改變讀寫屬性

<script>function doclick1(){
      
       var txtN = document.getElementById("contactus5"); 
       txtN.readOnly = false;
      } 
   </script>
<input type="text"  id="contactus5"  readonly>
<input type="button" name="button4" value="修改"  οnclick="doclick1();" />