javascript - Q.defer() 和 Promise() 的区别

Question

我试图理解为什么以下代码与 Q.defer() 和 Promise() 的行为不同

Case 1 : When I'm using Q.defer()

getDocument(id)
.then(function (response) {
   console.log('in first then')
   return 'from two';
}).then(function (response) {
   console.log(response)
});

var getDocument=function(){
  var b = Q.defer();
    b.resolve('from getDocument'); // here will do some async operation..this is just an example
  return b.promise;
}

输出:

in first then
undefined

Case 2: using Promise()

getDocument(id)
.then(function (response) {
   console.log('in first then')
   return 'from two';
}).then(function (response) {
   console.log(response)
});

var getDocument=function(){
  return Promise.resolve('from getDocument');
}

输出:

in first then        
from two

Question

1. 为什么输出有差异？
2. 我知道 Q.defer 是一种反模式，但是如何选择何时使用什么？

Answer 1

事实上，两个示例都返回相同的结果(相同的顺序)。检查这个codepen Example

var getDocument=function(){
  var b = Q.defer();
    b.resolve('Q from getDocument'); // here will do some async operation..this is just an example
  return b.promise;
}

getDocument(1)
.then(function (response) {
   console.log('Q in first then')
   return 'Q from two';
}).then(function (response) {
   console.log(response)
});

var getDocumentP=function(){
  return Promise.resolve('P from getDocument');
}

getDocumentP(1)
.then(function (response) {
   console.log('P in first then')
   return 'P from two';
}).then(function (response) {
   console.log(response)
});

2) 你可以在这里看到 Q.defer 的一些用法:Q.defer you´re doing it wrong