String.prototype.indexOf()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The indexOf()
method of String
values searches this string and returns the index of the first occurrence of the specified substring. It takes an optional starting position and returns the first occurrence of the specified substring at an index greater than or equal to the specified number.
Try it
Syntax
indexOf(searchString)
indexOf(searchString, position)
Parameters
searchString
-
Substring to search for. All values are coerced to strings, so omitting it or passing
undefined
causesindexOf()
to search for the string"undefined"
, which is rarely what you want. position
Optional-
The method returns the index of the first occurrence of the specified substring at a position greater than or equal to
position
, which defaults to0
. Ifposition
is greater than the length of the calling string, the method doesn't search the calling string at all. Ifposition
is less than zero, the method behaves as it would ifposition
were0
.-
'hello world hello'.indexOf('o', -5)
returns4
— because it causes the method to behave as if the second argument were0
, and the first occurrence ofo
at a position greater or equal to0
is at position4
. -
'hello world hello'.indexOf('world', 12)
returns-1
— because, while it's true the substringworld
occurs at index6
, that position is not greater than or equal to12
. -
'hello world hello'.indexOf('o', 99)
returns-1
— because99
is greater than the length ofhello world hello
, which causes the method to not search the string at all.
-
Return value
The index of the first occurrence of searchString
found, or -1
if not found.
Return value when using an empty search string
Searching for an empty search string produces strange results. With no second argument, or with a second argument whose value is less than the calling string's length, the return value is the same as the value of the second argument:
"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 a second argument whose value is greater than or equal to the string's length, the return 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, the method behaves as if it found an empty string just after the position specified in the second argument. In the latter instance, the method behaves as if it found an empty string at the end of the calling string.
Description
Strings are zero-indexed: The index of a string's first character is 0
, and the index of a string's last character is the length of the string minus 1.
"Blue Whale".indexOf("Blue"); // returns 0
"Blue Whale".indexOf("Wale"); // 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
When checking if a specific substring occurs within a string, the correct way to check is test whether the return value is -1
:
"Blue Whale".indexOf("Blue") !== -1; // true; found 'Blue' in 'Blue Whale'
"Blue Whale".indexOf("Wale") !== -1; // false; no 'Wale' in 'Blue Whale'
Examples
Using indexOf()
The following example uses indexOf()
to locate substrings in the string
"Brave new world"
.
const str = "Brave new world";
console.log(str.indexOf("w")); // 8
console.log(str.indexOf("new")); // 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")); // 19
console.log(myCapString.indexOf("cheddar")); // -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); // 4
Specifications
Specification |
---|
ECMAScript Language Specification # sec-string.prototype.indexof |
Browser compatibility
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