SOURCE

/*
*1、依次输出数组每一项
*2、每项按数组的大小依次循环输出
*/

var arr = [2, 1, 2];
var count = 0;

for (let i = 0; i < arr.length; i++) {
  //每一项内循定时环输出
  for (let n = 0; n < arr[i]; n++) {
    //console.log(n, i, arr.length, n + i * arr.length) let m = n + i * arr.length;

    setTimeout(function() {
      //console.log(i,n)
      var num = "<p>"+count + ":" + i + "--" + n+"</p>";
      document.getElementById("output").innerHTML = num;
    },
    count * 1000);
    count++;
  }

}

/*
while
do...while
*/
let user =0;
while(user > 0){
  console.log("Can i help you：",user);
  user --;
}

// do{
//   console.log("What can i help you：",user);
//   user --;
// }while(user > 0)

<div id="output">
</div>