ES Modules import and export in Node.js

Introduction

ES Modules let you share and use code between files easily. They help organize your code like separate boxes you can open and use when needed.

When you want to split your code into smaller, reusable parts.

When you need to use functions or data from another file.

When building a Node.js app that uses modern JavaScript features.

When you want to keep your code clean and easy to maintain.

When sharing code between different projects or files.

Syntax

Node.js

export const name = value;
export function myFunc() {}

import { name, myFunc } from './file.js';

// Default export
export default function() {}
import anyName from './file.js';

Use export to share variables, functions, or classes from a file.

Use import to bring those shared parts into another file.

Examples

Exports a constant named greeting that other files can import.

Node.js

export const greeting = 'Hello';

Exports a function named sayHi.

Node.js

export function sayHi() {
  console.log('Hi!');
}

Imports the greeting constant and sayHi function from messages.js.

Node.js

import { greeting, sayHi } from './messages.js';

Exports a default function without a name. It can be imported with any name.

Node.js

export default function() {
  console.log('Default export');
}

Sample Program

This example shows how to export a function and a constant from one file and import them in another. The app.js file uses the imported parts to print results.

Node.js

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

export const pi = 3.14159;

// file: app.js
import { add, pi } from './mathUtils.js';

console.log('Add 2 + 3 =', add(2, 3));
console.log('Value of pi:', pi);

Important Notes

File extensions like .js are required in import paths in Node.js ES Modules.

Use type: "module" in your package.json to enable ES Modules in Node.js.

Default exports let you import without curly braces, named exports require curly braces.

Summary

ES Modules help organize code by exporting and importing parts between files.

Use export to share code and import to use it elsewhere.

Remember to enable ES Modules in Node.js and include file extensions in imports.

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