倒计时的实现方式

// ES6 Class写法
class countDown{
  constructor(obj){
    this.obj = obj;
  }
  run(time,callback){
    let start = time;
    this.obj.innerText = start;
    this.clock = setInterval(() => {
      if(start === 0){
        this.destroy();
        if (callback){
          callback();
        }
      }
      else{
        start--;
        this.obj.innerText = start;
      }
    },1000);
  }
  destroy(){
    clearInterval(this.clock);
  }
}


// 普通函数写法
function countDown(obj,start,callback){
  obj.innerText = start;
  var timer = setInterval(function(){
    if(start === 0){
      clearInterval(timer);
      if (callback) {
        callback();
      }
    }
    else{
      start--;
      obj.innerText = start;
    }
  },1000);
}