1 <script type="text/javascript">
 2 
 3             window.onload = function () {
 4                 var btn = document.getElementById("btn");
 5 
 6                 btn.onclick = function () {
 7                     btn.type = "checkbox";
 8                 }
 9                 
10                 
11             }
12 </script>
13 
14 <input type="button" id="btn" />