<script>
//Promise代码部分（我选择狗带）
Promise = function() {
  this.queue = [];
  this.value = null;
  this.status = 'pending';// pending fulfilled rejected
};

Promise.prototype.getQueue = function() {
  return this.queue;
};
Promise.prototype.getStatus = function() {
  return this.status;
};
Promise.prototype.setStatus = function(s, value) {
  if (s === 'fulfilled' || s === 'rejected') {
    this.status = s;
    this.value = value || null;
    this.queue = [];
    var freezeObject = Object.freeze || function(){};
    freezeObject(this);// promise的状态是不可逆的
  } else {
    throw new Error({
      message: "doesn't support status: " + s
    });
  }
};
Promise.prototype.isFulfilled = function() {
  return this.status === 'fulfilled';
};
Promise.prototype.isRejected = function() {
  return this.status === 'rejected';
}
Promise.prototype.isPending = function() {
  return this.status === 'pending';
}
Promise.prototype.then = function(onFulfilled, onRejected) {
  var handler = {
    'fulfilled': onFulfilled,
    'rejected': onRejected
  };
  handler.deferred = new Deferred();

  if (!this.isPending()) {//这里允许先改变promise状态后添加回调
    utils.procedure(this.status, handler, this.value);
  } else {
    this.queue.push(handler);//then may be called multiple times on the same promise;规范2.2.6
  }
  return handler.deferred.promise;//then must return a promise;规范2.2.7
};

var utils = (function(){
  var makeSignaler = function(deferred, type) {
    return function(result) {
      transition(deferred, type, result);
    }
  };

  var procedure = function(type, handler, result) {
    var func = handler[type];
    var def = handler.deferred;

    if (func) {
      try {
        var newResult = func(result);
        if (newResult && typeof newResult.then === 'function') {//thenable
          // 此种写法存在闭包容易造成内存泄露，我们通过高阶函数解决
          // newResult.then(function(data) {
          //   def.resolve(data);
          // }, function(err) {
          //   def.reject(err);
          // });
          //PromiseA+规范，x代表newResult，promise代表def.promise
          //If x is a promise, adopt its state [3.4]:
          //If x is pending, promise must remain pending until x is fulfilled or rejected.
          //If/when x is fulfilled, fulfill promise with the same value.
          //If/when x is rejected, reject promise with the same reason.
          newResult.then(makeSignaler(def, 'fulfilled'), makeSignaler(def, 'rejected'));//此处的本质是利用了异步闭包
        } else {
          transition(def, type, newResult);
        }
      } catch(err) {
        transition(def, 'rejected', err);
      }
    } else {
      transition(def, type, result);
    }
  };

  var transition = function(deferred, type, result) {
    if (type === 'fulfilled') {
      deferred.resolve(result);
    } else if (type === 'rejected') {
      deferred.reject(result);
    } else if (type !== 'pending') {
      throw new Error({
        'message': "doesn't support type: " + type
      });
    }
  };

  return {
    'procedure': procedure
  }
})();

Deferred = function() {
  this.promise = new Promise();
};

Deferred.prototype.resolve = function(result) {
  if (!this.promise.isPending()) {
    return;
  }

  var queue = this.promise.getQueue();
  for (var i = 0, len = queue.length; i < len; i++) {
    utils.procedure('fulfilled', queue[i], result);
  }
  this.promise.setStatus('fulfilled', result);
};

Deferred.prototype.reject = function(err) {
  if (!this.promise.isPending()) {
    return;
  }

  var queue = this.promise.getQueue();
  for (var i = 0, len = queue.length; i < len; i++) {
    utils.procedure('rejected', queue[i], err);
  }
  this.promise.setStatus('rejected', err);
}
/*****************************上面看不懂，分割线************************************/
//测试部分
ajax = function(url) {
  var def = new Deferred();

  var xhr = new XMLHttpRequest();
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4) {
      if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) {
        def.resolve(xhr.responseText)
      } else {//简化ajax，没有提供错误回调
        def.reject(new Error({
          message: xhr.status
        }));
      }
    }
  };
  xhr.open('get', url, true);
  xhr.send(null);

  return def.promise;
}

ajax('test.php?act=1').then(function(data1) {
  console.log(data1);//处理data1
  return ajax('test.php?act=2');
}).then(function(data2) {
  console.log(data2);//处理data2
  return ajax('test.php?act=3');
}, function(err) {
  console.error(err);
}).then(function(data3) {
  console.log(data3);
  alert('success');
}, function(err) {
  console.error(err);
});
</script>

<?php
if($_GET['act']==1){
  echo json_encode(array("code"=>200));
}else if($_GET['act']==2){
  echo json_encode(array("code"=>300));
}else if($_GET['act']==3){
  echo json_encode(array("code"=>400));
}