未捕获的ReferenceError：未定义interval

Question

我正在我的网站中集成jQuery Countdown。倒计时停止后还会显示一个回调函数。

<script>
    // countdown script
        $("#countdown").countdown({
            date: "23 december 2014 23:59:59", // add your date here.
                format: "on"
        },
        function() { 
            alert("Campaing has been end!") 
        }); 
</script>

如果正常工作并且还显示了回调函数，那么它会在回调函数中给出此错误未捕获的ReferenceError：未在countdown.js第26行定义。

这是countdown.js

(function($) {
    $.fn.countdown = function(options, callback) {

        //custom 'this' selector
        thisEl = $(this);

        //array of custom settings
        var settings = { 
            'date': null,
            'format': null
        };

        //append the settings array to options
        if(options) {
            $.extend(settings, options);
        }

        //main countdown function
        function countdown_proc() {

            eventDate = Date.parse(settings['date']) / 1000;
            currentDate = Math.floor($.now() / 1000);

            if(eventDate <= currentDate) {
                callback.call(this);
                clearInterval(interval); //Line 26 Here
            }

            seconds = eventDate - currentDate;

            days = Math.floor(seconds / (60 * 60 * 24)); //calculate the number of days
            seconds -= days * 60 * 60 * 24; //update the seconds variable with no. of days removed

            hours = Math.floor(seconds / (60 * 60));
            seconds -= hours * 60 * 60; //update the seconds variable with no. of hours removed

            minutes = Math.floor(seconds / 60);
            seconds -= minutes * 60; //update the seconds variable with no. of minutes removed

            //conditional Ss
            if (days == 1) { thisEl.find(".timeRefDays").text("day"); } else { thisEl.find(".timeRefDays").text("days"); }
            if (hours == 1) { thisEl.find(".timeRefHours").text("hour"); } else { thisEl.find(".timeRefHours").text("hours"); }
            if (minutes == 1) { thisEl.find(".timeRefMinutes").text("minute"); } else { thisEl.find(".timeRefMinutes").text("minutes"); }
            if (seconds == 1) { thisEl.find(".timeRefSeconds").text("second"); } else { thisEl.find(".timeRefSeconds").text("seconds"); }

            //logic for the two_digits ON setting
            if(settings['format'] == "on") {
                days = (String(days).length >= 2) ? days : "0" + days;
                hours = (String(hours).length >= 2) ? hours : "0" + hours;
                minutes = (String(minutes).length >= 2) ? minutes : "0" + minutes;
                seconds = (String(seconds).length >= 2) ? seconds : "0" + seconds;
            }

            //update the countdown's html values.
            if(!isNaN(eventDate)) {
                thisEl.find(".days").text(days);
                thisEl.find(".hours").text(hours);
                thisEl.find(".minutes").text(minutes);
                thisEl.find(".seconds").text(seconds);
            } else { 
                alert("Invalid date. Here's an example: 12 Tuesday 2012 17:30:00");
                clearInterval(interval); 
            }
        }

        //run the function
        countdown_proc();

        //loop the function
        interval = setInterval(countdown_proc, 1000);

    }
}) (jQuery);

有没有办法结束这个错误。请帮助。