indexOf()

ES3+

Returns the index of the first occurrence of a specified value in a string.

Syntax

string.indexOf(searchValue, fromIndex)

Parameters

searchValue string

The value to search for

fromIndex number optional

Position to start searching from

Return Value

number

The index of the first occurrence, or -1 if not found

Examples

JavaScript

const str = 'Hello World';
console.log(str.indexOf('o'));
console.log(str.indexOf('x'));

Output:

// 4 -1

📌 When to Use

Use indexOf() when you need both to check for substring existence AND know its position. For simple existence checks, prefer includes(). Use indexOf() for parsing, extracting portions of strings, or finding multiple occurrences.

⚠️ Common Mistakes

Using indexOf() !== -1 for simple boolean checks when includes() is more readable and expresses intent better.

Forgetting that indexOf() returns -1 (not null, undefined, or false) when the substring is not found.

Not using the fromIndex parameter when searching for multiple occurrences, resulting in infinite loops or incorrect results.

✅ Best Practices

Use indexOf() when you need the position for subsequent operations like slice() or substring().

To find all occurrences, use a while loop with fromIndex: indexOf(search, lastIndex + 1).

Consider using search() with regex when you need case-insensitive matching or pattern-based searching.

⚡ Performance Notes

indexOf() uses an optimized string search algorithm in modern engines. For very long strings with many searches, building an index or using more specialized algorithms may be beneficial. The fromIndex parameter can significantly improve performance when searching for subsequent occurrences.

🌍 Real World Example

String Parsing and Extraction

indexOf() is essential for parsing strings to extract data, finding delimiters, and processing text formats like email addresses, URLs, or structured data.

// Parse email into username and domain
function parseEmail(email) {
  const atIndex = email.indexOf('@');
  if (atIndex === -1) {
    return null;
  }
  return {
    username: email.slice(0, atIndex),
    domain: email.slice(atIndex + 1)
  };
}

console.log(parseEmail('user@example.com'));
// { username: 'user', domain: 'example.com' }

// Find all occurrences of a substring
function findAllOccurrences(str, search) {
  const positions = [];
  let index = str.indexOf(search);

  while (index !== -1) {
    positions.push(index);
    index = str.indexOf(search, index + 1);
  }

  return positions;
}

console.log(findAllOccurrences('banana', 'a')); // [1, 3, 5]

// Extract query string parameters
function getQueryParam(url, param) {
  const queryStart = url.indexOf('?');
  if (queryStart === -1) return null;

  const query = url.slice(queryStart + 1);
  const paramStart = query.indexOf(param + '=');
  if (paramStart === -1) return null;

  const valueStart = paramStart + param.length + 1;
  const valueEnd = query.indexOf('&', valueStart);
  return valueEnd === -1
    ? query.slice(valueStart)
    : query.slice(valueStart, valueEnd);
}

console.log(getQueryParam('https://example.com?name=John&age=30', 'name'));
// 'John'

Related Methods

includes() search()