jQuery Uncaught TypeError：无法读取未定义的属性“fn”（匿名函数）

全部， 我下载的一些代码出现错误。这是代码：

/*----------------------------------------------------------------------*/
/* wl_Alert v 1.1 
/* description: Handles alert boxes
/* dependency: jquery UI Slider, fadeOutSlide plugin
/*----------------------------------------------------------------------*/


$.fn.wl_Alert = function (method) {
var args = arguments;
return this.each(function () {

    var $this = $(this);


    if ($.fn.wl_Alert.methods[method]) {
        return $.fn.wl_Alert.methods[method].apply(this, Array.prototype.slice.call(args, 1));
    } else if (typeof method === 'object' || !method) {
        if ($this.data('wl_Alert')) {
            var opts = $.extend({}, $this.data('wl_Alert'), method);
        } else {

            var opts = $.extend({}, $.fn.wl_Alert.defaults, method, $this.data());
        }
    } else {
        $.error('Method "' + method + '" does not exist');
    }


    if (!$this.data('wl_Alert')) {

        $this.data('wl_Alert', {});

        //bind click events to hide alert box
        $this.bind('click.wl_Alert', function (event) {
            event.preventDefault();

            //Don't hide if it is sticky
            if (!$this.data('wl_Alert').sticky) {
                $.fn.wl_Alert.methods.close.call($this[0]);
            }

            //prevent hiding the box if an inline link is clicked
        }).find('a').bind('click.wl_Alert', function (event) {
            event.stopPropagation();
        });
    } else {

    }
    //show it if it is hidden
    if ($this.is(':hidden')) {
        $this.slideDown(opts.speed / 2);
    }

    if (opts) $.extend($this.data('wl_Alert'), opts);
});

};

$.fn.wl_Alert.defaults = {
speed: 500,
sticky: false,
onBeforeClose: function (element) {},
onClose: function (element) {}
};
$.fn.wl_Alert.version = '1.1';


$.fn.wl_Alert.methods = {
close: function () {
    var $this = $(this),
        opts = $this.data('wl_Alert');
    //call callback and stop if it returns false
    if (opts.onBeforeClose.call(this, $this) === false) {
        return false;
    };
    //fadeout and call an callback
    $this.fadeOutSlide(opts.speed, function () {
        opts.onClose.call($this[0], $this);
    });
},
set: function () {
    var $this = $(this),
        options = {};
    if (typeof arguments[0] === 'object') {
        options = arguments[0];
    } else if (arguments[0] && arguments[1] !== undefined) {
        options[arguments[0]] = arguments[1];
    }
    $.each(options, function (key, value) {
        if ($.fn.wl_Alert.defaults[key] !== undefined || $.fn.wl_Alert.defaults[key] == null) {
            $this.data('wl_Alert')[key] = value;
        } else {
            $.error('Key "' + key + '" is not defined');
        }
    });

}
};

//to create an alert box on the fly
$.wl_Alert = function (text, cssclass, insert, after, options) {
//go thru all
$('div.alert').each(function () {
    var _this = $(this);
    //...and hide if one with the same text is allready set
    if (_this.text() == text) {
        _this.slideUp($.fn.wl_Alert.defaults.speed);
    }
});

//create a new DOM element and inject it
var al = $('<div class="alert ' + cssclass + '">' + text + '</div>').hide();
(after) ? al.appendTo(insert).wl_Alert(options) : al.prependTo(insert).wl_Alert(options);

//return the element
return al;
};

以前有人见过这种类型的错误吗？我该如何解决这样的事情？感谢您的任何建议！

尝试在 bootstrap.js 之前调用 jQuery 库

<script src="js/jquery-3.3.1.min.js"></script>
<script src="js/bootstrap.min.js"></script>