javascript - Selection making a field mandatory -

i new javascript , have been doing university assignment based around creating form in html , javascript. in assignment have been asked make field mandatory based on whether specific option chosen in previous drop-down box.

1. if 'uws student' or 'uws staff' selected drop-down list field 'student number/staff number' becomes mandatory otherwise not mandatory field

2. if 'uws student' or 'uws staff' selected field 'institution/company' must automatically filled in 'university of western sydney'

here segment have been using:

<b>registration category:</b>  <select> <option value="uws student">uws student</option> <option value="student @ institution">student @ institution</option> <option value="uws academic">uws academic</option> <option value="other uws staff">other uws staff</option> <option value="academic institution">academic       institution</option> <option value="professional">professional</option> <option value="retired">retired</option> </select>  <b>student number/staff number:</b> <input type="text" name="studentnumber/staffnumber"> <br> <b>institution/company:</b> <input type="text" id="txtinstcomp" name="institutioncompany">  </fieldset> 

note: have function on 'institution/company' field previous question make filed mandatory, if matters @ all:

function validatetext() { var institutioncompany=document.getelementbyid('txtinstcomp'); if (institutioncompany.value=="") {      alert("institution/company must filled out");      return false; } } 

any appreciated struggling grasp of javascript

you must add id attributes select , input text fields below.

<b>registration category:</b>  <select id="cat" onchange="populateinstitution();">      <option value="uws student">uws student</option>      <option value="student @ institution">            student @ institution</option>      <option value="uws academic">uws academic</option>      <option value="other uws staff">other uws staff</option>      <option value="academic institution">            academic institution</option>      <option value="professional">professional</option>      <option value="retired">retired</option> </select> <br> <b>student number/staff number:</b>  <input type="text" id="stnumber" name="studentnumber/staffnumber"> <br> <b>institution/company:</b>  <input type="text" id="txtinstcomp" name="institutioncompany"> <br> <input type="button" value="submit" onclick="validatetext()" /> 

and have javascript function 'populateinstitution' change event on select field

function validatetext() {     var institutioncompany = document.getelementbyid('txtinstcomp').value;     if (institutioncompany == "") {         alert("institution/company must filled out");         return false;     }     var cat = document.getelementbyid('cat').value;     if (cat == "uws student" || cat == "other uws staff") {         if (document.getelementbyid("stnumber").value == "") {             alert('studentnumber/staffnumber mandatory');             return;         }      } }  function populateinstitution() {     var cat = document.getelementbyid('cat').value;     if (cat == "uws student" || cat == "other uws staff") {          document.getelementbyid('txtinstcomp').value='university of western sydney';     }else{         document.getelementbyid('txtinstcomp').value='';     } }