CommonJS require and module.exports in Node.js

Introduction

CommonJS lets you split your code into files and use them together easily. It helps organize code by sharing functions or data between files.

You want to use code from another file in your Node.js project.

You want to share a function or object with other parts of your app.

You want to keep your code clean by separating features into different files.

You are working on a Node.js app that does not use ES modules.

You want to load modules synchronously in your Node.js scripts.

Syntax

Node.js

const moduleName = require('modulePath');

module.exports = valueToExport;

Use require() to load a module or file.

Use module.exports to share what a file provides.

Examples

Load a local file named math.js from the same folder.

Node.js

const math = require('./math');

Export a function named add from a file.

Node.js

module.exports = function add(a, b) {
  return a + b;
};

Export an object with data.

Node.js

module.exports = {
  name: 'Alice',
  age: 30
};

Load a built-in Node.js module named fs.

Node.js

const fs = require('fs');

Sample Program

This example shows how to export a function from math.js and use it in app.js. The app.js file loads the add function and prints the result of adding 5 and 7.

Node.js

// math.js
function add(a, b) {
  return a + b;
}

module.exports = add;

// app.js
const add = require('./math');

console.log(add(5, 7));

Important Notes

The require function loads modules synchronously, so it pauses code until the module is ready.

You can export any value with module.exports: functions, objects, strings, numbers, etc.

File paths in require for local files need ./ or ../ to work correctly.

Summary

CommonJS uses require() to load modules and module.exports to share code.

It helps organize Node.js code by splitting it into reusable files.

Use relative paths with require for your own files, and module names for built-in or installed packages.

Practice

1. What does module.exports do in a Node.js file?

easy

A. It deletes the current module from memory.

B. It imports code from another module.

C. It runs the module as a standalone program.

D. It defines what the module shares when required by another file.

Solution

1. Step 1: Understand module.exports role

   module.exports sets the object or value that other files receive when they use require() on this module.

2. Step 2: Differentiate from require()

   require() is used to import, while module.exports is used to export code from a module.

3. Final Answer:

   It defines what the module shares when required by another file. -> Option D

4. Quick Check:

   module.exports = export code [OK]

Hint: Remember: module.exports shares, require() imports [OK]

Common Mistakes:

• Confusing require() with module.exports
• Thinking module.exports runs code
• Assuming module.exports deletes modules

2. Which of the following is the correct syntax to import a local module named utils.js using CommonJS?

easy

A. const utils = require('./utils');

B. const utils = require('utils');

C. import utils from './utils';

D. const utils = import('./utils');

Solution

1. Step 1: Identify local module import syntax

   Local files require a relative path starting with './' or '../' in require().

2. Step 2: Check each option

   const utils = require('./utils'); uses require('./utils'), which correctly imports the local utils.js file. const utils = require('utils'); misses './', so it looks for a package. import utils from './utils'; uses ES module syntax, not CommonJS. const utils = import('./utils'); uses dynamic import, not CommonJS.

3. Final Answer:

   const utils = require('./utils'); -> Option A

4. Quick Check:

   Local modules need './' in require() [OK]

Hint: Use './' prefix for local files in require() [OK]

Common Mistakes:

• Omitting './' for local modules
• Using ES module import syntax in CommonJS
• Using import() instead of require()

3. Given the following two files, what will be logged when node app.js runs?

// math.js
module.exports.add = (a, b) => a + b;
module.exports.sub = (a, b) => a - b;

// app.js
const math = require('./math');
console.log(math.add(5, 3));
console.log(math.sub(5, 3));

medium

A. undefined and undefined

B. 8 and 2

C. Error: add is not a function

D. 5 and 3

Solution

1. Step 1: Understand exports in math.js

   math.js exports two functions: add and sub, which add and subtract two numbers.

2. Step 2: Trace app.js calls

   app.js requires math.js and calls math.add(5, 3) which returns 8, and math.sub(5, 3) which returns 2.

3. Final Answer:

   8 and 2 -> Option B

4. Quick Check:

   5+3=8 and 5-3=2 [OK]

Hint: Check exported function names and call with correct args [OK]

Common Mistakes:

• Expecting undefined because of wrong export syntax
• Confusing module.exports with exports shorthand
• Forgetting to require the module

4. What is the error in the following code snippet?

// greet.js
exports = function() { return 'Hello'; };

// app.js
const greet = require('./greet');
console.log(greet());

medium

A. Cannot find module './greet'.

B. SyntaxError due to missing module.exports.

C. greet is not a function because exports was overwritten incorrectly.

D. No error; it logs 'Hello'.

Solution

1. Step 1: Analyze exports assignment in greet.js

   Assigning directly to exports replaces the local exports variable but does not change module.exports, so require() gets an empty object.

2. Step 2: Understand require() result in app.js

   Since module.exports was not changed, greet is an empty object, not a function, so calling greet() causes an error.

3. Final Answer:

   greet is not a function because exports was overwritten incorrectly. -> Option C

4. Quick Check:

   Overwrite exports breaks module.exports [OK]

Hint: Always assign to module.exports, not exports directly [OK]

Common Mistakes:

• Assigning function directly to exports instead of module.exports
• Expecting exports and module.exports to be the same after reassignment
• Ignoring that require() returns module.exports

5. You want to export a single class from a module so that requiring it returns the class directly. Which is the correct way to do this in CommonJS?

class User {
  constructor(name) {
    this.name = name;
  }
}

// What should you write here?

hard

A. module.exports = User;

B. exports.User = User;

C. module.exports.User = User;

D. export default User;

Solution

1. Step 1: Understand exporting a single value

   To export a single class so require() returns it directly, assign it to module.exports.

2. Step 2: Compare options

   module.exports = User; assigns User directly to module.exports, so require('./module') returns the class. module.exports.User = User; and exports.User = User; export an object with User property, so require() returns an object, not the class itself. export default User; uses ES module syntax, invalid in CommonJS.

3. Final Answer:

   module.exports = User; -> Option A

4. Quick Check:

   Single export = module.exports = value [OK]

Hint: Assign single export directly to module.exports [OK]

Common Mistakes:

• Using exports.User instead of module.exports for single export
• Mixing ES module syntax with CommonJS
• Expecting require() to return class when exporting as property