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    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
    Optional. The location within the calling string to start the search from. It can be any integer. The default value is 0. If fromIndex < 0 the entire string is searched (same as passing 0). If fromIndex >= str.length, the method will return -1 unless searchValue is an empty string in which case str.length is returned.

    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 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'));
    // 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 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
    

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition.
    ECMAScript 5.1 (ECMA-262)
    The definition of 'String.prototype.indexOf' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'String.prototype.indexOf' in that specification.
    Release Candidate  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

    See also

    Document Tags and Contributors

    Last updated by: stevemao,
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