I'm currently learning how to use Ajax but i already have a problem :

1 HTML :

```
<body>
<form>

    Nom d'utilisateur : <input type="text" id="username" name="username"/>
    <input type="submit" id="submit" />
</form>

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script src="script.js"></script>

```

JS :

```
$(document).ready(function(){

$("form").submit(function(e){
    e.preventDefault();
    var don=$('#username').val();
    $.ajax({
        url:'test.php',
        type:'post',
        data:don,
        success: function(html){
            alert(html);
        }

    });
});

```

});

PHP :

```
<?php
if(!empty($_POST['username'])){
    $response="yep";
}
else{
    $response="nope";
}

echo $response;

```

?>

As you can see, it is really simple. But it drives me crazy, i didn't understand why i always have the response "nope". 
Can you help me ?
Thank you