如何用 JavaScript 动态创建分钟与秒的倒数计时器

<span id="minute">5</span>分
<span id="second">0</span>秒

<script>
var minute = document.getElementById("minute");
var second = document.getElementById("second");


var timer = setInterval (function(){
                        second.innerHTML--;
                        if(minute.innerHTML == 0 && minute.innerHTML == 0){
                                clearInterval(timer);
                                alert("完成");
                        }

                        if(second.innerHTML<0){
                            minute.innerHTML--; 
                            second.innerHTML = 59;
                        }

},1000);

</script>