Javascript – Stop form submit depending on ajax response [duplicate]

As the ajax call is asynchronous, you have to prevent the form from submitting, and then when a result is returned, you check if it matches the condition and submit the form with the native submit handler, avoiding the preventDefault() in the jQuery event handler :

$("form").submit(function(e) {
    e.preventDefault();

    var self = this,
        tray = $('select[name=tray_id]').val();

    $.ajax({
        type: "POST",
        url: "modules/reserve-check.php",
        data: {tray_id: tray},
        cache: false
    }).done(function(result) {
        if (result == "") self.submit();
    }).fail(function() {
        alert('error');
    });
});