ES Modules import and export in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of ES Modules in Node.js?

ES Modules allow you to organize and reuse code by importing and exporting functions, objects, or values between files in a clear and standardized way.

beginner

How do you export a single function as the default export in an ES Module?

Use export default functionName; to export a single function as the default export from a module.

beginner

What is the syntax to import a default export from another module?

Use <code>import anyName from './module.js';</code> where <code>anyName</code> is your chosen name for the default export.

intermediate

How do you export multiple named values from a module?

Use <code>export const name1 = value1;</code> or <code>export { name1, name2 };</code> to export multiple named values.

intermediate

How can you import multiple named exports from a module?

Use <code>import { name1, name2 } from './module.js';</code> to import specific named exports.

Which keyword is used to export a default value in ES Modules?

A require

B export const

C module.exports

D export default

The export default keyword exports a single default value from a module.

How do you import a named export called calculate from a module?

A import { calculate } from './module.js';

B import calculate from './module.js';

C const calculate = require('./module.js');

D export { calculate } from './module.js';

Named exports are imported using curly braces: import { calculate } from './module.js';.

What file extension should you use for ES Modules in Node.js by default?

A .mjs

B .cjs

C .js

D .json

The .mjs extension explicitly marks a file as an ES Module in Node.js.

Which statement is true about mixing CommonJS and ES Modules in Node.js?

A You can freely mix <code>require</code> and <code>import</code> in the same file.

B ES Modules can import CommonJS modules, but CommonJS cannot import ES Modules directly.

C CommonJS modules are the same as ES Modules.

D You must rename all files to .cjs to use ES Modules.

ES Modules can import CommonJS modules, but CommonJS modules cannot directly import ES Modules without special handling.

How do you export a variable named count as a named export?

A export default count;

B import count from './module.js';

C export { count };

D module.exports = count;

Use export { count }; to export count as a named export.

Explain how to export and import both default and named values using ES Modules in Node.js.

Think about how you share one main thing versus multiple things from a file.

Describe the difference between ES Modules and CommonJS modules in Node.js and how they interact.

Consider the keywords and compatibility between the two module systems.

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