JavaScript | lastIndex Property

Last Updated : 29 Apr, 2019

The lastIndex Property in JavaScript is used to specify the index at which to start the next match. If “g” modifier is not present this property will not work.
It can be used to return the position of character immediately after the previous match.

Syntax:

RegexObj.lastIndex

Example-1: This example checks if the string contains “Geek” in it.

<!DOCTYPE html> 
<html> 
  
<head> 
    <title> 
      lastIndex Property 
  </title> 
</head> 
  
<body style="text-align:center"> 
    <h1 style="color:green"> 
      GeeksforGeeks 
  </h1> 
    <h2> 
      lastIndex Property 
  </h2> 
    <button onclick="geek()"> 
      Click it! 
  </button> 
  
    <script> 
        function geek() { 
            var str =  
                "GeeksforGeeks is the"+ 
                " computer science portal"+ 
                " for geeks"; 
            
            var patt1 = /Geek/g; 
            var ans = "'Geek' found. Indexes are: "; 
            // check "geek" in string. 
            while (patt1.test(str) == true) { 
                ans += patt1.lastIndex + " "; 
            } 
            alert(ans); 
        } 
    </script> 
</body> 
  
</html> 

Output:
Before clicking the button:
lastIndex
After clicking the button:

Example-2: This example checks if the string contains “abc” in it.

<!DOCTYPE html> 
<html> 
  
<head> 
    <title> 
      lastIndex Property 
  </title> 
</head> 
  
<body style="text-align:center"> 
    <h1 style="color:green"> 
      GeeksforGeeks 
  </h1> 
    <h2>lastIndex Property</h2> 
    <button onclick="geek()"> 
      Click it! 
  </button> 
  
    <script> 
        function geek() { 
            var str =  
                "GeeksforGeeks is"+ 
                " the computer science"+ 
                " portal for geeks"; 
            
            var patt1 = /abc/g; 
            var ans =  
                "'Geek' found. Indexes are: "; 
            var ans1 =  
                "'Geek' found. Indexes are: "; 
            var len = ans.length; 
            
            // check "geek" in string. 
            while (patt1.test(str) == true) { 
                ans1 += patt1.lastIndex + " "; 
            } 
            var x = ans1.length; 
            if (len === x) 
                alert("RegExp is not "+ 
                      "present in the string."); 
            else 
                alert(ans1); 
        } 
    </script> 
</body> 
  
</html> 

Output:
Before clicking the button:
lastIndex
After clicking the button:

Supported Browsers: The browsers supported by JavaScript lastIndex Property are listed below:

Google Chrome
Apple Safari
Mozilla Firefox
Opera
Internet Explorer

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