CommonJS Modules: require and module.exports

CommonJS is a module system primarily designed for the Node.js environment. It allows splitting code into smaller files and reusing them across different parts of an application. CommonJS became the de-facto standard for Node.js applications and was widely adopted before the introduction of ES Modules (ESM).

In CommonJS, each file is a separate module with its own scope. A module can export data, functions, or objects, which can then be imported into other files.

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How CommonJS Works

In CommonJS, the require() function is used to import modules, while the module.exports object is used for exporting.

  
// greet.js file
function greet(name) {
  return `Hello, ${name}!`;
}

module.exports = greet;
  

  
// main.js file
const greet = require('./greet');
console.log(greet('Ani')); // Hello, Ani!
  

As we can see, the greet.js file exports the greet function via module.exports, and main.js imports it using the require() function.

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The Difference Between module.exports and exports

CommonJS provides two ways to export: module.exports and exports. Initially, exports is just a reference to module.exports. However, caution is required:

  
// EXAMPLE: Correct usage
exports.greet = function(name) {
  return `Hello, ${name}!`;
};
  

  
// EXAMPLE: Incorrect usage
exports = function(name) {
  return `Hello, ${name}!`;
};
  

In the second example, the exports reference is reassigned to a new function, and Node.js won’t know what to export. Therefore, if you want to export the entire module, always use module.exports.

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Key Features of CommonJS Modules

• Synchronous Import — The require() function imports modules synchronously.
• Caching — Once a module is imported, it is cached, so subsequent require() calls return the same object.
• Primarily Used in Node.js — CommonJS is tightly coupled with Node.js’s server-side environment.

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Limitations of CommonJS

• Does Not Work in Browsers — CommonJS modules are designed for servers and don’t work directly in browsers without bundlers (e.g., Webpack).
• Synchronous Nature — Modules in browsers should ideally be loaded asynchronously, but CommonJS does not support this natively.

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Conclusion

CommonJS was the first widely adopted module system in JavaScript. It works exceptionally well in Node.js and remains a crucial part of server-side applications. In the next article, we’ll explore ES Modules—the modern, official module system for JavaScript.