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Understanding JavaScript Modules: .mjs, .cjs, and .js

Overview​

Understanding the differences between .mjs, .cjs, and .js files is crucial for working effectively with JavaScript modules, especially when dealing with module resolution and transpiling. This document provides a brief overview and links to external resources for in-depth understanding.

Quick Reference​

  • MJS (.mjs): Use when you want to ensure a file is treated as an ECMAScript module, especially in projects that mix module systems.
  • CJS (.cjs): Use when you need a file to be explicitly treated as a CommonJS module in an ESM project.
  • JS (.js): Use with a consistent "type" field in your package.json to define the module system for your project.

Key Concepts​

  1. Module Resolution: Understanding how Node.js and build tools resolve modules to avoid conflicts and errors.
  2. Transpiling: Using tools like Babel when you need to support environments that don't natively understand your chosen module system.

External Resources​

Official Documentation​

Understanding ECMAScript Modules​

Blogs and Articles​

Transpiling and Bundling​

Checklist for Implementation​

  • Understand the differences between MJS, CJS, and JS modules
  • Choose the appropriate module system for your project
  • Configure package.json "type" field if using .js files
  • Set up transpiling tools if needed for target environments
  • Ensure consistent module usage across the project
  • Test module resolution in different environments

Notes​

This documentation serves as a starting point for understanding JavaScript modules. For specific implementation details or best practices, always refer to the most up-to-date official documentation and consider the specific requirements of your project.