Testing exceptions in async functions with jest

Sync world #

Write test that check a function throws can be daunting. This is a first attempt:

try {
  functionThatThrows();
} catch (error) {
  expect(error).toEqual(...)
}

The above code have several problems. The most important is that if the function doesn't throw, the test passes. Bad.

Fortunately, jest has you covered:

expect(functionThatThrows).toThrow(...)

Async world #

But what happens if the "functionThatThrows" is async (returns a promise)?

This doesn't work:

// DO NOT try this at home:
expect(await functionThatThrows()).toThrow(...)

But there are other alternatives explained here. My favourite is:

await expect(asyncFunction()).rejects.toThrow(...)

A real world example:

await expect(UserFactory.create({ name: null })).rejects.toThrow(
  /NULL into column 'name'/
);

Happy testing 🖖