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.