Promise 串行

Promise 串行执行简单方法：
v1.0

function serialLoad(callback) {
      if (callback && typeof callback == "function") {
          return new Promise(resolve => callback(resolve))
      }
  }
  serialLoad(function (resolve) {
      console.log("start");
      setTimeout(() => resolve(2), 200);
  }).then(res => serialLoad(function (resolve) {
      console.log(res);
      setTimeout(() => resolve(4), 500);
  })).then(res => serialLoad(function (resolve) {
      console.log(res);
      setTimeout(() => resolve(6), 300);
  })).then(res => serialLoad(function (resolve) {
      console.log(res, "end");
  })).then(res => serialLoad(function (resolve) {
      console.log(res, "finish");
  }));