MJS

mjs commonly refers to the .mjs file extension used to designate JavaScript files that use the ECMAScript module (ESM) format. In practice, .mjs signals that a file should be treated as an ES module rather than a traditional CommonJS module.

In Node.js, the .mjs extension was introduced to signal ESM before broader support was standardized. Historically, Node.js treated .js as CommonJS by default, so developers used .mjs to opt into ESM. Modern Node.js can treat JavaScript files as modules via package.json with "type": "module" or by using the .mjs extension, and some packages also use the .cjs extension for CommonJS. Browsers load ES modules via script type="module" and can import modules from files with the .mjs extension in many setups, though relative extensionless imports may be resolved by bundlers or other tooling.

ESM supports import and export syntax, and in supported environments it enables features such as top-level await and strict module scope. Static imports are resolved at parse time, while dynamic import('path') returns a promise. Tooling such as bundlers and transpilers often treat .mjs as indicating ES module semantics to ensure proper module loading and scoping.

Usage and interoperability: while .mjs remains in use, many projects opt into ES modules by configuring package.json with "type": "module", which allows using .js for ESM in accordance with the project setup. The .mjs extension is one indicator of module format, alongside configuration-driven approaches that unify behavior across different environments and tooling. See also ECMAScript modules, CommonJS, and JavaScript module extensions.