是“.then（function（a）{return a;}）”承诺的无操作？ [英] Is ".then(function(a){ return a; })" a no-op for promises?

问题描述

我正在阅读本教程关于Bookshelf 。 Bookshelf使用Bluebird承诺。有很多例子看起来像这样：

I'm reading this tutorial about Bookshelf. Bookshelf uses Bluebird promises. There's quite a few examples that look something like this:

var getEvents = function(participantId) {  
  return new models.Participant()
    .query({where: {id: participantId}})
    .fetch({withRelated: ['events'], require: true})
    .then(function(model) {
      return model;
    });
};

我对承诺仍然不满意，但从目前为止我所学到的这看起来很奇怪。我的问题是，上述函数是否与直接返回 fetch（）完全相同，并且离开最后的然后（）：

I'm still not comfortable with promises, but from what I've learned so far this seems odd. My question is, is the above function exactly the same as returning fetch() directly and leaving off the final then():

var getEvents = function(participantId) {  
  return new models.Participant()
    .query({where: {id: participantId}})
    .fetch({withRelated: ['events'], require: true});
};

也就是说，它仍然做同样的事情，返回相同的承诺，可以同样调用方式等？

That is, it still does the same thing, returns the same promise, can be called in the same way, etc?

据我所知，传递给然后的函数的参数得到的返回值链中的先前承诺。所以，在我看来，像 .then（function（a）{return a;}）一般来说只是一个无操作。对吗？

From what I understand, the parameter to the function passed to then gets the return value of the previous promise in the chain. So, it seems to me like .then(function (a) { return a; }) in general is just a no-op. Right?

如果它们不相同，有什么区别？发生了什么，为什么作者这样写呢？

If they aren't the same, what's the difference? What's going on and why did the author write it that way?

推荐答案

在我看来似乎 .then（function（a）{return a;}）只是一个无操作。对吗？

It seems to me like .then(function (a) { return a; }) is just a no-op. Right?

是。 ¹

这是没用的，应该省略。

Yes.¹

It is useless and should be omitted.

发生了什么以及为什么作者这样写呢？

What's going on and why did the author write it that way?

这是一个大错。或者作者不理解承诺。

It's a blunder. Or the author didn't understand promises.

_{1：如果它们不相同，有什么区别？}

<sub>与往常一样，有一些边缘情况。真的很奇怪。没有人应该使用（没有广泛的评论）：

a）它返回一个新的promise实例，一个不同的对象，以避免共享。但是， .then（）也会。

b） a 再次进行测试thenable的烦躁。如果它自实现以来突然成为一种承诺，那么它现在将被等待。这当然很糟糕。</sub>

_{As always, there are some edge cases. Really weird ones. That no-one should use (without extensive commenting):
a) it returns a new promise instance, a distinct object, to avoid sharing. However, .then() would as well.
b) a is tested again for its thenable-ness. If it suddenly became a promise since the fulfillment, it now will be awaited. This would be awful of course.}

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