Textbox enable /disable on drop down change

Question

Hi,  i want to use javascript code to enable/disable the textbox on selection of value from drop down list. My javascript code is not working.

 

 

<script type="text/javascript">
 function changetextbox() { 
if (document.getElementById("mfi_4_a_i").value === "noy")
 {         document.getElementById("sdd").disable='true';
 } else {  
       document.getElementById("sdd").disable='false'; }
 } </script>