Unit test tips & tricks — Avoid timeout in async test

We write unit test everywhere in OpenPaaS, some of them are asynchronous, let’s see an example:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    expect(configuration.modules).to.have.length(2);
    done();
  });

It won’t be a problem if it passes, but what if it fails?

Error: timeout of 20000ms exceeded. Ensure the done() callback is being called in this test.

Well, the test case got hang for 20s then printed out an error message that is not helpful at all. Why the done() was not called? Did the promise reject? Did the promise take too much time to resolve? You start debuging the failed test by commenting the expect statement:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    //expect(configuration.modules).to.have.length(2);
    done();
  });

And it passed!

Done, without errors.

Obviously, the problem comes from the expect statement, you continue with:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    console.log('Length:', configuration.modules.length);
    //expect(configuration.modules).to.have.length(2);
    done();
  });
Length: 1

Cool! Finally, you found the source of the problem. You fix your code or your mock, the test comes green, you feel so happy.

Well, it would not be easy like that when your test fails on your CI service, especially with random failure. You may already knew, sometimes we have random failure and the error message is just Error: timeout of 20000ms exceeded. It drives you crazy because you don't know why is that, the error message is not helpful. When you try to debug it on your local machine, you cannot reproduce the failure because it's simply a random failure!

Here is the secret: always call done when error occurs (of course call it with useful error message)

Now, let’s modify your test like this:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    expect(configuration.modules).to.have.length(2);
    done();
  })
  .catch(done);
AssertionError: expected [ Array(1) ] to have a length of 2 but got 1

The error message is much more helpful now. But hey, what if the promise rejects with undefined error message? Your test will become a success even though it should be a failure. To avoid that, always make sure you call done with something:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    expect(configuration.modules).to.have.length(2);
    done();
  })
  .catch(done.bind(null, 'this promise should resolve'));
Error: done() invoked with non-Error: this promise should resolve

But the error is not really helpful, it doesn’t tell you why it didn’t resolve.

This is better:

getModule()
  .findByDomainId()
  .then(function(configuration) {
    expect(configuration.modules).to.have.length(2);
    done();
  })
  .catch(function(err) {
    done(err || 'this promise should resolve');
  });
AssertionError: expected [ Array(1) ] to have a length of 2 but got 1

That’s quite cumbersome, let’s try ES6 syntax:

getModule()
  .findByDomainId()
  .then(configuration => {
    expect(configuration.modules).to.have.length(2);
    done();
  })
  .catch(err => done(err || 'should resolve'));
AssertionError: expected [ Array(1) ] to have a length of 2 but got 1

That’s perfect now.

Happy unit testing!