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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.

Note: For the Array method, see Array.prototype.indexOf().
Syntax
str.indexOf(searchValue[, fromIndex])
Parameters
searchValue
A string representing the value to search for.
fromIndex Optional
An integer representing the index at which to start the search; the default value is 0. For fromIndex values lower than 0 or greater than str.length, the search starts at index 0 and str.length respectively.
Return value
The index of the first occurrence of searchValue, or -1 if not found.
An empty string searchValue will match at any index between 0 and str.length

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
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