ES Modules import and export in Node.js - Mini Project: Build & Apply

ES Modules import and export

📖 Scenario: You are building a small Node.js project that uses ES Modules to organize code. You want to split your code into two files: one for utility functions and one for the main program.

🎯 Goal: Create a module file that exports a function, then import and use that function in the main file using ES Modules syntax.

📋 What You'll Learn

Create a file utils.js that exports a function called greet

The greet function takes a name parameter and returns a greeting string

Create a file main.js that imports the greet function from utils.js

Call the greet function in main.js with the argument 'Alice'

💡 Why This Matters

🌍 Real World

Organizing code into modules helps keep projects clean and reusable. ES Modules are the standard way to share code in modern JavaScript and Node.js.

💼 Career

Understanding ES Modules is essential for JavaScript developers working on scalable applications, libraries, or any code that benefits from modular design.

1

Create the utils module with a greet function

Create a file called utils.js and write a function named greet that takes a parameter name and returns the string `Hello, ${name}!`. Export this function using export function greet(name) syntax.

Node.js

# Your code here

Hint

Use export function greet(name) { ... } to export the function directly.

2

Create the main file and import greet

Create a file called main.js. Import the greet function from ./utils.js using import { greet } from './utils.js' syntax.

Node.js

# Your code here

Hint

Use import { greet } from './utils.js'; to import the function.

3

Call the greet function with 'Alice'

In main.js, call the imported greet function with the argument 'Alice' and assign the result to a variable named message.

Node.js

import { greet } from './utils.js';

# Your code here

Hint

Use const message = greet('Alice'); to call the function and store the result.

4

Export the message variable from main.js

In main.js, export the variable message using export { message } syntax so it can be used by other modules.

Node.js

import { greet } from './utils.js';

const message = greet('Alice');

# Your code here

Hint

Use export { message }; to export the variable.

Practice

1. What is the main purpose of using export and import in Node.js ES Modules?

easy

A. To create new variables inside a file

B. To run JavaScript code faster in Node.js

C. To convert JavaScript code into another language

D. To share code between different files by exporting and importing parts

Solution

1. Step 1: Understand the role of export

   The export keyword allows parts of a file (like functions or variables) to be shared with other files.

2. Step 2: Understand the role of import

   The import keyword is used to bring those shared parts into another file to use them.

3. Final Answer:

   To share code between different files by exporting and importing parts -> Option D

4. Quick Check:

   Export and import = share code [OK]

Hint: Export shares code, import uses it elsewhere [OK]

Common Mistakes:

• Thinking export/import changes code speed
• Confusing export with variable creation
• Believing export/import converts languages

2. Which of the following is the correct syntax to export a function named greet in an ES Module file?

easy

A. export function greet() {}

B. module.exports = greet;

C. export = greet;

D. exports.greet = function() {}

Solution

1. Step 1: Identify ES Module export syntax

   In ES Modules, use export function functionName() {} to export a function.

2. Step 2: Check other options for CommonJS syntax

   Options B and D use CommonJS style, not ES Modules. export = greet; is invalid syntax.

3. Final Answer:

   export function greet() {} -> Option A

4. Quick Check:

   ES Modules export function = export function [OK]

Hint: Use 'export function' for ES Modules exports [OK]

Common Mistakes:

• Using CommonJS syntax in ES Modules
• Writing invalid export syntax
• Confusing export default with named export

3. Given the following files, what will be logged when running node main.js?

// utils.js
export const value = 5;
export function double(x) { return x * 2; }

// main.js
import { value, double } from './utils.js';
console.log(double(value));

medium

A. 10

B. 5

C. undefined

D. SyntaxError

Solution

1. Step 1: Understand the imports and exports

   The file utils.js exports a constant value = 5 and a function double that multiplies input by 2.

2. Step 2: Trace the code in main.js

   It imports value and double, then calls double(value) which is double(5), returning 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   double(5) = 10 [OK]

Hint: Import exports correctly, then call functions with values [OK]

Common Mistakes:

• Forgetting to add .js extension in import
• Confusing export default with named exports
• Expecting value instead of double(value)

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

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

// app.js
import { add } from './math';
console.log(add(2, 3));

medium

A. Function add is not exported correctly

B. Cannot import named exports with curly braces

C. Missing file extension in import statement

D. Using CommonJS syntax in ES Modules

Solution

1. Step 1: Check import statement syntax

   The import statement uses import { add } from './math'; but misses the .js extension required in Node.js ES Modules.

2. Step 2: Confirm export syntax is correct

   The function add is correctly exported with export function add(), so no error there.

3. Final Answer:

   Missing file extension in import statement -> Option C

4. Quick Check:

   Node.js ES Modules need file extensions [OK]

Hint: Always include .js extension in ES Module imports [OK]

Common Mistakes:

• Omitting file extensions in import paths
• Confusing CommonJS and ES Module syntax
• Assuming named exports don't use curly braces

5. You have two files:

// data.js
const secret = 'hidden';
export const visible = 'shown';
export default function getSecret() { return secret; }

// index.js
import getSecret, { visible } from './data.js';
console.log(visible);
console.log(getSecret());

What will be the output when running node index.js?

hard

A. SyntaxError due to mixed import syntax

B. shownhidden

C. shownundefined

D. hiddenshown

Solution

1. Step 1: Understand default and named exports

   getSecret is the default export function returning secret. visible is a named export with value 'shown'.

2. Step 2: Analyze import and console.log calls

   import getSecret, { visible } correctly imports default and named exports. Logging visible prints 'shown'. Calling getSecret() returns 'hidden'.

3. Final Answer:

   shown
   hidden -> Option B

4. Quick Check:

   Default + named import works = shown, hidden [OK]

Hint: Default import first, named imports in braces [OK]

Common Mistakes:

• Mixing default and named imports incorrectly
• Expecting secret variable to be exported directly
• Confusing order of import syntax