Check null field using getElemenytById in Java script.

<script type='text/javascript'>
function notEmpty(elem, helperMsg){
if(elem.value.length == 0){
alert(helperMsg);
elem.focus();
return false;
}
return true;
}
</script>
<form>
Employee Name: <input type='text' id='req1'/>
<input type='button'
onclick="notEmpty(document.getElementById('req1'), 'Employee Name should not be empty')"
value='Submit' />
</form>