JavaScript

JavaScript is a high-level, often just-in-time compiled programming language that enables interactive features on the World Wide Web. It runs in web browsers and, through environments such as Node.js, on servers and in various runtime contexts. JavaScript is one of the core technologies of the web, alongside HTML and CSS, and is used to create dynamic user interfaces, validate forms, fetch data, and build entire applications.

The language was created in 1995 by Brendan Eich at Netscape; it was originally called Mocha, then LiveScript, and finally JavaScript. It was standardized as ECMAScript by Ecma International, with annual updates. Modern JavaScript (ES6/ES2015 and later) introduced let and const, classes, modules, iterators, generators, Promises, and async/await, among other features.

JavaScript is dynamically typed and uses a single inheritance model based on prototypes. It supports multiple programming styles, including imperative, functional, and object-oriented patterns. It uses first-class functions and closures, and interacts with the browser via the Document Object Model (DOM) and Web APIs. Asynchronous programming is common, with callbacks, promises, and async/await.

In browsers, JavaScript can manipulate the DOM, handle events, and communicate with servers using fetch or XMLHttpRequest. On the server, Node.js provides a non-blocking I/O environment and a rich ecosystem of modules via npm. The language relies on modules (ES modules) and tooling such as transpilers (Babel) and bundlers (Webpack). Security considerations include XSS, same-origin policy, and careful handling of user input.

JavaScript has a large ecosystem of frameworks and libraries (React, Angular, Vue) and continues to evolve with new language features and platform-specific capabilities.