String.prototype.indexOf()

indexOf(searchString, fromIndex?)

Returns: number · Added in ves6 · Updated March 13, 2026 · String Methods

javascript es6 string search

The indexOf() method searches for the first occurrence of a substring within a string, starting from the beginning. It returns the index position where the substring starts, or -1 if the substring is not found. This is one of the most commonly used string methods in JavaScript for finding substrings.

Syntax

string.indexOf(searchString)
string.indexOf(searchString, fromIndex)

searchString: The substring to search for. If omitted, indexOf() returns 0 when searching for an empty string, or -1 otherwise.
fromIndex (optional): The index to start searching from. Defaults to 0. Negative values are treated as 0.
Returns: A number representing the index of the first occurrence, or -1 if not found.

TypeScript Signature

interface String {
  indexOf(searchString: string, fromIndex?: number): number;
}

Basic Examples

const text = "hello world";

console.log(text.indexOf("hello"));  // 0 (starts at index 0)
console.log(text.indexOf("world"));  // 6
console.log(text.indexOf("!"));      // -1 (not found)
console.log(text.indexOf("o"));      // 4 (first occurrence)

Finding Multiple Occurrences

To find all occurrences of a substring, you need a loop:

const text = "hello hello hello";
const search = "hello";
let positions = [];
let idx = text.indexOf(search);

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

console.log(positions);  // [0, 6, 12]

Using the fromIndex Parameter

The second parameter controls where the search starts:

const text = "hello world hello";

console.log(text.indexOf("hello"));        // 0 (searches from start)
console.log(text.indexOf("hello", 1));    // 6 (skips first occurrence)
console.log(text.indexOf("world", 10));  // -1 (starts past where world is)

Practical Use Case: Checking for Keywords

A common pattern is to check if a string contains a word or phrase:

function containsKeyword(text, keyword) {
  return text.indexOf(keyword) !== -1;
}

console.log(containsKeyword("JavaScript is great", "Script")); // true
console.log(containsKeyword("JavaScript is great", "Type"));    // false

Case Sensitivity

The indexOf() method is case sensitive:

const text = "JavaScript JavaScript";

console.log(text.indexOf("Java"));    // 0
console.log(text.indexOf("java"));    // -1 (lowercase does not match)
console.log(text.indexOf("SCRIPT"));  // -1 (uppercase does not match)

// For case-insensitive search, convert to lowercase first
const lower = text.toLowerCase();
console.log(lower.indexOf("java"));   // 0

Case-Insensitive Search Helper

function indexOfIgnoreCase(text, search) {
  return text.toLowerCase().indexOf(search.toLowerCase());
}

console.log(indexOfIgnoreCase("Hello WORLD", "hello")); // 0
console.log(indexOfIgnoreCase("Hello WORLD", "WORLD")); // 6

Edge Cases

const text = "hello";

// Empty string returns fromIndex (or 0)
console.log(text.indexOf(""));         // 0
console.log(text.indexOf("", 3));     // 3

// Negative fromIndex is treated as 0
console.log(text.indexOf("h", -5));    // 0

// fromIndex greater than string length
console.log(text.indexOf("h", 100));   // -1

// Search string longer than target
console.log(text.indexOf("hello world")); // -1

indexOf() vs includes()

Method	Returns	Use Case
`indexOf()`	`number` (index or -1)	When you need the position
`includes()`	`boolean`	When you only need to check existence

const text = "hello world";

console.log(text.indexOf("world"));   // 6 (found, returns position)
console.log(text.includes("world"));  // true (just checks existence)

Browser Compatibility

Browser	Version
Chrome	1+
Firefox	1+
Safari	1+
Edge	12+
Node.js	0.1.0+

Universal support across all modern and legacy JavaScript environments.