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.
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Array.prototype.indexOf()
.Syntax
str.indexOf(searchValue [, fromIndex])
Parameters
searchValue
-
The string value to search for.
If no string is explicitly provided, searchValue will be coerced to "
undefined
", and this value will be searched for instr
.So, for example:
'undefined'.indexOf()
will return0
, asundefined
is found at position0
in the stringundefined
.'undefine'.indexOf()
however will return-1
, asundefined
is not found in the stringundefine
. fromIndex
Optional-
An integer representing the index at which to start the search. Defaults to
0
.For
fromIndex
values lower than0
, or greater thanstr.length
, the search starts at index0
andstr.length
, respectively.For example,
'hello world'.indexOf('o', -5)
will return4
, as it starts at position0
, ando
is found at position4
. On the other hand,'hello world'.indexOf('o', 11)
(and with anyfromIndex
value greater than11
) will return-1
, as the search is started at position11
, a position after the end of the string.
Return value
The index of the first occurrence of searchValue
, or
-1
if not found.
An empty string searchValue
produces strange results. With no
fromIndex
value, or any fromIndex
value
lower than the string's length
, the returned value is the same as the
fromIndex
value:
'hello world'.indexOf('') // returns 0
'hello world'.indexOf('', 0) // returns 0
'hello world'.indexOf('', 3) // returns 3
'hello world'.indexOf('', 8) // returns 8
However, with any fromIndex
value equal to or greater than the
string's length
, the returned value is the string's
length
:
'hello world'.indexOf('', 11) // returns 11
'hello world'.indexOf('', 13) // returns 11
'hello world'.indexOf('', 22) // returns 11
In the former instance, JS seems to find an empty string just after the specified index value. In the latter instance, JS seems to be finding an empty string at the end of the searched string.
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('Whale', 7) // returns -1
'Blue Whale'.indexOf('') // returns 0
'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
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
~('Blue Whale'.indexOf('Bloe')) // 0, which is falsy
Examples
Using indexOf()
The following example uses indexOf()
to locate values in the string
"Brave new world"
.
const str = 'Brave new world'
console.log('Index of first w from start is ' + str.indexOf('w')) // logs 8
console.log('Index of "new" from start is ' + str.indexOf('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
.
const myString = 'brie, pepper jack, cheddar'
const 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
:
const str = 'To be, or not to be, that is the question.'
let count = 0
let position = str.indexOf('e')
while (position !== -1) {
count++
position = str.indexOf('e', position + 1)
}
console.log(count) // displays 4
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'String.prototype.indexOf' in that specification. |
Browser compatibility
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