The defaultChecked property is a Boolean value that reports which radio buttons contain the HTML CHECKED attribute.
If the CHECKED attribute is contained in the Radio object, true is returned. Otherwise, false is returned.
<html> <head> <title> Example of the radio defaultChecked property</title> </head> <body> <script language="JavaScript"> <!-- function checkBox(){ if(document.form1.button1.defaultChecked == true){ alert("Box1 is checked by default"); } else if(document.form1.button2.defaultChecked == true){ alert("Box 2 is checked by default"); } } --> </script> <form name="form1"> <input type="radio" name=button1>Box 1 <br> <input type="radio" name=button2 CHECKED>Box 2 <br><br> <input type="button" value="Get Default" onClick='checkBox()'> </form> </body> </html>