If Email Null Then Stop Submitting Data To PHP

I am new to javascript. I was trying to passe text input to javascript. But code is not working.

Here is the code
<script type="text/javascript">
function val(){
if(document.a.email.value=="" ||document.a.email.value==null ){
alert("sssss");
return false;
}
if(document.a.email.value!="" ||document.a.email.value!=null ){
alert("sssss"+document.a.email.value);
return true;
}}
</script>

And here is the HTML code,
<form action="a.php" method="POST" onsubmit="return val();">
<strong>E-Mail Address</strong> *</td><td><input name="email" type="text" value="" />
<input type="submit" value="submit" />
</form>

Any reason not to view the alert message.. and I want if the email is null to stop submitting the data to a.php.