html代码

分：<input type="text" value="0">
秒：<input type="text" value="0">
<button>开始计时</button>

javascript代码

var input = document.getElementsByTagName('input');
var min = 0;
var miao = 0;
var button = document.getElementsByTagName('button')[0];

function start() {
   
            var timer = setInterval(function () {
   
                miao++;
                if (miao == 60) {
   
                    miao = 0;
                    min++;
                }
                input[1].value = miao;
                input[0].value = min;
                if (min == 3) {
   
                    clearInterval(timer);
                }
            }, 10);
        }
button.onclick = function() {
   
      start();
 }