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
. IffromIndex <= 0
the entire string is searched. IffromIndex >= str.length
, the string is not searched and-1
is returned. UnlesssearchValue
is an empty string, thenstr.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
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |