ECMAScript (/ˈɛkməskrɪpt/; ES) is a standard for scripting languages, including JavaScript, JScript, and ActionScript. It is best known as a JavaScript standard intended to ensure the interoperability of web pages across different web browsers. It is standardized by Ecma International in the document ECMA-262.

ECMAScript is commonly used for client-side scripting on the World Wide Web, and it is increasingly being used for server-side applications and services using runtime environments such as Node.js, Deno and Bun.

ECMA-262, or the ECMAScript Language Specification, defines the ECMAScript Language, or just ECMAScript. ECMA-262 specifies only language syntax and the semantics of the core application programming interface (API), such as Array, Function, and globalThis, while valid implementations of JavaScript add their own functionality such as input/output and file system handling.

The ECMAScript specification is a standardized specification of a scripting language developed by Brendan Eich of Netscape; initially named Mocha, then LiveScript, and finally JavaScript. In December 1995, Sun Microsystems and Netscape announced JavaScript in a press release. In November 1996, Netscape announced a meeting of the Ecma International standards organization to advance the standardization of JavaScript. The first edition of ECMA-262 was adopted by the Ecma General Assembly in June 1997. Several editions of the language standard have been published since then. The name "ECMAScript" was a compromise between the organizations involved in standardizing the language, especially Netscape and Microsoft, whose disputes dominated the early standards sessions. Eich commented that "ECMAScript was always an unwanted trade name that sounds like a skin disease." ECMAScript has been formalized through operational semantics by work at Stanford University and the Department of Computing, Imperial College London for security analysis and standardization. "ECMA" stood for "European Computer Manufacturers Association" until 1994.

Ecma's Technical Committee 39 (TC39) is responsible for the maintenance of ECMAScript. New proposals to the language go through a staged process, with each stage representing the completeness of the proposal's specification. Consensus must be reached within the committee to advance a proposal to the next stage. Proposals that reach stage 4, the final stage, will be included into the next version of the standard. Since the release of version 6 in June 2015, new major versions have been finalized and published every June.

The ECMAScript language includes structured, dynamic, functional, and prototype-based features.

ECMAScript provides a clear and consistent syntax for defining variables, functions, expressions, and control flow constructs such as conditionals and loops. This structure allows developers to write organized and maintainable code.

In ECMAScript, variables are not bound to a specific data type. A variable can hold any type of data, and its type can change during program execution. This flexibility simplifies rapid development but requires careful handling to avoid type-related errors.