Question

I want to change written value of JavaScript after I change the value of ComboBox, but it didn’t work. Here is my code :

<script>var selGate = "empty"</script>
   <SELECT NAME = "cmbGate" STYLE ="width: 600px" SIZE = 15 onchange="chGate()">
   <OPTION Value = "opt1">Option 1</OPTION>
   <OPTION Value = "opt2">Option 2</OPTION>
   </SELECT>                    
   <BR>
   <form name="frGate">
   Selected Gate is: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;    
   <script>document.write(selGate)</script>
   </form>
            <script>
                function chGate()
                {
                    selGate = document.getElementsByName("cmbGate").selectorText;
                    frGate.submit();
                }
            </script>                  

Answer 1

You function would look like this:

function chGate()
{
    var e = document.getElementById("cmbGate");//get the combobox
    var selGate = e.options[e.selectedIndex].value;//get selected value
    //you can also do use                  ^.text =>to get the text instead
    document.getElementById("selected").innerHTML = "Selected Gate is:"+selGate;
                                     //^^ set the text to the selected value
    frGate.submit();
}

You html should change like this:

==> Change the select

<SELECT id="cmbGate" STYLE ="width: 600px" SIZE = 15 onchange="chGate()">
//      ^^ set the id

==> Change the form

<form name="frGate">
    <div id="selected">Selected Gate is:</div>
    //    ^^ set an id here also so you can update it 
</form> 

See a working demo:
http://jsfiddle.net/Mr2CF/2/