JavaScript String.prototype.indexOf() - Index Of Explained with Examples

The indexOf() method returns the first index at which a specified string can be found in the given String object. If the string is not present, it returns -1.

Syntax

str.indexOf(searchValue[, fromIndex])

Parameters

  • searchValue Substring for which you are looking. If this is empty ( '' ) and there is no fromIndex argument, indexOf() will return 0.
  • fromIndex Optional. The index at which you want to start the search from. The default value is 0.
    • If fromIndex is negative, the string will be searched from the beginning.
    • If fromIndex is less than the string’s length:
      • If searchValue is not an empty string ( '' ), the string will be searched from fromIndex .
      • If searchValue is an empty string ( '' ), the method returns fromIndex .
    • If fromIndex is greater than or equal to the string’s length:
      • If searchValue is not an empty string ( '' ), the string is not searched and the method returns -1.
      • If searchValue is an empty string ( '' ), the method returns the string’s length.

Description

The indexOf() method checks the string from left to right. The index of the first character is 0 and the index of the last character is string.length - 1 . The method checks each substring against searchValue using strict equality ( === ), which means that this method is case sensitive. Once it finds a substring that returns true , it returns the index of its first character.

Examples

'Blue Whale'.indexOf('Blue');     // returns  0
'Blue Whale'.indexOf('Blute');    // returns -1
'Blue Whale'.indexOf('blue');     // 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
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