path.extname for file extensions in Node.js - Step-by-Step Execution

Concept Flow - path.extname for file extensions

Input: file path string

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Find last '.' in string

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If '.' found and not last char

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Extract substring from '.' to end

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Return file extension

The function takes a file path string, finds the last dot, and returns the substring from that dot to the end as the file extension. If no dot or dot is last, it returns an empty string.

Execution Sample

Node.js

const path = require('path');
const ext = path.extname('folder/file.txt');
console.log(ext);

This code extracts the file extension from 'folder/file.txt' and prints it.

Execution Table

Step	Input String	Action	Result	Output
1	'folder/file.txt'	Find last '.'	Index 11
2	Index 11	Check if '.' is last char	No, last char index 14
3	Index 11	Extract substring from '.' to end	'.txt'
4	'.txt'	Return extension	'.txt'	'.txt'
5	'folder/file'	Find last '.'	No '.' found
6	No '.'	Return empty string	''	''
7	'file.'	Find last '.'	Index 4
8	Index 4	Check if '.' is last char	Yes, last char index 4
9	'.' at end	Return empty string	''	''

💡 Execution stops after returning the extension or empty string depending on input.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	After Step 9
inputString	'folder/file.txt'	'folder/file.txt'	'folder/file.txt'	'folder/file.txt'	'folder/file'	'file.'
lastDotIndex	undefined	11	11	11	-1	4
extension	''	''	'.txt'	'.txt'	''	''

Key Moments - 2 Insights

Why does path.extname return an empty string when the file ends with a dot?

Because the function checks if the dot is the last character (see Step 8). If yes, it returns an empty string since no extension follows the dot.

What happens if the input string has no dot at all?

The function finds no dot (Step 5), so it returns an empty string (Step 6). This means no extension is present.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at Step 4 for input 'folder/file.txt'?

A '.txt'

B 'txt'

C ''

D '.file'

💡 Hint

Check the 'Output' column at Step 4 in the execution_table.

At which step does path.extname return an empty string for input 'folder/file'?

A Step 3

B Step 9

C Step 6

D Step 1

💡 Hint

Look for the step where no '.' is found and empty string is returned.

If the input is 'file.', what does path.extname return according to the table?

A '.file'

B ''

C '.'

D 'file'

💡 Hint

See Steps 7-9 where the dot is last character and empty string is returned.

Concept Snapshot

path.extname(pathString)
- Finds last '.' in pathString
- Returns substring from '.' to end as extension
- Returns '' if no '.' or '.' is last char
- Useful to get file extensions
- Example: path.extname('file.txt') returns '.txt'

Full Transcript

The path.extname function takes a file path string and finds the last dot character. If a dot exists and is not the last character, it extracts and returns the substring from that dot to the end, which is the file extension. If no dot is found or the dot is the last character, it returns an empty string. For example, 'folder/file.txt' returns '.txt', 'folder/file' returns '', and 'file.' returns ''. This helps identify file extensions safely.