javascript - show/hide form input fields based on menu selection -

i'm trying input hidden field data load based on menu selections. can't work code below. can me find out i'm going wrong? i'd try , code working.

i have javascript in header.

<script type="text/javascript">     function toggle() {         //alert(thing.options[thing.selectedindex].innerhtml);         var w = document.getelementbyid("pt").selectedindex;         var selected_text = document.getelementbyid("pt").options[w].text;         if (selected_text == "home") {             //alert(selected_text)             document.getelementbyid("hiddenbox").innerhtml = '<input type="hidden" name="sfrtype[]" value="res-single family"><input type="hidden" name="sfrtype[]" value="res-farm-ranch">'         } else if (selected_text == "lot") {             // alert(selected_text)             document.getelementbyid("hiddenbox").innerhtml = '<input type="hidden" id="amin_acres" name="amin_acres" value="0.001">'         } else if (selected_text == "acreage") {             //alert(selected_text)             document.getelementbyid("hiddenbox").innerhtml = '<input type="hidden" name="amin_acres" value="2">'         } else {             document.getelementbyid("hiddenbox").innerhtml = '';         }     } </script> 

here's field form menu html:

<select name="pt" id="pt" onchange="toggle()" size="5" multiple="multiple">     <option value="sfr" selected="selected">home</option>     <option value="ld">lot</option>     <option value="ld">acreage</option>     <option value="com">commercial</option> </select>  

so if acreage selected, input hidden field <input type="hidden" name="amin_acres" value="2"> load. used work , stopped working.

be sure add div

<div id="hiddenbox"></div>

<script type="text/javascript"> function toggle() { //alert(thing.options[thing.selectedindex].innerhtml); var w = document.getelementbyid("pt").selectedindex; var selected_text = document.getelementbyid("pt").options[w].text;  if (selected_text == "home") {     //alert(selected_text)   document.getelementbyid("hiddenbox").innerhtml = '<input  name="sfrtype[]" value="res-single family"><input type="hidden" name="sfrtype[]" value="res-farm-ranch">'   }     else if  (selected_text == "lot") {    // alert(selected_text)   document.getelementbyid("hiddenbox").innerhtml = '<input  id="amin_acres" name="amin_acres" value="0.001">'   }     else if (selected_text == "acreage") {     //alert(selected_text)   document.getelementbyid("hiddenbox").innerhtml = '<input  name="amin_acres" value="2">'  } else {   document.getelementbyid("hiddenbox").innerhtml ='';  } }   </script>