Get The Extension - FreeCodeCamp Daily Challenge

Extension Extractor - FreeCodeCamp Daily Challenge

This problem challenges us to extract the extension from a given filename. We’ll explore an efficient step-by-step solution, handling special cases and common edge cases.

📋 Problem Statement

Extension Extractor

Given a string representing a filename, return its extension.

Specifications:

• The extension is the part of the name that comes after the last dot (.)
• If the file contains no dot or ends with a dot, return "none"
• The extension must preserve case sensitivity

🧠 Problem Analysis

Understanding the Problem

We need to identify and extract a file’s extension. However, several special cases must be handled:

Resolution Strategy

The solution is based on three fundamental steps:

1. Locate the last dot in the filename
2. Validate that a valid extension exists
3. Extract the corresponding substring

flowchart TD
    A[Start: Receive filename] --> B[Find last dot]
    B --> C{Dot exists?}
    C -->|No dot found| D[Return 'none']
    C -->|Yes| E{Dot at end?}
    E -->|Final dot| D
    E -->|Valid| F[Extract substring after dot]
    F --> G[Return extension]

    style A fill:#e1f5ff
    style D fill:#ffe1e1
    style G fill:#e1ffe1

🛠️ Solution Development

JavaScript Implementation

function getExtension(filename) {
  // Find the position of the last dot
  const lastDotIndex = filename.lastIndexOf('.')

  // Validate cases where there's no valid extension
  if (lastDotIndex === -1 || lastDotIndex === filename.length - 1) {
    return 'none'
  }

  // Extract and return the extension
  return filename.substring(lastDotIndex + 1)
}

Detailed Code Explanation

1. Last dot search

const lastDotIndex = filename.lastIndexOf('.')

The lastIndexOf('.') method traverses the string from end to beginning, returning the position of the last dot found, or -1 if it doesn’t exist.

2. Special case validation

if (lastDotIndex === -1 || lastDotIndex === filename.length - 1) {
  return 'none';
}

• lastDotIndex === -1: No dot exists in the name
• lastDotIndex === filename.length - 1: The dot is the last character

3. Extension extraction

return filename.substring(lastDotIndex + 1);

substring(lastDotIndex + 1) extracts from the position after the dot to the end of the string.

📊 Test Cases

Complete Test Suite

// Case 1: Standard extension
console.log(getExtension('document.txt')) // 'txt'

// Case 2: No extension
console.log(getExtension('README')) // 'none'

// Case 3: Case preservation
console.log(getExtension('image.PNG')) // 'PNG'

// Case 4: File starting with dot (Unix hidden file)
console.log(getExtension('.gitignore')) // 'gitignore'

// Case 5: Multiple dots in name
console.log(getExtension('archive.tar.gz')) // 'gz'

// Case 6: Ends with dot
console.log(getExtension('final.draft.')) // 'none'

// Case 7: Single character extension
console.log(getExtension('script.c')) // 'c'

// Case 8: Long extension
console.log(getExtension('backup.backup')) // 'backup'

Results Table

📈 Complexity Analysis

Time Complexity: O(n)

• lastIndexOf('.'): O(n) - traverses the string once
• substring(): O(k) where k is the extension length
• Worst case: O(n)

Space Complexity: O(k)

• Where k is the extension length
• substring() creates a new string
• Worst case: O(n) if the entire string is the extension

Optimization

This solution is already optimal. We cannot improve time complexity because we need to examine the string at least once to find the last dot.

🎯 Alternative Solutions

Alternative 1: Using split()

function getExtensionAlt1(filename) {
  const parts = filename.split('.')

  if (parts.length === 1 || filename.endsWith('.')) {
    return 'none'
  }

  return parts[parts.length - 1]
}

Advantages: More declarative code Disadvantages: Less efficient (O(n) additional space)

Alternative 2: Using Regular Expressions

function getExtensionAlt2(filename) {
  const match = filename.match(/\.([^.]+)$/)
  return match ? match[1] : 'none'
}

Advantages: Compact and expressive Disadvantages: May be less readable for some developers

💡 Key Concepts Learned

1. lastIndexOf() is ideal for searching from the end
2. Early validation prevents errors and simplifies logic
3. substring() is efficient for extracting string parts
4. Edge cases are crucial in string manipulation problems

🤔 Final Reflections

This problem illustrates the importance of:

• Identifying special cases before implementing
• Choosing the right tools (lastIndexOf vs split)
• Writing defensive code that handles all scenarios

String manipulation is fundamental in programming. Mastering methods like lastIndexOf(), substring(), and understanding when to use them makes the difference between robust and fragile code.

🔗 Additional Resources

• MDN: String.prototype.lastIndexOf()
• MDN: String.prototype.substring()
• FreeCodeCamp JavaScript Algorithms