How to test the type of a thrown exception in Jest

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 is AVA and I can test it as a second argument t.throws method, like here:

it('should throw Error with message \'UNKNOWN ERROR\' when no params were passed', (t) => {
  const error = t.throws(() => {
    throwError();
  }, TypeError);

  t.is(error.message, 'UNKNOWN ERROR');
});

I started rewriting my tests in Jest and couldn't find how to easily do that. Is it even possible?

In Jest you have to pass a function into expect(function).toThrow(<blank or type of error>).

Example:

test("Test description", () => {
  const t = () => {
    throw new TypeError();
  };
  expect(t).toThrow(TypeError);
});

Or if you also want to check for error message:

test("Test description", () => {
  const t = () => {
    throw new TypeError("UNKNOWN ERROR");
  };
  expect(t).toThrow(TypeError);
  expect(t).toThrow("UNKNOWN ERROR");
});

If you need to test an existing function whether it throws with a set of arguments, you have to wrap it inside an anonymous function in expect().

Example:

test("Test description", () => {
  expect(() => {http.get(yourUrl, yourCallbackFn)}).toThrow(TypeError);
});

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: https://stackoverflow.com/questions/46042613/how-to-test-type-of-thrown-exception-in-jest/58103698#58103698

EDIT 2022:

To use this approach and not trigger no-conditional-expect rule (if you're using eslint-plugin-jest), documentation of this rule suggest to use error wrapper:

class NoErrorThrownError extends Error {}

const getError = async <TError>(call: () => unknown): Promise<TError> => {
  try {
    await call();

    throw new NoErrorThrownError();
  } catch (error: unknown) {
    return error as TError;
  }
};

describe('when the http request fails', () => {
  it('includes the status code in the error', async () => {
    const error = await getError(async () => makeRequest(url));

    // check that the returned error wasn't that no error was thrown
    expect(error).not.toBeInstanceOf(NoErrorThrownError);
    expect(error).toHaveProperty('statusCode', 404);
  });
});

See: no-conditional-expect docs

I use a slightly more concise version:

expect(() => {
  // Code block that should throw error
}).toThrow(TypeError) // Or .toThrow('expectedErrorMessage')

From my (albeit limited) exposure to Jest, I have found that expect().toThrow() is suitable if you want to only test an error is thrown of a specific type:

expect(() => functionUnderTest()).toThrow(TypeError);

Or an error is thrown with a specific message:

expect(() => functionUnderTest()).toThrow('Something bad happened!');

If you try to do both, you will get a false positive. For example, if your code throws RangeError('Something bad happened!'), this test will pass:

expect(() => functionUnderTest()).toThrow(new TypeError('Something bad happened!'));

The answer by bodolsog which suggests using a try/catch is close, but rather than expecting true to be false to ensure the expect assertions in the catch are hit, you can instead use expect.assertions(2) at the start of your test where 2 is the number of expected assertions. I feel this more accurately describes the intention of the test.

A full example of testing the type and message of an error:

describe('functionUnderTest', () => {
    it('should throw a specific type of error.', () => {
        expect.assertions(2);

        try {
            functionUnderTest();
        } catch (error) {
            expect(error).toBeInstanceOf(TypeError);
            expect(error).toHaveProperty('message', 'Something bad happened!');
        }
    });
});

If functionUnderTest() does not throw an error, the assertions will be be hit, but the expect.assertions(2) will fail and the test will fail.

Modern Jest allows you to make more checks on a rejected value. For example, you could test status code of http exception:

const request = Promise.reject({statusCode: 404})
await expect(request).rejects.toMatchObject({ statusCode: 500 });

will fail with error

Error: expect(received).rejects.toMatchObject(expected)

- Expected
+ Received

  Object {
-   "statusCode": 500,
+   "statusCode": 404,
  }

Further to Peter Danis' post, I just wanted to emphasize the part of his solution involving "[passing] a function into expect(function).toThrow(blank or type of error)".

In Jest, when you test for a case where an error should be thrown, within your expect() wrapping of the function under testing, you need to provide one additional arrow function wrapping layer in order for it to work. I.e.

Wrong (but most people's logical approach):

expect(functionUnderTesting();).toThrow(ErrorTypeOrErrorMessage);

Right:

expect(() => { functionUnderTesting(); }).toThrow(ErrorTypeOrErrorMessage);

It's very strange, but it should make the testing run successfully.

I manage to combine some answers and end up with this:

it('should throw', async () => {
    await expect(service.methodName('[email protected]', 'unknown')).rejects.toThrow(
      HttpException,
    );
  });

In case you are working with Promises:

await expect(Promise.reject(new HttpException('Error message', 402)))
  .rejects.toThrowError(HttpException);

I haven't tried it myself, but I would suggest using Jest's toThrow assertion. So I guess your example would look something like this:

it('should throw Error with message \'UNKNOWN ERROR\' when no parameters were passed', (t) => {
  const error = t.throws(() => {
    throwError();
  }, TypeError);

  expect(t).toThrowError('UNKNOWN ERROR');
  //or
  expect(t).toThrowError(TypeError);
});

Again, I haven't test it, but I think it should work.

Jest has a method, toThrow(error), to test that a function throws when it is called.

So, in your case you should call it so:

expect(t).toThrowError(TypeError);

The documentation.

Check out toThrow method.

You must wrap the code in an additional function callback!

You should check both: the error message and its type.

For example:

expect(
  () => { // additional function wrap
    yourCodeToTest();
  }
).toThrow(
  new RangeError('duplicate prevArray value: A')
);

Because of additional callback wrap, the code will not be run immediately, so jest will be able to catch it.

You should always check the error message to be sure you are checking the correct throw case and not getting another error your code may throw.

It is also nice to check the error type, so the client code may rely on it.

The documentation is clear on how to do this. Let's say I have a function that takes two parameters and it will throw an error if one of them is null.

function concatStr(str1, str2) {
  const isStr1 = str1 === null
  const isStr2 = str2 === null
  if(isStr1 || isStr2) {
    throw "Parameters can't be null"
  }
  ... // Continue your code

Your test

describe("errors", () => {
  it("should error if any is null", () => {
    // Notice that the expect has a function that returns the function under test
    expect(() => concatStr(null, "test")).toThrow()
  })
})

I have successfully used this

await expect(
      async () => await apiCalls()
    ).rejects.toThrow();

I ended up writing a convenience method for our test-utils library

/**
 *  Utility method to test for a specific error class and message in Jest
 * @param {fn, expectedErrorClass, expectedErrorMessage }
 * @example   failTest({
      fn: () => {
        return new MyObject({
          param: 'stuff'
        })
      },
      expectedErrorClass: MyError,
      expectedErrorMessage: 'stuff not yet implemented'
    })
 */
  failTest: ({ fn, expectedErrorClass, expectedErrorMessage }) => {
    try {
      fn()
      expect(true).toBeFalsy()
    } catch (err) {
      let isExpectedErr = err instanceof expectedErrorClass
      expect(isExpectedErr).toBeTruthy()
      expect(err.message).toBe(expectedErrorMessage)
    }
  }

A good way is to create custom error classes and mock them. Then you can assert whatever you want.

MessedUpError.ts

type SomeCrazyErrorObject = {
  [key: string]: unknown,
}

class MessedUpError extends Error {
  private customErrorData: SomeCrazyErrorObject = {};

  constructor(err?: string, data?: SomeCrazyErrorObject) {
    super(err || 'You messed up');

    Object.entries(data ?? {}).forEach(([Key, value]) => {
      this.customErrorData[Key] = value;
    });
    Error.captureStackTrace(this, this.constructor);
  }

  logMe() {
    console.log(this.customErrorData);
  }
}

export default MessedUpError;

messedUpError.test.ts

import MessedUpError from './MessedUpError';

jest.mock('./MessedUpError', () => jest.fn().mockImplementation((...args: any[]) => ({
  constructor: args,
  log: () => {},
})));

type MessedUpErrorContructorParams = Expand<typeof MessedUpError['prototype']>
const MessedUpErrorMock = MessedUpError as unknown as jest.Mock<MessedUpError, [MessedUpErrorContructorParams]>;

const serverErrorContructorCall = (i = 0) => ({
  message: MessedUpErrorMock.mock.calls[i][0],
  ...MessedUpErrorMock.mock.calls[i][1] || {},
});

beforeEach(() => {
  MessedUpErrorMock.mockClear();
});

test('Should throw', async () => {
  try {
    await someFunctionThatShouldThrowMessedUpError();
  } catch {} finally {
    expect(MessedUpErrorMock).toHaveBeenCalledTimes(1);
    const constructorParams = serverErrorContructorCall();
    expect(constructorParams).toHaveProperty('message', 'You messed up');
    expect(constructorParams).toHaveProperty('customErrorProperty', 'someValue');
  }
});

The assertions always go inside the finally clause. This way it will always be asserted. Even if the test does not throw any errors.

Try:

expect(t).rejects.toThrow()