const list = [];
for (let i = 0; i < 100; ++i) {
  list.push(i+'c');
}
var promise = new Promise(function (resolve, reject) {
  setTimeout(function () {
    console.log('3秒');
    return resolve('3秒');
  }, 3000);
});
function PromiseForEach(arr, cb) {
  let realResult = []
  let result = Promise.resolve()
  arr.forEach(function (a, index) {
    result = result.then(function () {
      return cb(a).then(function (res) {
        realResult.push(res);
      });
    });
  });

  return result.then(function () {
    return realResult;
  });
}
promise.then(function (d) {
  PromiseForEach(list, function (number) {
    return new Promise(function (resolve, reject) {
      setTimeout(function () {
        console.log(number);
        return resolve(number);
      }, 100);
    });
  })
}).then(function (data) {
  console.log("成功");
  console.log(data);
}).catch(function (err) {
  console.log("失败");
  console.log(err);
});
//预期的打印结果是
//3秒
// console.log(number);
//成功
//data
//然而实际的打印结果是
//3秒
//成功
//undefined
//最后执行了 console.log(number);