JQuery :: How To Get ID Of DIV By Using Attribute Function

index.php
if (idx == 8) {
var str="<?php echo $username1; ?>";
document.write(str);
jQuery.get("usermessage.php?username="+str, function(data) {
document.write(data);
var msgIDss = $(".messagepiece:last").attr("id");
document.write(msgIDss);
});
}

usermessage.php
<?php
$username1=$_GET["username"];
$query2 = "SELECT id, message, datetime FROM messages WHERE username='{$username1}' ORDER BY id";
$result2 = mysql_query($query2,$connection) or die (mysql_error());
confirm_query($result2);
$num = mysql_num_rows($result2);
while($userinfo = mysql_fetch_array($result2)){
$msgID= $userinfo['id'];
echo "<div class="messagepiece" id="" . $msgID . "">" . $userinfo['id'] . $userinfo['message'] . "<br><span style="font-size:13px; color:orange;">" . $userinfo['datetime'] . "</span></div>|";
}?>

My problem is document.write(msgIDss)
display empty / blank value.
On index.php, I have tested document.write(data);
And it works. I have tested document.write(str)
And it work too. I also tested open usermessage.php manually and it success display all the div(s), so usermessage.php page is not the problem. I guess the problem is on this line of code var msgIDss = $(".messagepiece:last").attr("id")