<form action="navigation.html" method="post" id="query-form" class="form-signin">

    <input type="text" name="username" class="form-control" id="username" placeholder="Username">
    <input type="password" name="password" class="form-control" id="password" placeholder="Password">
    <button type="submit" class="btn btn-lg btn-primary btn-block" id="submit">
        Prisijungti
    </button>
</form>


<script>
jQuery(document).ready(function($) {
    jQuery("#submit").click(function() {
        var username = jQuery("#username").val();
        var password = jQuery("#password").val();
        jQuery('#username, #password').removeClass("has_error");
        if (username === '') {
            jQuery('#username').addClass("has_error");
            return false;
        } else {
            jQuery('#username').removeClass("has_error");
        }
        if (password === '') {
            jQuery('#password').addClass("has_error");
            return false;
        } else {
            jQuery('#password').removeClass("has_error");
        }
    });
});
</script>

jQuery("#submit").click(function(e) {
    e.preventDefault();
    var username = jQuery("#username").val();
    var password = jQuery("#password").val();

    //rest of code

    $('#query-form').submit(); // once checks complete
});

$(function() {
    $("#submit").click(function() {
        var $inputs = $(this).siblings('input'),
            isValid = true;
        $inputs.each(function() {
          var hasError = !$(this)[0].value.length;
          isValid = isValid && !hasError;
          $(this).toggleClass('has_error', hasError);
        });

        return isValid;
    });
});