A .cjs file is a JavaScript file used in the CommonJS module system, a standard for organizing and sharing JavaScript code, particularly in server-side environments like Node.js. Unlike the more modern ECMAScript modules (ESM) which use .mjs extensions, CommonJS modules use the require() function to import dependencies and module.exports (or exports) to expose functionality. This system allows developers to break down large applications into smaller, reusable modules, improving code organization, maintainability, and reusability. CommonJS was instrumental in the early development of Node.js and remains widely used in existing Node.js projects. While ESM is now the preferred standard, understanding CommonJS is crucial for working with legacy codebases and certain Node.js environments. The .cjs extension explicitly signals that the file should be treated as a CommonJS module, ensuring proper loading and execution by the JavaScript runtime. It helps avoid ambiguity, especially in environments where both CommonJS and ESM modules are supported.