无极地吞下摩卡/柴断言

Question

I have a function in my project to send a bunch of emails one after the other. So, I compile these lists into an array of promises, and use Promise.all(...) to send them all and end the function after. I am going to test this, however, and it seems as though Promise is 'swallowing' my error. Here is a snippet from my code:

module.exports.checkForLoanEmailAndSendAll = function(next) {
    // Some setup code
    var emailPromises = [];
    Object.keys(userLoanMap).forEach(function(user) {
      emailPromises.push(new Promise((resolve, reject) => {
        sendSingleLoanEmail(user, userLoanMap[user], loanEmailObj, function(error) {
          if (error) {
            reject(error);
          } else {
            resolve();
          }
        });
      }));
    });

    Promise.all(emailPromises).then(function() {
      next();
    }, function(error) {
      next(error);
    });
}

next in this case is just a standard callback that returns the error if there's an error, or null if not. In my test I have this code (which calls the previous snippet)

it('sends loan emails to appropriate users if there is a email to send today', (done) => {
    Emailer.checkForLoanEmailAndSendAll(function(error) {
        should.not.exist(error);
        var sentMail = nodemailerMock.mock.sentMail();
        sentMail.length.should.be.eql(2);
        // more should assertions
        done();
    });
});

What happens is if, say sentMail.length.should.be.eql(2); fails, then it will result in an UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): AssertionError: instead of being handled normally by mocha and failing the test.

How do I get Promise to not swallow this error and let mocha to handle it?

Answer 1

Here's the immediate problem you are facing. The sequence of events is:

1. Promise.all(emailPromises) resolves, so it calls the success callback you passed to .then .

2. next() is called.

3. sentMail.length.should.be.eql(2); fails, which throws an exception.

4. Because the exception happened in a .then handler, it is assimilated to a promise rejection.

From this point on, there's nothing which is able to handle the rejection. You have this:

Promise.all(emailPromises).then(function() {
  next();
}, function(error) {
  next(error);
});

But you should note that when you call .then(success, failure) the failure handler will catch anything that happens before success is called. If success itself fails, then failure won't be called .

As a minimal change you could rewrite it as:

Promise.all(emailPromises).then(function() {
  next();
}).catch(function(error) {
  next(error);
});

This would catch any errors that happen when next is called by the .then handler.

Ultimately, however, I would rewrite the whole thing so that it uses promises through and through. Then you could just wait on the promise in your Mocha test and return that promise to Mocha. It would be much nicer overall.