JavaScript string manipulation is a fundamental skill that every developer encounters regularly, and two methods that often cause confusion are substring() and substr(). While they might seem interchangeable at first glance, these methods have distinct behaviors that can lead to unexpected results if not properly understood. This post will break down the key differences between these methods, demonstrate their practical applications, and help you choose the right one for your specific use cases.

How These Methods Work

Both substring() and substr() extract portions of strings, but they interpret their parameters differently. Understanding this fundamental difference is crucial for avoiding bugs in your applications.

The substring() method takes two parameters: a start index and an end index. It returns the portion of the string between these two positions, excluding the character at the end index.

string.substring(startIndex, endIndex)

The substr() method, on the other hand, takes a start index and a length parameter. It returns a substring beginning at the specified index and extending for the given number of characters.

string.substr(startIndex, length)

Here’s a critical point that trips up many developers: substr() is deprecated according to the ECMAScript specification, though it’s still widely supported for backward compatibility. The MDN documentation explicitly recommends avoiding substr() in new code.

Key Differences and Comparison

Feature	substring()	substr()
Second Parameter	End index (exclusive)	Length of substring
Negative Values	Treated as 0	Start index can be negative (counts from end)
Parameter Swapping	Automatically swaps if start > end	No automatic swapping
Standards Status	Standard (ES1)	Deprecated (Legacy)
Browser Support	Universal	Universal but not recommended

Practical Examples and Use Cases

Let’s examine how these methods behave in real-world scenarios with concrete examples:

const text = "Hello, World!";

// Basic usage
console.log(text.substring(0, 5));    // "Hello"
console.log(text.substr(0, 5));       // "Hello"

// Different second parameters
console.log(text.substring(7, 12));   // "World" (positions 7 to 11)
console.log(text.substr(7, 5));       // "World" (start at 7, take 5 chars)

// Negative values
console.log(text.substring(-3, 5));   // "Hello" (negative treated as 0)
console.log(text.substr(-6, 5));      // "World" (starts 6 chars from end)

// Parameter order matters
console.log(text.substring(12, 7));   // "World" (parameters swapped automatically)
console.log(text.substr(12, 7));      // "" (empty string, no chars after position 12)

Common real-world applications include:

• Extracting file extensions: filename.substring(filename.lastIndexOf('.') + 1)
• Truncating text for previews: article.substring(0, 150) + "..."
• Parsing URLs or extracting subdomains
• Processing CSV data or fixed-width text formats

Performance Considerations and Benchmarks

Performance differences between these methods are generally negligible for most applications. However, substring() typically performs slightly better due to better optimization in modern JavaScript engines.

// Performance test example (run in browser console)
const testString = "A".repeat(10000);
const iterations = 100000;

console.time("substring");
for (let i = 0; i < iterations; i++) {
  testString.substring(100, 200);
}
console.timeEnd("substring");

console.time("substr");
for (let i = 0; i < iterations; i++) {
  testString.substr(100, 100);
}
console.timeEnd("substr");

In typical scenarios, both methods execute in microseconds, making performance a non-issue for most applications.

Best Practices and Common Pitfalls

Follow these guidelines to avoid common mistakes and write maintainable code:

• Use substring() for new projects - It's the standard-compliant choice
• Validate your indices - Always check that start and end positions are within bounds
• Consider slice() as an alternative - It handles negative indices more intuitively than substring()
• Document your intentions - When extracting substrings, comment why you chose specific indices

Common pitfalls to watch out for:

const text = "JavaScript";

// Pitfall 1: Confusing length vs end index
// Wrong: trying to get first 4 characters
console.log(text.substring(0, 4));  // "Java" ✓
console.log(text.substr(0, 4));     // "Java" ✓

// Pitfall 2: Negative indices behavior
console.log(text.substring(-3));    // "JavaScript" (not last 3 chars!)
console.log(text.substr(-3));       // "ript" ✓

// Pitfall 3: Parameter order confusion
console.log(text.substring(4, 0));  // "Java" (swapped automatically)
console.log(text.substr(4, 0));     // "" (empty string)

Migration Strategy from substr() to substring()

If you're working with legacy code that uses substr(), here's how to migrate safely:

// Original substr() usage
const original = text.substr(startPos, length);

// Convert to substring()
const converted = text.substring(startPos, startPos + length);

// Handle negative start positions
function safeSubstring(str, start, length) {
  if (start < 0) {
    start = Math.max(0, str.length + start);
  }
  return str.substring(start, start + length);
}

// Usage
const result = safeSubstring("Hello World", -5, 5); // "World"

Alternative Methods and Modern Approaches

Modern JavaScript offers additional string manipulation methods that might better suit your needs:

const text = "Hello, World!";

// slice() - similar to substring but handles negatives like substr
console.log(text.slice(0, 5));     // "Hello"
console.log(text.slice(-6, -1));   // "World"

// Template literals for complex string building
const start = 7;
const end = 12;
const extracted = text.slice(start, end);
const result = `Extracted: "${extracted}" from position ${start} to ${end}`;

// Regular expressions for pattern-based extraction
const emailRegex = /([a-zA-Z0-9._%+-]+)@([a-zA-Z0-9.-]+\.[a-zA-Z]{2,})/;
const email = "contact@example.com";
const [, username, domain] = email.match(emailRegex) || [];
console.log(username); // "contact"

For more complex string operations, consider using utility libraries like Lodash's string methods or the native String prototype methods.

The choice between string manipulation methods ultimately depends on your specific use case, but sticking with standardized methods like substring() and slice() will ensure your code remains maintainable and future-proof. Remember that clarity and consistency in your codebase are more valuable than micro-optimizations in most scenarios.

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