string.indexOf

The indexOf() method returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex. Returns -1 if the value is not found.

Syntax

str.indexOf(searchValue[, fromIndex])

Parameters

searchValue

A string representing the value to search for.

fromIndex Optional

The index at which to start the searching forwards in the string. It can be any integer. The default value is 0. If fromIndex <= 0 the entire string is searched. If fromIndex >= str.length, the string is not searched and -1 is returned. Unless searchValue is an empty string, then str.length is returned.

Return value

The index of the first occurrence of the specified value; -1 if not found.

Description

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character of a string called stringName is stringName.length - 1.

'Blue Whale'.indexOf('Blue');     // returns  0
'Blue Whale'.indexOf('Blute');    // returns -1
'Blue Whale'.indexOf('Whale', 0); // returns  5
'Blue Whale'.indexOf('Whale', 5); // returns  5
'Blue Whale'.indexOf('');         // returns  0
'Blue Whale'.indexOf('', 9);      // returns  9
'Blue Whale'.indexOf('', 10);     // returns 10
'Blue Whale'.indexOf('', 11);     // returns 10

Case-sensitivity

The indexOf() method is case sensitive. For example, the following expression returns -1:

'Blue Whale'.indexOf('blue'); // returns -1

Checking occurrences

Note that '0' doesn't evaluate to true and '-1' doesn't evaluate to false. Therefore, when checking if a specific string exists within another string the correct way to check would be:

'Blue Whale'.indexOf('Blue') !== -1; // true
'Blue Whale'.indexOf('Bloe') !== -1; // false

Examples

Using indexOf() and lastIndexOf()

The following example uses indexOf() and lastIndexOf() to locate values in the string "Brave new world".

var anyString = 'Brave new world';

console.log('The index of the first w from the beginning is ' + anyString.indexOf('w'));
// logs 8
console.log('The index of the last w from the beginning is ' + anyString.lastIndexOf('w'));
// logs 10

console.log('The index of "new" from the beginning is ' + anyString.indexOf('new'));
// logs 6
console.log('The index of "new" from the end is ' + anyString.lastIndexOf('new'));
// logs 6

indexOf() and case-sensitivity

The following example defines two string variables. The variables contain the same string except that the second string contains uppercase letters. The first console.log() method displays 19. But because the indexOf() method is case sensitive, the string "cheddar" is not found in myCapString, so the second console.log() method displays -1.

var myString    = 'brie, pepper jack, cheddar';
var myCapString = 'Brie, Pepper Jack, Cheddar';

console.log('myString.indexOf("cheddar") is ' + myString.indexOf('cheddar'));
// logs 19
console.log('myCapString.indexOf("cheddar") is ' + myCapString.indexOf('cheddar'));
// logs -1

Using indexOf() to count occurrences of a letter in a string

The following example sets count to the number of occurrences of the letter e in the string str:

var str = 'To be, or not to be, that is the question.';
var count = 0;
var pos = str.indexOf('e');

while (pos !== -1) {
  count++;
  pos = str.indexOf('e', pos + 1);
}

console.log(count); // displays 4

Specifications

Specification	Status	Comment
ECMAScript 1st Edition (ECMA-262)	Standard	Initial definition.
ECMAScript 5.1 (ECMA-262) The definition of 'String.prototype.indexOf' in that specification.	Standard
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'String.prototype.indexOf' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'String.prototype.indexOf' in that specification.	Draft

Browser compatibility

See also

• String.prototype.charAt()
• String.prototype.lastIndexOf()
• String.prototype.split()
• Array.prototype.indexOf()