Netscape released its web browser product Netscape Navigator 2. As a result, JavaScript entered version 1.0.

Netscape released JavaScript 1.1 in Netscape Navigator 3. In the meantime, Microsoft introduced a web browser product called Internet Explorer 3 (IE 3), which competed with Netscape. However, IE came with its own JavaScript implementation called JScript. Microsoft used the name JScript to avoid possible license issues with Netscape.

Hence, two different JavaScript versions were in the market:

  • JavaScript in Netscape Navigator
  • JScript in Internet Explorer.

JavaScript had no standards that governed its syntax and features. And the community decided that it was time to standardize the language.

In 1997, JavaScript 1.1 was submitted to the European Computer Manufacturers Association (ECMA) as a proposal. Technical Committee #39 (TC39) was assigned to standardize the language to make it a general-purpose, cross-platform, and vendor-neutral scripting language.

TC39 developed ECMA-262, a standard for defining a new scripting language named ECMAScript (often pronounced Ek-ma-script).

After that, the International Organization for Standardization and International Electrotechnical Commissions (ISO/IEC) adopted ECMAScript (ISO/IEC-16262).

JavaScript overview

To define a variable in JavaScript, you use the var keyword. For example:

var x = 10;
var y = 20; 

ES6 added a new way to declare a variable with the let keyword:

let x = 10;
let y = 20; 

There are differences between var and let. And it’s a good practice to use the let keyword to declare variables.

To declare a function, you use the function keyword. The following example defines a function that calculates the sum of two arguments:

function add( a, b ) {
   return a + b;

To call the add() function, you use the following syntax:

let result = add(x, y);

To log the result into the console window of the web browser, you use the console.log() :


Now, you should see 30 in the console window.

JavaScript provides you with condition statements such as if-else and switch statements. For example:

let a = 20,
    b = 30;

function divide(a, b) {
    if(b == 0) {
       throw 'Division by zero';
    return a / b;

In the divide() function, we check whether the de-numerator (b) is zero. If yes, we throw an exception. Otherwise, we return the result of a / b.

To declare an array, you use the following syntax:

let items = []; 

To declare an array with some initial elements, you specify the elements in the square brackets:

let items = [1, 2, 3];

You can access the number of elements in the items array through its length property:

// 3

To iterate over the elements of the items array, you use the for loop statement as follows:

for(let i = 0; i < items.length; i++) {

Or use the for...of loop in ES6:

for(let item of items) {

JavaScript is an evolving language. It has many other features that you’ll learn in the following Chapter.

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