Question

If a user selects a certain option value in one select field, how do you then make a text field hidden, s it will not be needed. This must be done before the form is submitted?

For example in the following select field, a user select chooses value='a' then how would this text field become hidden:

     <select name="form" id="aForm">
        <option value="a">choice1</option>
        <option value="b">choice2</option>
        <option value="c">choice3</option>
     </select>

     <input type="text" style="width:285px" name="textField" id="textField"/>

Answer 1

$("#aForm").on("change", function() { 
    if ($(this).val() == "a") 
        $("#textField").hide();
    else 
        $("#textField").show(); 
});

Here is also a jsfiddle

I assumed that you will show the textfield for any other value than a.

If you’re using plain JavaScript and not jQuery

function hideTF() {
    var s = document.getElementById("aForm");
    document.getElementById("textField").style.display 
         = (s.selectedIndex > 0  && s.options[s.selectedIndex] == 'a'
         ? "none" : "block");
}
var s = document.getElementById("aForm");
if (s.attachEvent)
    s.attachEvent("onchange", hideTF);
else
    s.addEventListener("change", hideTF, false);