CommonJS require and module.exports in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this CommonJS module import?

Consider two files:
math.js:

module.exports = { add: (a, b) => a + b };

app.js:

const math = require('./math');
console.log(math.add(2, 3));

What will be printed when running node app.js?

Node.js

module.exports = { add: (a, b) => a + b };

const math = require('./math');
console.log(math.add(2, 3));

A undefined

B 5

C TypeError: math.add is not a function

D NaN

💡 Hint

Remember that module.exports defines what is returned by require.

The math.js exports an object with an add function. Requiring it returns that object, so math.add(2, 3) returns 5.

❓ Predict Output

intermediate

What error does this code raise?

Given the file greet.js:

exports = function() { return 'Hello'; };

And app.js:

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

What happens when running node app.js?

Node.js

exports = function() { return 'Hello'; };

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

A TypeError: greet is not a function

B Hello

C SyntaxError

D undefined

💡 Hint

Assigning to exports alone does not change module.exports.

Assigning directly to exports breaks the link to module.exports, so require returns an empty object. Calling greet() causes a TypeError.

❓ component_behavior

advanced

How does module caching affect this code?

Consider counter.js:

let count = 0;
module.exports.increment = () => ++count;
module.exports.get = () => count;

And app.js:

const c1 = require('./counter');
const c2 = require('./counter');

c1.increment();
c2.increment();
console.log(c1.get());

What is the output when running node app.js?

Node.js

let count = 0;
module.exports.increment = () => ++count;
module.exports.get = () => count;

const c1 = require('./counter');
const c2 = require('./counter');

c1.increment();
c2.increment();
console.log(c1.get());

A 2

B TypeError

C 0

D 1

💡 Hint

Modules are cached and shared across requires.

Both c1 and c2 reference the same cached module instance, so increments accumulate. After two increments, count is 2.

📝 Syntax

advanced

Which option correctly exports a single function?

You want to export a single function from logger.js so that requiring it returns that function directly. Which code does this correctly?

A exports = function(msg) { console.log(msg); };

B exports.log = function(msg) { console.log(msg); };

C module.exports.log = function(msg) { console.log(msg); };

D module.exports = function(msg) { console.log(msg); };

💡 Hint

To export a single function, assign it to module.exports directly.

Assigning a function directly to module.exports makes require return that function. Other options export an object or break the export.

🔧 Debug

expert

Why does this module export fail to work as expected?

Given data.js:

const data = { value: 42 };
exports = data;

And app.js:

const imported = require('./data');
console.log(imported.value);

What is the output and why?

Node.js

const data = { value: 42 };
exports = data;

const imported = require('./data');
console.log(imported.value);

A TypeError because imported is not an object

B 42 because exports points to data object

C undefined because exports reassignment does not change module.exports

D SyntaxError due to invalid exports assignment

💡 Hint

Remember the difference between exports and module.exports references.

Reassigning exports breaks the link to module.exports, so require returns the original empty object, making imported.value undefined.

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