JavaScript lastIndexOf() | Find Position of Last Occurrence of a Value

The JavaScript lastIndexOf() method returns the position of last occurrence of 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 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 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 first character of last occurrence of specified substring or value. Otherwise it returns -1 in case if specified substring does not exists in the given string.

JavaScript lastIndexOf() Example

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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