data returned from ajax always failing in == comparison?

A new problem emerged from this thread: jquery form validator never makes ajax call, in that the data returned from the ajax request is accurate, but it always fails on a comparison. This is the code that makes the ajax request:

var pass_form = $('#pass_form');
pass_form.submit( valid_pass_sett );

function valid_pass_sett() {
    //remove old errors - snipped
    pass_old = $('input[name=pass_old]').val();
    pass_new = $('input[name=pass_new]').val();
    pass_confirm_new = $('input[name=pass_confirm_new]').val();

    if (pass_old === "") {
        //display error on form - snipped
        return false;
    } else if (pass_new === "") {
        //display error on form - snipped
        return false;
    } else if (pass_new != pass_confirm_new) {
        //display error on form - snipped
        return false;
    } else if (pass_new.length < 8) {
        //display error on form - snipped
        return false;
    } else {
        $.post("http://www.example.com/ajax/validate.php",{ // async validation
            type: 'valid_old_change_pass', 
            pass_old: pass_old,
            pass_new: pass_new
        }, valid_pass_combo_callback);
        alert('after the ajax call...');
    }
    return false;  // cancel form submission
}

this is the code that the request is submitted to:

$username = $_SESSION['username'];
$pass_old = $_POST['pass_old'];
$pass_new = $_POST['pass_new'];
if (empty($pass_old) || empty($pass开发者_开发百科_new)) {
    echo "invalid";
} else if (!User::valid_user_pass($username, $pass_old)) {
    echo "invalid_old";
} else if (!Sanitize::is_legal_password($pass_new)) {
    echo "invalid_new";
} else {
    echo "valid";
}

and this is the callback function that processes it; the callback function is the one where the comparison always fails.

function valid_pass_combo_callback( data ) {

//breakpoint is set here
    if (data == 'valid') {
        //only if the form is valid!
        pass_form[0].unbind('submit').submit();
    }
    else if (data == "invalid_old") {
        //display error on form - snipped
    }
    else if (data == "invalid_new") {
        //display error on form - snipped
    }
    else {
        //it always jumps to here..., even though data *is* the correct value
    }
}

I debugged this code, and validate.php is returning "invalid_old" which is correct (based on the test data I'm entering). So, data is storing "invalid_old" according to Firebug; however, the code always jumps down to the last else statement. Why is the comparison always failing?

---

Try placing an alert(data.toString()); above your checks temporarily to see what it returns. If it's not the string itself, then it's not returning the data you're expecting.

---

As stated in the comment, adding JQuery's trim function fixed the problem, as shown:

function valid_pass_combo_callback( data ) {
    trimmed_data = $.trim(data);
    if (trimmed_data == 'valid') {
        //only if the form is valid!
        pass_form[0].unbind('submit').submit();
    }
    else if (trimmed_data == "invalid_old") {
        //display error on form - snipped
    }
    else if (trimmed_data == "invalid_new") {
        //display error on form - snipped
    }
    else {
        //it always jumps to here..., even though data *is* the correct value
    }
}