CommonJS require and module.exports in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does require() do in CommonJS?

require() loads and runs a module file, returning the exported values from that module.

beginner

How do you export a single function or object in CommonJS?

You assign it to module.exports. For example: module.exports = myFunction;

intermediate

What is the difference between exports and module.exports?

exports is a shortcut to module.exports. Reassigning exports breaks the link, so only module.exports controls what is exported.

beginner

Can you require a JSON file using require() in CommonJS?

Yes, require() can load JSON files and returns the parsed object.

intermediate

What happens if you call require() multiple times for the same module?

The module is loaded once and cached. Subsequent require() calls return the cached exports, not re-running the module code.

Which statement correctly exports a function named sayHello in CommonJS?

A exports = sayHello;

B module.exports = sayHello;

C export default sayHello;

D require('sayHello');

Only module.exports = sayHello; correctly exports the function. Reassigning exports breaks the export link.

What does require('./math') return if math.js uses module.exports = { add, subtract }?

A Undefined

B A string with the file path

C An object with <code>add</code> and <code>subtract</code> functions

D A function named <code>math</code>

The require() call returns the object assigned to module.exports, which contains the add and subtract functions.

If you want to export multiple values, which is the best way in CommonJS?

A Use <code>require()</code> inside the module

B Call <code>exports = value1, value2</code>

C Use <code>export { value1, value2 }</code>

D Assign an object to <code>module.exports</code> with all values

Assigning an object with multiple properties to module.exports is the standard way to export multiple values.

What happens if you do exports = { a: 1 } instead of module.exports = { a: 1 }?

A The module exports remain unchanged (empty object)

B The module exports the object { a: 1 }

C An error is thrown

D The module exports a function

Reassigning exports breaks the link to module.exports, so the exported object stays as the default empty object.

How does CommonJS handle circular dependencies between modules?

A It returns a partially loaded module object during the cycle

B It throws an error immediately

C It ignores the second require call

D It reloads the module twice

CommonJS returns a partially loaded module object during circular dependencies to avoid infinite loops.

Explain how require() and module.exports work together in CommonJS modules.

Think about how you share code between files.

Describe the difference between exports and module.exports and why it matters.

Consider what happens if you assign a new object to exports.

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