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Definition: JavaScript


A widely used programming language that is embedded within the HTML of Web pages. Developed in 1995 and supported by all Web browsers, JavaScript enables interactive functions to be added to Web pages, which are otherwise static. JavaScript evolved from Netscape's LiveScript language, not from Java.

HTML5
As of HTML Version 5, JavaScript was formalized as part of HTML, thus turning HTML from a presentation language into a programming language. See HTML5.

JavaScript Is Plain Text
JavaScript source code is plain text that resides between Begin-Script and End-Script HTML tags in the Web page. Along with myriad other functions, JavaScript is used to enable interactive page elements such as navigation buttons and drop-down menus. It is also used to identify the page to analytics servers that capture traffic statistics. See JScript, Dojo and VBScript.

JavaScript Vs. Java
Because JavaScript is executed merely by retrieving a page from any website, it cannot be used to access local files and probe the user's computer (JavaScript is "sandboxed"). Java, on the other hand, is a full-blown programming language that can manipulate any resource in the computer. Although there are commonalities, JavaScript is not a subset of Java, but they both may be used in the same Web-based system. For example, JavaScript could display a data entry form and validate the input, while a Java program in the server processes the information. In addition, JavaScript remains as human-readable text, while Java source code is converted into a binary format (see bytecode). See Java and servlet.

A Low-Level Replacement for JavaScript
JavaScript is an interpreted language, which means the source code remains as readable text the way the programmer wrote it. A runtime engine has to analyze (parse) the text statements before it can execute them. Twenty years after the creation of JavaScript, WebAssembly was created to provide a lower-level bytecode language that is executed much faster than the source code (see WebAssembly).




JavaScript Is Always Visible
JavaScript code is embedded in a Web page and easily viewed as in this Opera browser on a Mac. The blue text is the HTML page rendering language, and the brown is JavaScript. In this example, the JavaScript causes navigation buttons to change color when the mouse rolls over them and to jump to another page when clicked. See HTML.