$(document).ready(function() {

    var addItem = function() {
      $('.mainList').append('<li class="listItem"><input id="check" class="checkBox" type="checkbox"><input class="textInp" type="text" placeholder="..."></li>');
    };
    
    var itemDone = function() {
        if ($(this).is(':checked')) {
            alert("Checked!");
        } else {
            alert("Unchecked!");
        }
    };
    
    $('.mainList').on('click', '#check', itemDone);
    $('.addItem').click(addItem);
});
$(document).ready(function() {

    var addItem = function() {
      $('.mainList').append('<li class="listItem"><input id="check" class="checkBox" type="checkbox"><input class="textInp" type="text" placeholder="Add Item..."></li>');
    };
    
    var itemDone = function() {
        var $this = $(this);
        $this.parent().fadeOut(600, function() {
            $this.parent().remove();
        });
    };
    
    $('.mainList').on('click', '#check', itemDone);
    $('.addItem').click(addItem);
});
Make list items fade out on check.
Make list items fade out on check.
