<label>计时:</label> 
<input type="text" name="" id="timer"/> 
<button onclick="pause(this)" id="pause" state="on">暂停</button>
<button onclick="restart()">重新开始</button>

var ele_timer = document.getElementById("timer");
var n_sec = 0; //秒
var n_min = 0; //分
var n_hour = 0; //时

//60秒 === 1分
//60分 === 1小时
function timer() {
 return setInterval(function () {

  var str_sec = n_sec;
  var str_min = n_min;
  var str_hour = n_hour;
  if ( n_sec < 10) {
   str_sec = "0" + n_sec;
  }
  if ( n_min < 10 ) {
   str_min = "0" + n_min;
  }

  if ( n_hour < 10 ) {
   str_hour = "0" + n_hour;
  }

  var time = str_hour + ":" + str_min + ":" + str_sec;
  ele_timer.value = time;
  n_sec++;
  if (n_sec > 59){
   n_sec = 0;
   n_min++;
  }
  if (n_min > 59) {
   n_sec = 0;
   n_hour++;
  }


 }, 1000);
}

var n_timer = timer();

//暂停和继续
function pause(self) {
  var state = self.getAttribute("state");
  if (state === "on") {
   clearInterval(n_timer);
   self.textContent = "继续";
   self.setAttribute("state", "off");
  } else {
   n_timer = timer();
   self.textContent = "暂停";
   self.setAttribute("state", "on");
  }
}

function restart() {
 clearInterval(n_timer);
 n_sec = 0;
 n_min = 0;
 n_hour = 0;
 n_timer = timer();

 var ele_pause = document.getElementById("pause");
 ele_pause.textContent = "暂停";
 ele_pause.setAttribute("state", "on");
}