JavaScript lastIndexOf(): Find the Position of the Last Occurrence of a Value

The JavaScript lastIndexOf() method returns the position of the last occurrence of a specified value in a given string. For example:

HTML with JavaScript Code

<!DOCTYPE html>
<html>
<body>

   <p id="xyz"></p>

   <script>
      let mystr = "JavaScript is fun, is not it?";
      let pos = mystr.lastIndexOf("is");

      document.getElementById("xyz").innerHTML = pos;
   </script>
   
</body>
</html>

Output

Since indexing starts with 0, therefore, in the string "JavaScript is fun, is not it?"

'J' is at index no. 0.
'a' is at index no. 1.
'v' is at index no. 2.
and so on.

Similarly, the last "is" starts from index no. 19. That is, 'i' is at index no. 19, and 's' is at index no. 20.

JavaScript lastIndexOf() syntax

The syntax of the lastIndexOf() method in JavaScript is:

string.lastIndexOf(subString, startIndex)

The startIndex argument is optional. Its default value is the length of the string or string.length - 1.

The lastIndexOf() method returns the index number of the first character of the last occurrence of a specified substring or value. Otherwise, it returns -1 if the specified substring does not exist in the given string.

JavaScript lastIndexOf() example

Consider the following code as an example demonstrating the lastIndexOf() method in JavaScript:

HTML with JavaScript Code

<!DOCTYPE html>
<html>
<body>

   <p>Index number of last 'i' = <span id="m"></span></p>
   <p>Index number of last 'i' before index no.24 = <span id="n"></span></p>
   <p>Index number of last 'i' before index no.15 = <span id="o"></span></p>

   <script>
      let x = "JavaScript is fun, is not it?";

      let a = x.lastIndexOf("i");
      document.getElementById("m").innerHTML = a;

      let b = x.lastIndexOf("i", 24);
      document.getElementById("n").innerHTML = b;

      let c = x.lastIndexOf("i", 15);
      document.getElementById("o").innerHTML = c;
   </script>
   
</body>
</html>

Output

Index number of last 'i' =

Index number of last 'i' before index no.24 =

Index number of last 'i' before index no.15 =

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