SOURCE

let num=6;
 let interval=setInterval (function () {
  
  num-=1;
  console.log(num)
  if(num<=1){
      clearInterval(interval)
  }
}, 1000);        


var countTime=function(lefttime){
      
       var leftm = Math.floor(lefttime/(1000*60)%60);  //计算分钟数
       var lefts = Math.floor(lefttime/1000%60);  //计算秒数
      
           if(leftm<10){
               if(lefts<10){
                   return "0"+ leftm + ":" + "0"+lefts;
               }else{
                    return "0"+ leftm + ":" + lefts;
               }
           }else{
               if(lefts<10){
                   return leftm + ":" + "0"+lefts;
               }else{
                    return  leftm + ":" + lefts;
               }
               
           }
       
      
   
}