facebook option list selectedIndex not working

Hy I have t his script:

<script> 
function get(obj) {
    return document.getElementById(obj);
}

function getCrer(sel) {
  var value =  get(sel).options[get(sel).selectedIndex].value;  
  get('cbt').value=value;
}
</script>

<select id="combo1" onchange="getCrer(this)">
<option value="">Select</option>
<option value=0>Text1</option>
<option 开发者_JAVA技巧value=5>Text2</option>
<option value=9>Text3</option>
</select>
<input name="cbt" type="text" id="cbt"/>

I like to work in facebook so when I select from the list the calue shoul go to the input text cbt...but not working in fb. why?

---

When your onchange event fires, it's passing in the Select Object to getCrer, not the Object's ID, so your get method will return null. Try this instead:

function getCrer(sel) {
  var value =  sel.options[sel.selectedIndex].value;  
  get('cbt').value=value;
}