CJs

CJS most commonly refers to CommonJS, a project and set of specifications for creating portable, server-side JavaScript modules. Initiated in 2009 by the CommonJS Initiative, it aimed to standardize how JavaScript code is authored, loaded, and packaged outside the browser. The core idea of CommonJS is a synchronous, single-file module format in which each module runs in its own scope and communicates through a defined interface. A module typically uses require('dep') to load dependencies and exposes its public API by assigning to module.exports or to exports.

In practice, CommonJS is best known for its use in Node.js, which implements CommonJS module semantics as the primary module system for server-side JavaScript. This has led to widespread adoption in npm packages and many tooling ecosystems. The standard contrasts with browser-oriented module systems such as AMD and with the modern JavaScript ES modules (ESM), which use import/export and often asynchronous loading. In recent years, Node and the broader ecosystem have increasingly supported ES modules, but CommonJS remains widely used, especially in existing projects and packages.

The .cjs file extension is used in Node.js to explicitly denote a file containing CommonJS modules, distinguishing it from ES modules that use .mjs or .js with a type: module designation. While the CommonJS module format is mature, it has not become a universal browser standard, which has led developers to rely on bundlers and transpilers to target browsers.