CommonJSModule

CommonJSModule refers to a module that follows the CommonJS module specification, a standard that originated in the CommonJS project to enable modular JavaScript, particularly in server-side environments such as Node.js. A CommonJS module exports its API by assigning to module.exports or by attaching properties to exports. The module system is designed around synchronous loading and a clear, file-based scope.

When a CommonJS module runs, its code executes inside a wrapper function that provides five local variables: require, module, exports, __filename, and __dirname. The exports object is initially identical to module.exports, and developers commonly attach exported properties to exports or reassign module.exports to export a single function or object.

Loading a module happens via require. The resolver can handle core built-in modules, relative file paths, and installed packages. Modules are cached after their first evaluation, so subsequent requires return the same exported value without re-execution. This caching, along with the synchronous loading model, influences how modules respond to dependencies, including the potential for partially initialized values in circular dependencies.

CommonJSModules are widely used in Node.js and have influenced many bundlers and tooling for browser environments, though they differ from the ES Modules standard which uses import/export syntax. While effective for server-side modularity, CommonJS modules require environments or tooling that provide a compatible require function and module wrapper, or transpilation for broader browser use.