CommonJS require and module.exports in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the 'fs' module using CommonJS require.

Node.js

const fs = [1]('fs');

Drag options to blanks, or click blank then click option'

Arequire

Bimport

Cinclude

Dload

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'import' instead of 'require'

Using 'include' or 'load' which are not valid in CommonJS

In CommonJS, require is used to import modules.

2fill in blank

medium

Complete the code to export a function named 'greet' using module.exports.

Node.js

module.exports = [1];

Drag options to blanks, or click blank then click option'

Agreet()

Bgreet

C{ greet }

Dfunction greet()

Attempts:

3 left

💡 Hint

Common Mistakes

Using greet() which calls the function instead of exporting it

Using braces which exports an object, not the function itself

You assign the function name directly to module.exports to export it.

3fill in blank

hard

Fix the error in importing the default export from 'math.js' using require.

Node.js

const add = [1]('./math.js');

Drag options to blanks, or click blank then click option'

Afetch

Bimport

Crequire

Dinclude

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'import' which is ES Modules syntax

Using 'fetch' or 'include' which are invalid here

Use require to import modules in CommonJS, not import.

4fill in blank

hard

Fill both blanks to export an object with two functions, 'start' and 'stop'.

Node.js

module.exports = { [1], [2] };

Drag options to blanks, or click blank then click option'

Astart

Bstop

Cbegin

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong function names that do not exist

Forgetting to export both functions

Export an object with keys matching the function names you want to export.

5fill in blank

hard

Fill all three blanks to import 'readFile', export a function 'writeFile', and assign it to module.exports.

Node.js

const { [1] } = require('fs');

function [2](path, data) {
  // write data to file
}

module.exports = [3];

Drag options to blanks, or click blank then click option'

AreadFile

BwriteFile

DreadFileSync

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up imported and exported function names

Not exporting the function correctly

You import readFile from 'fs', define writeFile function, and export it.

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