An MJS file is a source code file containing an ES Module (ECMAScript Module) for use with a Node.js application. MJS files are written in JavaScript, and may also use the .JS extension outside of the Node.js context. ES Modules allow web and application developers to organize code into smaller reusable components.

ECMAScript 6 (ES6) introduced the specification for ES Modules, providing a standard for implementing modules in JavaScript. As of 2018, all major web browsers support ES Modules. However, the popularity of modularized JavaScript pre-dates ES6. Node.js, a JavaScript runtime environment, used CommonJS as the specification for modules. Because so many existing applications were built with CommonJS, when Node.js added support for native ES modules, it controversially introduced the MJS file extension to differentiate the two and prevent applications from breaking. NOTE: Some developers informally refer to MJS files as "Michael Jackson Script" files.