WebIDL

Web IDL, short for Web Interfaces Description Language, is a formal language used to describe the interfaces of web platform APIs and how they are exposed to JavaScript in browsers. It provides a precise, machine-readable description of objects, their methods, properties, constants, and the types they use. Web IDL specifications describe the surface area of web APIs so that browser implementations can generate the appropriate bindings to the JavaScript environment.

The language defines several core concepts, including interfaces, interface mixins, dictionaries, enums, typedefs, and callbacks. Interfaces declare members such as operations (methods) and attributes (properties with getters and optional setters). Dictionaries describe structured argument bundles with required and optional fields. Enums specify a fixed set of string values, while callbacks define function types. Inheritance between interfaces is allowed, and the type system includes primitive types like boolean, octet, short, long, float, double, and DOMString, as well as sequences and other user-defined types.

Usage and role in the web ecosystem: Web IDL files appear in web specifications to declare the API surface that web pages interact with. Browser vendors implement these interfaces in their engines, using the IDL descriptions to generate language bindings and ensure interoperability with JavaScript. The language serves as a stable contract across implementations, guiding API evolution while enabling consistent behavior for developers.

Relationship to JavaScript and the web platform: Web IDL is not a programming language for applications; it is a descriptive language used by standards bodies and browser developers to define APIs. It underpins how the DOM, Web Platform features, and other browser APIs are exposed to scripts, providing a common foundation for cross-browser compatibility.