path.extname for file extensions in Node.js - Mini Project: Build & Apply

Using path.extname to Extract File Extensions in Node.js

📖 Scenario: You are building a simple Node.js script to help organize files by their extensions. To do this, you need to extract the file extension from each filename.

🎯 Goal: Create a Node.js script that uses path.extname to get the file extension from a list of filenames.

📋 What You'll Learn

Create an array called files with the exact filenames: 'report.pdf', 'photo.jpg', 'archive.zip', 'notes.txt'

Create a variable called extToFind and set it to '.jpg'

Use a for loop with the variable file to iterate over files and create an array extensions that stores the extension of each file using path.extname(file)

Add a final line that exports the extensions array using module.exports = extensions

💡 Why This Matters

🌍 Real World

Extracting file extensions is useful when organizing files, validating uploads, or processing files differently based on type.

💼 Career

Understanding how to use Node.js built-in modules like path and how to manipulate arrays is essential for backend JavaScript development.

1

Create the list of filenames

Create an array called files with these exact filenames: 'report.pdf', 'photo.jpg', 'archive.zip', 'notes.txt'

Node.js

// Your code here

Hint

Use square brackets [] to create an array and separate items with commas.

2

Add a variable for the extension to find

Create a variable called extToFind and set it to the string '.jpg'

Node.js

const files = ['report.pdf', 'photo.jpg', 'archive.zip', 'notes.txt'];
// Your code here

Hint

Use const to declare a variable and assign the string '.jpg'.

3

Extract extensions using path.extname in a loop

Use a for loop with the variable file to iterate over files. Inside the loop, use path.extname(file) to get the extension and store all extensions in an array called extensions.

Node.js

const files = ['report.pdf', 'photo.jpg', 'archive.zip', 'notes.txt'];
const extToFind = '.jpg';

// Your code here

Hint

Remember to require('path') at the top. Use extensions.push(path.extname(file)) inside the loop.

4

Export the extensions array

Add a final line that exports the extensions array using module.exports = extensions

Node.js

const path = require('path');
const files = ['report.pdf', 'photo.jpg', 'archive.zip', 'notes.txt'];
const extToFind = '.jpg';

const extensions = [];
for (const file of files) {
    extensions.push(path.extname(file));
}

// Your code here

Hint

Use module.exports = extensions to make the array available outside this file.

Practice

1. What does path.extname('example.txt') return in Node.js?

easy

A. '.txt'

B. 'txt'

C. 'example.txt'

D. '' (empty string)

Solution

1. Step 1: Understand what path.extname does

   The path.extname method extracts the file extension including the dot from a filename string.

2. Step 2: Apply to 'example.txt'

   Since the file name is 'example.txt', the extension is '.txt' including the dot.

3. Final Answer:

   '.txt' -> Option A

4. Quick Check:

   Extension includes dot = '.txt' [OK]

Hint: Remember extname returns extension with dot [OK]

Common Mistakes:

• Forgetting the dot in the extension
• Returning the whole filename
• Returning empty string for files with extension

2. Which of the following is the correct syntax to get the extension of a file named 'photo.jpeg' using Node.js path module?

easy

A. path.extname('photo.jpeg')

B. path.extension('photo.jpeg')

C. path.getExt('photo.jpeg')

D. path.ext('photo.jpeg')

Solution

1. Step 1: Recall the correct method name

   The Node.js path module provides the method extname to get file extensions.

2. Step 2: Check the syntax

   The correct syntax is path.extname('filename'). Other options are invalid method names.

3. Final Answer:

   path.extname('photo.jpeg') -> Option A

4. Quick Check:

   Correct method is extname() [OK]

Hint: Use path.extname() exactly as named [OK]

Common Mistakes:

• Using incorrect method names like extension or ext
• Missing parentheses after method name
• Confusing with other path methods

3. What will be the output of the following code?

const path = require('path');
console.log(path.extname('archive.tar.gz'));

medium

A. '.tar.gz'

B. '' (empty string)

C. 'tar.gz'

D. '.gz'

Solution

1. Step 1: Understand how extname handles multiple dots

   The path.extname method returns the substring from the last dot to the end of the string.

2. Step 2: Apply to 'archive.tar.gz'

   The last dot is before 'gz', so the extension returned is '.gz'.

3. Final Answer:

   '.gz' -> Option D

4. Quick Check:

   Extension is from last dot = '.gz' [OK]

Hint: extname returns from last dot to end [OK]

Common Mistakes:

• Assuming it returns multiple extensions
• Returning empty string for multiple dots
• Including the whole suffix after first dot

4. Identify the error in this code snippet:

const path = require('path');
const ext = path.extname('document');
console.log(ext);

medium

A. It returns 'document' as extension

B. It throws an error because 'document' has no extension

C. It returns an empty string because no extension exists

D. It returns undefined

Solution

1. Step 1: Check the filename for extension

   The filename 'document' has no dot, so no extension exists.

2. Step 2: Understand extname behavior with no extension

   path.extname returns an empty string when no extension is found, not an error or undefined.

3. Final Answer:

   It returns an empty string because no extension exists -> Option C

4. Quick Check:

   No dot means empty string [OK]

Hint: No dot means extname returns empty string [OK]

Common Mistakes:

• Expecting an error for missing extension
• Expecting undefined instead of empty string
• Confusing filename with extension

5. You want to filter an array of filenames to only include '.js' files. Which code snippet correctly uses path.extname to do this?

const path = require('path');
const files = ['app.js', 'index.html', 'script.ts', 'readme'];
const jsFiles = files.filter(???);
console.log(jsFiles);

hard

A. file => path.extname(file) === ''

B. file => path.extname(file) === '.js'

C. file => path.extname(file) === '.JS'

D. file => path.extname(file) === 'js'

Solution

1. Step 1: Understand the filter condition

   We want to keep files whose extension is exactly '.js' including the dot and case-sensitive.

2. Step 2: Use path.extname correctly

   The correct comparison is path.extname(file) === '.js'. Other options either miss the dot, use wrong case, or check for empty extension.

3. Final Answer:

   file => path.extname(file) === '.js' -> Option B

4. Quick Check:

   Filter by exact '.js' extension [OK]

Hint: Compare extname result including dot and case [OK]

Common Mistakes:

• Omitting the dot in extension comparison
• Ignoring case sensitivity
• Filtering by empty string instead of '.js'