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JavaScript IndexOf

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The JavaScript indexof method is used to return the index position of the first occurrence of a specified string. If the specified string is not found, indexof function will return -1.

In this article we will show you, How to write IndexOf Function in JavaScript with example. The basic syntax of the JavaScript indexof function is as shown below:

String_Object.indexof(Substring, Starting_Position)
  • String_Object: Please select the valid String Object or literal.
  • Substring: Please specify the string you want to search for inside the string_Object.
  • Starting_Position: This is an optional parameter. If you want to specify the starting point (starting index position) then, Please specify the index value here.

NOTE:

  • If we specify the Negative number as Starting_Position, JavaScript IndexOf() will start looking from Zero.
  • If we specify the Out of range Index as Starting_Position, IndexOf() will start looking from Highest index number.

JavaScript indexof Example

The following set of examples will help you understand the indexof Function.

<!DOCTYPE html>
<html>
<head>
    <title> JavaScript IndexOf </title>
</head>
<body>
    <h1> JavaScript IndexOf </h1>
<script>
 var Str1 = "Learn JavaScript at Tutorial Gateway.org";
 var Str2 = "We are abc working at abc company";
 var Str3 = Str1.indexOf("Script");
 var Str4 = Str1.indexOf("abc"); // Non existing item
 var Str5 = Str2.indexOf("abc");
 var Str6 = Str1.indexOf("Script", 5);
 var Str7 = Str2.indexOf("abc", 10);
 
 document.write("<b> Index position of Script is:</b> " + Str3);
 document.write("<br \> <b> Index position of abc is:</b> " + Str4);
 document.write("<br \> <b> Index position of abc is:</b> " + Str5);
 document.write("<br \> <b> Index position of Script is:</b> " + Str6);
 document.write("<br \> <b> Index position of abc is:</b> " + Str7);
</script>
</body>
</html>

TIP: The index position in JavaScript indexof() Function will start from 0, Not 1.

OUTPUT

JavaScript IndexOf

ANALYSIS

In this indexof example, Within the Script tag, First we declared two String variables Str1, Str2 and assigned corresponding values using following statement

 var Str1 = "Learn JavaScript at Tutorial Gateway.org";
 var Str2 = "We are abc working at abc company";

Following statement will find the index position of a substring ‘Script’ and store the index value in Str3.

var Str3 = Str1.indexOf("Script");

NOTE: You should count the space as One Character.

In the next line we are looking for non existing item “abc” inside the Str1. Since JavaScript indexof function doesn’t find the substring, it is returning -1 as output

 var Str4 = Str1.indexOf("abc"); // Non existing item

In the next line, we are looking for “abc” inside the Str2 using IndexOf function.

var Str5 = Str2.indexOf("abc");

From the above statement you can observe that, although term abc is repeated multiple times, Javascript indexof function written the index position of a first occurrence. Now, let us change the starting position from default 0 to 10

 var Str7 = Str2.indexOf("abc", 10);

Above statement will return the first occurrence of string abc starting at index position 10.

The following statements will display the content output to the respective browser

 document.write("<b> Index position of Script is:</b> " + Str3);
 document.write("<br \> <b> Index position of abc is:</b> " + Str4);
 document.write("<br \> <b> Index position of abc is:</b> " + Str5);
 document.write("<br \> <b> Index position of Script is:</b> " + Str6);
 document.write("<br \> <b> Index position of abc is:</b> " + Str7);