2.   function compose (fns ) {
    4.     let i = -1;
    6.     return function () {
    8.      function run(index) {
    10.         let fn = fns[index];
    12.         if (typeof fn !== "function") {
    13.           return;
    14.         }
    16.         return fn(run.bind(null,index+1));
    17.       }
    19.       return run(i+1);
    20.     }
    21.   }
    23.   class TaskController {
    25.     taskQueue = [];
    27.     constructor() {
    29.       setTimeout(() => {
    30.         compose(this.taskQueue)();
    31.       });
    32.     }
    34.     task(fn) {
    35.       this.taskQueue.push(fn);
    36.       return this;
    37.     }
    39.     // 多少毫秒以后执行下一个函数
    40.     timeout(ms) {
    41.       this.taskQueue.push(
    42.         next => setTimeout(next, ms)
    43.       );
    44.       return this;
    45.     }
    47.     timeoutFirst(ms, fn) {
    48.       this.taskQueue.push(
    49.         next => fn(), setTimeout(next, ms)
    50.       );
    51.       return this;
    52.     }
    53.   }
    56.   function builder() {
    58.     return new TaskController();
    59.   }