How to test the type of a thrown exception in Jest

Question

I\'m working with some code where I need to test the type of an exception thrown by a function (is it TypeError, ReferenceError, etc.?).

My current testing framework

Answer 1

It is a little bit weird, but it works and IMHO is good readable:

it('should throw Error with message \'UNKNOWN ERROR\' when no parameters were passed', () => {
  try {
      throwError();
      // Fail test if above expression doesn't throw anything.
      expect(true).toBe(false);
  } catch (e) {
      expect(e.message).toBe("UNKNOWN ERROR");
  }
});

The Catch block catches your exception, and then you can test on your raised Error. Strange expect(true).toBe(false); is needed to fail your test if the expected Error will be not thrown. Otherwise, this line is never reachable (Error should be raised before them).

@Kenny Body suggested a better solution which improve a code quality if you use expect.assertions():

it('should throw Error with message \'UNKNOWN ERROR\' when no parameters were passed', () => {
  expect.assertions(1);
  try {
      throwError();
  } catch (e) {
      expect(e.message).toBe("UNKNOWN ERROR");
  }
});

See the original answer with more explanations: How to test the type of a thrown exception in Jest