How to ensure Jest fails on "unhandledRejection"?

Question

Our unit tests run in containers in our continuous delivery pipelines.

Sometimes, we don't handle rejections in our unit tests, however, I don't think it's correct and in my opinion the pipeline should fail.

How can I make sure that when I execute jest and during the tests an unhandledRejection event occurs, jest will exit with error?

I tried to hook event listeners in a setup script:

/* jest.config.js */
module.exports = {
    setupFiles: ['<rootDir>/test/setup.ts'],
    // ...
}

and in that setup script, I can detect unhandledRejection events, but unfortunately, I can't make jest break

process.on('unhandledRejection', () => {
    console.error('Now what?');
    console.error('I want to make sure that jest exits with an error!');
});

Answer 1

You can create a jest config file jest.config.js and put the following entry into it:

module.exports = {
  ...
  setupFiles: ['<rootDir>/test/setup.js'],
  ...
};

And then in your setup.js file, you can do something like this:

process.on('unhandledRejection', (err) => {
  fail(err);
});

And unhandledRejection will fail a test, though there are two caveates to be aware of:

1. Unhandled rejections from promises that reject when there is no test running will end the process. This is probably what you want and expect.
2. Unhandled rejections from promises that reject when a new test is running (not the one that initiated the promise) will fail the new test, not the original test. This is confusing and can make for difficult to track bugs.

As a commenter mentioned above, if your tests are well written, then you should never hit this scenario, but you don't always have that much control.

Answer 2

Adding the following flags when running jest did the trick for me:

--detectOpenHandles --forceExit --runInBand