String.prototype.indexOf()

Summary

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

The location within the calling string to start the search from. It can be any integer between 0 and the length of the string. The default value is 0.

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("", 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

Example: 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"));
// Displays 8
console.log("The index of the first w from the end is " + anyString.lastIndexOf("w")); 
// Displays 10

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

Example: 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 log method displays 19. But because the indexOf method is case sensitive, the string "cheddar" is not found in myCapString, so the second 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"));    
// Displays 19
console.log('myCapString.indexOf("cheddar") is ' + myCapString.indexOf("cheddar")); 
// Displays -1

Example: Using indexOf to count occurrences of a letter in a string

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

count = 0;
pos = str.indexOf("x");

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

Specifications

Browser compatibility

See also

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