NodeJS UnhandledPromiseRejectionWarning [英] NodeJS UnhandledPromiseRejectionWarning

问题描述

所以，我正在测试一个依赖于事件发射器的组件。为此，我想出了一个使用Promise with Mocha + Chai的解决方案：

So, I'm testing a component that relies on an event-emitter. To do so I came up with a solution using Promises with Mocha+Chai:

it('should transition with the correct event', (done) => {
  const cFSM = new CharacterFSM({}, emitter, transitions);
  let timeout = null;
  let resolved = false;
  new Promise((resolve, reject) => {
    emitter.once('action', resolve);
    emitter.emit('done', {});
    timeout = setTimeout(() => {
    if (!resolved) {
      reject('Timedout!');
    }
    clearTimeout(timeout);
  }, 100);
}).then(((state) => {
      resolved = true;
      assert(state.action === 'DONE', 'should change state');
      done();
    }))
    .catch((error) => {
      assert.isNotOk(error,'Promise error');
      done();
    });
  });
});

在控制台上我得到'UnhandledPromiseRejectionWarning'即使拒绝函数被调用，因为它立即显示消息'AssertionError：Promise error'

On the console I'm getting an 'UnhandledPromiseRejectionWarning' even though the reject function is getting called since it instantly shows the message 'AssertionError: Promise error'

（节点：25754）UnhandledPromiseRejectionWarning：未处理的承诺
拒绝（拒绝ID： 2）：AssertionError：Promise错误：预期
{Object（message，showDiff，...）}是假的
1）应该转换正确的事件

(node:25754) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): AssertionError: Promise error: expected { Object (message, showDiff, ...) } to be falsy 1) should transition with the correct event

然后，在2秒后我得到

错误：超过2000ms超时。确保在此测试中调用done（）回调是
。

Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test.

自执行catch回调以来，这甚至更怪异（我认为由于某种原因，断言失败阻止了其余的执行）

Which is even weirder since the catch callback was executed.(I think that for some reason the assert failure prevented the rest of the execution)

现在有趣的是，如果我注释掉断言.isNotOk（错误...）测试运行正常，没有控制台中的任何警告。它在执行捕获的意义上仍然失败。

但是，我仍然无法理解这些错误。有人可以开导我吗？

Now the funny thing, if I comment out the assert.isNotOk(error...) the test runs fine with out any warning in the console. It stills 'fails' in the sense that it executes the catch.
But still, I can't understand these errors with promise. Can someone enlighten me?

推荐答案

问题是由此引起的：

.catch((error) => {
  assert.isNotOk(error,'Promise error');
  done();
});

如果断言失败，则会抛出错误。此错误将导致 done（）永远不会被调用，因为代码在它之前出错了。这就是导致超时的原因。

If the assertion fails, it will throw an error. This error will cause done() never to get called, because the code errored out before it. That's what causes the timeout.

未处理的承诺拒绝也是由失败的断言引起的，因为如果在一个错误中引发错误 catch（） handler，并且没有后续的 catch（）处理程序，错误将被吞噬（如本文中所述）。 UnhandledPromiseRejectionWarning 警告提醒您注意这一事实。

The "Unhandled promise rejection" is also caused by the failed assertion, because if an error is thrown in a catch() handler, and there isn't a subsequent catch() handler, the error will get swallowed (as explained in this article). The UnhandledPromiseRejectionWarning warning is alerting you to this fact.

一般情况下，如果您想测试基于承诺的代码在摩卡，你应该依靠摩卡本身已经能够处理承诺的事实。您不应该使用 done（），而是从测试中返回一个承诺。然后摩卡会发现任何错误。

In general, if you want to test promise-based code in Mocha, you should rely on the fact that Mocha itself can handle promises already. You shouldn't use done(), but instead, return a promise from your test. Mocha will then catch any errors itself.

喜欢这样：

it('should transition with the correct event', () => {
  ...
  return new Promise((resolve, reject) => {
    ...
  }).then((state) => {
    assert(state.action === 'DONE', 'should change state');
  })
  .catch((error) => {
    assert.isNotOk(error,'Promise error');
  });
});

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