<script type='text/javascript'>
function isEmpty(elem, errMsg){
	if(elem.value.length == 0){
		alert(errMsg);
		elem.focus();
		return true;
	}
	return false;
}
</script>
<form>
Name : <input type='text' id='name'/>
<input type='button' 
	onclick="isEmpty(document.getElementById('name'), 'Please Enter a Value')"
	value='Check Field' />
</form>