includes()

ES6+

Determines whether one string may be found within another string.

Syntax

string.includes(searchString, position)

Parameters

searchString string

The string to search for

position number optional

Position to start searching from

Return Value

boolean

true if the string is found, otherwise false

Examples

JavaScript

const str = 'Hello World';
console.log(str.includes('World'));
console.log(str.includes('world'));

Output:

// true false

📌 When to Use

Use includes() for simple substring existence checks when you only need a boolean result. It provides cleaner and more readable code than indexOf() !== -1 for conditional logic.

⚠️ Common Mistakes

Forgetting that includes() is case-sensitive - "Hello".includes("hello") returns false. Use toLowerCase() on both strings for case-insensitive search.

Using includes() when you actually need the position - includes() returns boolean only, use indexOf() if you need the position.

Not understanding that includes() does not support regex patterns - use match() or search() for pattern matching.

✅ Best Practices

Use includes() in if statements for cleaner conditional code: if (str.includes(search)) instead of if (str.indexOf(search) !== -1).

Combine with toLowerCase() for case-insensitive checks: str.toLowerCase().includes(search.toLowerCase()).

Use the position parameter when you want to search from a specific index, useful for finding subsequent occurrences.

⚡ Performance Notes

includes() and indexOf() have nearly identical performance as they both scan the string from the starting position. For multiple substring checks, consider building a Set or using regex alternation for better performance.

🌍 Real World Example

Content Filtering and Search Features

includes() is commonly used for search functionality, filtering lists, content moderation, and checking if URLs or strings contain specific patterns.

// Simple search filter for products
function searchProducts(products, query) {
  const lowerQuery = query.toLowerCase();
  return products.filter(product =>
    product.name.toLowerCase().includes(lowerQuery) ||
    product.description.toLowerCase().includes(lowerQuery)
  );
}

const products = [
  { name: 'iPhone 15', description: 'Apple smartphone' },
  { name: 'Galaxy S24', description: 'Samsung smartphone' },
  { name: 'MacBook Pro', description: 'Apple laptop' }
];

console.log(searchProducts(products, 'apple'));
// Returns iPhone 15 and MacBook Pro

// Check if URL is from allowed domains
function isAllowedUrl(url, allowedDomains) {
  return allowedDomains.some(domain =>
    url.toLowerCase().includes(domain.toLowerCase())
  );
}

const allowed = ['example.com', 'trusted.org'];
console.log(isAllowedUrl('https://api.example.com/data', allowed)); // true
console.log(isAllowedUrl('https://malicious.com/data', allowed)); // false

// Content moderation - check for banned words
function containsBannedWords(text, bannedWords) {
  const lowerText = text.toLowerCase();
  return bannedWords.some(word =>
    lowerText.includes(word.toLowerCase())
  );
}

const banned = ['spam', 'scam', 'fake'];
console.log(containsBannedWords('This is a scam!', banned)); // true

Related Methods

indexOf() startsWith() endsWith()