Question

The following is not working as expected, it’s not picking up the alert message when you click on ‘.arrow-_down_img’

        function slideDown_Menu(){
                    $('.main_nav').slideDown('slow', function(){
                        $('.arrow_down_img').removeClass('arrow_down_img').addClass('arrow_up_img');
                    });
        }

        function slideUp_Menu(){
                    $('.main_nav').slideUp('slow', function(){
                        $('.arrow_up_img').removeClass('arrow_up_img').addClass('arrow_down_img');
                    });
        }

        $('.arrow_down_img').click(function(){
            // evt.preventDefault();
            // slideDown_Menu();
            alert('test');

        });

        $('.arrow_up_img').click(function(evt){
            evt.preventDefault();
            slideUp_Menu();
        });

HTML

        <div class="arrow_up">
            <div class="arrow_up_img"></div>
        </div>

Answer 1

As there is no element with class of .arrow_down_img on DOMReady, you should delegate the event, from one of static parents of the element or document object.

$(document).on('click', '.arrow_down_img', function(evt){
    // evt.preventDefault();
    // slideDown_Menu();
    alert('test');
});

Also note that in your click handler evt is undefined.