JavaScript is an interpreted programming language that is mostly used to turn static web pages into dynamic and interactive pages after a web browser has finished downloading a web page. For example, a clock displayed on a page that updates itself to show the current time on the user's computer. JavaScript's design was influenced by many programming languages, including C, but is meant to be more usable by non-programmers. JavaScript is not based on or related to Java; this is a common misunderstanding.

JavaScript is often included inside an HTML file or linked to from an HTML file and run locally by a web browser. This means that servers are free to work on something else rather than processing instructions for each client. This has made JavaScript a more popular choice than languages that require a server to do the processing.