从中间件中的Promise调用"next()"会导致“下一个不应被多次调用" [英] Calling 'next()' from promises in a middleware causes 'next shouldn't be called more than once'

问题描述

最近，我将代码从Express更改为Restify.老实说，我不确定它以前是否曾经发生过，但我想确实是这样.

基本上，在我的中间件中，我调用一个承诺方法，当它解析后，我调用next，并在下一个中间件中执行其他操作.当它被拒绝时，在某些情况下，我也想正确地调用next.否则，它必须调用将err传递给next的错误中间件.

somePromise()
.then(()=>{
    next();
})
.catch((err)=>{
    if(err.someatt) next();
    else next(err)
});

它可以很好地与somePromise的预期结果一起使用.问题是next受then-catch链约束.并且在下一个中间件中引发错误时，它会调用catch方法并再次调用next！

我发现next具有属性called，并且在再次调用next之前将其设置为false时，我摆脱了错误.但这当然是一种反模式.而且我在另一个也使用了promises的中间件中遇到了同样的问题(按预期方式调用next，然后在catch语句中再次调用它).

还有其他人有这样的问题吗?

解决方案

将您的链更改为此:

somePromise().then(() => {
  next();
}, err => {
  // error occurred in somePromise()
  if(err.someatt) next();
  else next(err);
}).catch(err => {
  // error occurred in .then()'s next()
  // don't call next() again
});

用作.catch()回调，但仅针对链中上级引发的错误而调用，而不针对相邻的.then()回调中引发的错误而调用.

从这个很棒的答案借来的非常有用的流程图说明了.then(onFulfilled, onRejected)和.then(onFulfilled).catch(onRejected)之间的区别:

Recently I changed my code from Express to Restify. I'm honestly not sure if it used to happen before, but I guess it did.

Basically in my middleware I call a promisified method and when it resolves I call next and do other stuff in the next middleware. When it is rejected I also want to call next with no errors in some cases. Otherwise it must call the error middleware passing err to next.

somePromise()
.then(()=>{
    next();
})
.catch((err)=>{
    if(err.someatt) next();
    else next(err)
});

It works fine with the expected results of somePromise. The problem is that next is bound by the then-catch chain. And when an error is thrown in the next middleware it invokes the catch method and calls next again!

I found out that next has an attribute called and when I turn it to false before calling next again I get rid of the the errors. But of course it is an antipattern. And I'm having the same problem in a different middleware that I also used promises (calling next as expected and then calling it again in the catch statement).

Anyone else had a problem like that?

解决方案

Change your chain to this:

somePromise().then(() => {
  next();
}, err => {
  // error occurred in somePromise()
  if(err.someatt) next();
  else next(err);
}).catch(err => {
  // error occurred in .then()'s next()
  // don't call next() again
});

The optional second argument of .then() acts as a .catch() callback, but is only invoked for errors thrown higher up in the chain, and is not invoked for errors thrown in the adjacent .then() callback.

A very helpful flowchart borrowed from this awesome answer demonstrates the difference between .then(onFulfilled, onRejected) and .then(onFulfilled).catch(onRejected):

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