f1().then(f2);

//constructor
var Promise = function() {
 this.callbacks = [];
}
Promise.prototype = {
 construct: Promise,
 resolve: function(result) {
  this.complete("resolve", result);
 },
 reject: function(result) {
  this.complete("reject", result);
 },
 complete: function(type, result) {
  while (this.callbacks[0]) {
   this.callbacks.shift()[type](result);
  }
 },
 then: function(successHandler, failedHandler) {
  this.callbacks.push({
   resolve: successHandler,
   reject: failedHandler
  });
  return this;
 }
}
// test
var promise = new Promise();
var delay1 = function() {
 setTimeout(function() {
  promise.resolve('数据1');
 }, 1000);
 return promise;
};
var callback1 = function(re) {
 re = re + '数据2';
 console.log(re);
};
delay1().then(callback1)

var promise = new Promise();
var delay1 = function() {
 setTimeout(function() {
  promise.resolve('数据1');
 }, 1000);
 return promise;
};
var callback1 = function(re) {
 re = re + '数据2';
 console.log(re);
 promise.resolve(re);
};
var callback2 = function(re) {
 console.log(re + '数据3');
};
delay1().then(callback1).then(callback2);

//第一步
var delay1 = function() {
 setTimeout(function() {
  promise.resolve('数据1');
 }, 1000);
 return promise;
};

//第二步
var callback1 = function(re) {
 
 re = re + '数据2';
 console.log(re);
 promise.resolve(re);
};

//第三步
delay1().then(callback1).then(callback2);

//第四步
setTimeout(function() {
 promise.resolve('数据1');
}, 1000);

//第五步
resolve: function(result) {
 this.complete("resolve", result);
},
//第六步：循环执行回调，将上一个回调的结果传递给下一个回调
complete: function(type, result) {
 while (this.callbacks[0]) {
  this.callbacks.shift()[type](result);
 }
},