Let’s understand the difference between CJS & MJS

The terms CJS (CommonJS) and MJS (ES Module) refer to two module systems used in JavaScript to organize code into reusable components. Here’s a comparison between the two:

1. CommonJS (CJS)

Syntax: CommonJS uses require() to load modules …


This content originally appeared on DEV Community and was authored by Syed Ammar

The terms CJS (CommonJS) and MJS (ES Module) refer to two module systems used in JavaScript to organize code into reusable components. Here's a comparison between the two:

1. CommonJS (CJS)

  • Syntax: CommonJS uses require() to load modules and module.exports or exports to export them.
  • Used in: It’s the module system primarily used in Node.js prior to the introduction of ES modules.
  • Synchronous Loading: CommonJS modules are loaded synchronously, meaning they block execution until the module is loaded. This is ideal for server-side applications but less suitable for client-side code where async loading is preferred.

  • Example:

     // Import
 const fs = require('fs');

 // Export
 module.exports = function () {
   console.log("Hello from CJS");
 };

2. ES Modules (MJS)

  • Syntax: ES Modules use import and export statements.
  • Used in: Modern JavaScript environments, both in browsers and Node.js (with the .mjs extension or using "type": "module" in package.json).
  • Asynchronous Loading: ES modules are asynchronously loaded, which is better suited for client-side environments.

  • Example:

     // Import
 import fs from 'fs';

 // Export
 export function greet() {
   console.log("Hello from MJS");
 }

Key Differences:

  1. Loading Mechanism:

    • CJS: Modules are loaded synchronously.
    • MJS: Modules are loaded asynchronously, which makes them non-blocking and more efficient in certain scenarios (especially in the browser).

  2. Syntax:

    • CJS: Uses require() and module.exports.
    • MJS: Uses import and export.

  3. Compatibility:

    • CJS: Widely supported in Node.js, but less compatible with browsers (without bundlers).
    • MJS: Native support in modern browsers and Node.js (from version 12+), aligning with the ES6 module standard.

  4. Default Exports:

    • CJS: Can export an object or a function directly as a module.
    • MJS: Supports both named and default exports, allowing more flexibility in exporting multiple functions or values.

When to Use:

  • CJS (CommonJS): If working with older Node.js projects or existing libraries that are based on the CommonJS module system.
  • MJS (ES Modules): When building modern applications, especially for client-side development or Node.js projects that target modern runtimes.

In modern development, ES Modules are becoming the standard, but many legacy projects still rely on CommonJS.


This content originally appeared on DEV Community and was authored by Syed Ammar


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