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.
str.indexOf(searchValue[, fromIndex]
)
searchValue
fromIndex
fromIndex < 0
the entire string is searched (same as passing 0). If fromIndex >= str.length
, the method will return -1 unless searchValue
is an empty string in which case str.length
is returned.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('', 9); // returns 9 'Blue Whale'.indexOf('', 10); // returns 10 'Blue Whale'.indexOf('', 11); // returns 10
The indexOf()
method is case sensitive. For example, the following expression returns -1:
'Blue Whale'.indexOf('blue'); // returns -1
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
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 first w from the end 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-sensitivityThe following example defines two string variables. The variables contain the same string except that the second string contains uppercase letters. The first 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
indexOf()
to count occurrences of a letter in a stringThe 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
Created by Mozilla Contributors, license: CC-BY-SA 2.5