使用 jest 验证异步函数抛出非错误对象

Question

Using the jest framework, how can I verify that my async function throws something other than an Error ?

In my examples below, the first one works as expected, as the function being tested throws an Error . The second example, where the function throws a string , doesnt work - jest doesnt verify that the function throws.

// Works as expected
test('verify error thrown', async () => {
    const expected = new Error('actual-error');
    const fn = async () => { throw expected; };
    await expect(fn()).rejects.toThrow(expected);
});

// Fails with: Received function did not throw
test('verify non-error thrown', async () => {
    const expected = 'non-error';
    const fn = async () => { throw expected; };
    await expect(fn()).rejects.toThrow(expected);
});

Answer 1

I don't think you need 'rejects' in either case. According to the docs it's not clear if the toThrow method expects results to be derived from Error . There is a regex form which might validate against a string.

rejects says that it unwraps the rejection so it can be matched, and that might give you the error message string from Error , or a string if the result was a string.

Answer 2

Dave's point was absolutely right; using rejects.toThrow was not the right way to go here.

Instead, I'm managed to verify the expected behaviour by simply catching the error and verifying it's value.

test('verify non-error thrown', async () => {
    const expected = 'non-error';
    const fn = async () => { throw expected; };
    await fn().catch(error => expect(error).toStrictEqual(expected));
    expect.assertions(1);
});

Note that expect.assertions(1) is important here. Without this, if the code being tested was updated to not throw, the test would still pass.