Lesson 1
Last Updated : February, 2023
Javascript - Program the Web
This course covers the basics of programming in Javascript.

JavaScript Overview

JavaScript is a high-level programming language that is widely used for creating interactive and dynamic web pages. It is a scripting language that allows you to create and control dynamic website content, validate user input, and create animations, among other things. JavaScript is a client-side language, which means it runs on the user’s browser, unlike server-side languages like PHP or Python.

Some of the key features of JavaScript include:

Dynamic web content: JavaScript allows you to create dynamic and interactive web pages that can respond to user actions and events.

Object-oriented programming: JavaScript is an object-oriented language, which means you can create reusable code in the form of objects and classes.

Event-driven programming: JavaScript is an event-driven language, which means it can execute code in response to user actions and events.

Cross-platform compatibility: JavaScript runs on all modern web browsers, making it a popular choice for web development.

Here’s an example of a basic JavaScript program that displays a message on the web page:

// Define a function to display a message
function showMessage() {
  var message = "Hello, world!";
  document.getElementById("output").innerHTML = message;

// Call the function when the page loads
window.onload = showMessage;

In this example, we define a function called showMessage() that sets the value of an HTML element with the ID “output” to the message “Hello, world!”. We then call this function when the page loads using the window.onload event.

JavaScript can also be used to validate user input, create animations, and interact with web APIs, among other things. It is a versatile language that is essential for modern web development.

How JavaScript Runs

JavaScript is a programming language that is executed by a browser or server. When a web page loads, the browser reads the HTML and CSS files and builds the Document Object Model (DOM) and the CSS Object Model (CSSOM) respectively. Once the DOM and CSSOM have been constructed, the browser executes any JavaScript code that is present on the page.

JavaScript code can be included in a web page in several ways. One way is to include it directly in the HTML file using the <script> tag. Another way is to include it in a separate JavaScript file and link to it in the HTML file using the <script> tag.

When the browser encounters a script tag, it downloads the script file and executes it. JavaScript is an interpreted language, meaning that it is executed line by line, in order. The browser reads each line of code, interprets it, and executes it. If there are any errors in the code, the browser will stop executing the script and display an error message.

JavaScript is a single-threaded language, which means that it can only execute one line of code at a time. However, JavaScript is also asynchronous, which means that it can execute code out of order when certain conditions are met. This is often used in conjunction with callback functions and promises to handle asynchronous operations such as network requests or user input.

In summary, JavaScript runs in a browser or server environment and is executed line by line. It can be included in a web page using the <script> tag or a separate file. It is a single-threaded language, but can handle asynchronous operations using callback functions and promises.