What Is JavaScript?
JavaScript as the Language of the Web
What is JavaScript? It is a programming language that adds behavior to web pages. HTML gives a page its structure and CSS controls its appearance, while JavaScript responds to user actions, updates content without reloading the page and sends data to and from servers.
JavaScript was created in 1995 for the Netscape browser. Today it is the only programming language that all major browsers run natively, so code written for it works without installing anything on a visitor's device.
JavaScript is not the same as Java, which is a separate language. The names are similar for historical reasons, but the two languages differ in design and use.
ECMAScript and Where JavaScript Runs
The language is standardized as ECMAScript, a specification maintained by Ecma International. New features are added in yearly editions, and browsers and runtimes implement them over time. In everyday use, people say JavaScript for both the language and its standard.
Browsers run JavaScript on the visitor's device, which is called client-side code. JavaScript can also run on servers, most commonly through Node.js, where it powers web servers, APIs and build tools. The same language can therefore be used on both sides of a website. Developers who want extra safety in large projects often use TypeScript, covered in TypeScript vs JavaScript.
Example: A Contact Form Checking Input
A contact form on a website uses JavaScript to check that an email address looks valid before the form is sent. The check happens instantly in the browser, so the visitor sees a message without waiting for a server reply.
The server still checks the data again, because code running in a browser can be changed by the visitor. The browser check improves the experience, while the server check protects the site.
Frequently asked questions
Is JavaScript the same as HTML or CSS?
No. HTML is a markup language for page structure, CSS styles that structure, and JavaScript adds logic and interaction. Each handles a separate job, and most websites use all three together. The comparison is explained in HTML vs CSS vs JavaScript.
Do I need to install JavaScript to use it?
Not to run it in a web page, because every browser includes a JavaScript engine. To run JavaScript outside a browser, such as on a server or in a command line tool, you need a runtime like Node.js. This is why hosting for JavaScript applications can differ from basic shared hosting.