问如何让JavaScript倒计时器在点击按钮时一次又一次地工作？

EN

    var timer;
var countdownfunction;
var countDownTime;

timer = function (){

    clearInterval(countdownfunction);

    countDownTime = $("#timer").attr("data-timer-val");

    var countdownfunction = setInterval(function() {

        if(countDownTime < 10 ){
            var temp = "0" + countDownTime;
            $("#time").text(temp);
        }else{
            $("#time").text(countDownTime);
        }


        countDownTime = countDownTime - 1;



        if (countDownTime < 0) {
            clearInterval(countdownfunction);
            $("#time-expired").text("Please click next problem.");
        }

    }, 1000);
} ;


$(document).ready(function(){
    timer();
});


$(document).ready(function(){

    $("#next-problem").on('click', function(){

        $.ajax({
            type : 'POST',

            url : '/test/next-problem/',

            success : function(data){
                // console.log(data);
                $("#problem-content").html(data);
                clearInterval(countdownfunction);
                timer();

            },

        });

    });

});