Jest: rejects/resolves does not fail the test on non-promise value

[alexilyaev]

Do you want to request a _feature_ or report a _bug_?
Bug

What is the current behavior?
Passing a non-promise value to rejects or resolves does not fail the test

If the current behavior is a bug, please provide the steps to reproduce and
either a repl.it demo through https://repl.it/languages/jest or a minimal
repository on GitHub that we can yarn install and yarn test.

Test code

describe('someMethod', () => {
  it('should reject', () => {
    expect(123).rejects.toThrow('error');
  });
});

Also this one:

describe('someMethod', () => {
  it('should reject', () => {
    const fn = new Promise((resolve, reject) => {
      resolve(1234);
    });

    expect(fn).rejects.toThrow('error');
  });
});

Output

(node:93202) UnhandledPromiseRejectionWarning: Error: expect(received).rejects.toThrow()

received value must be a Promise.
Received:
  number: 123
(node:93202) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
 PASS  app/remote-control/index.spec.js
  test
    someMethod
      ✓ should reject (1ms)

  console.warn node_modules/bluebird/js/release/debuggability.js:873
    Unhandled rejection error


Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.226s, estimated 1s

What is the expected behavior?
Should fail the test

Please provide your exact Jest configuration and mention your Jest, node,
yarn/npm version and operating system.
Node v9.3.0
Jest v22.1.2
Yarn v1.3.2
Mac OS X 10.12.6

[SimenB]

Good catch!

This function should not be async: https://github.com/facebook/jest/blob/36c705bb1eac380e0926487bbe8b8db0aa2fa2e8/packages/expect/src/index.js#L159-L189

[incleaf]

Hi, I'd like to take a look and submit a PR for this issue.

[SimenB]

Go for it!

[alexilyaev]

Added another test code in the description that's not as expected

[SimenB]

Note that the OP uses resolves/rejects incorrectly - it either has to be returned or awaited. See docs. If that is done, the test behaves as expected.

I still think we should give a proper synchronous error before switching into async mode, though.

There is no way for jest to detect this scenario (help welcome in the lint plugin: https://github.com/jest-community/eslint-plugin-jest/issues/54)

[alexilyaev]

Oh wow, what a bummer, totally forgot about it.
Would be awesome if Jest would let me know!

[SimenB]

It should be pretty easy using the eslint plugin. Help welcome 🙂

I don't think it's possible to detect in Jest itself as jest is runtime only - no static analysis (beyond what we do in the babel plugin)

[alexilyaev]

I see.
Might take a try at the eslint rule later.

Is there still something to do in this ticket then?

[SimenB]

Will close once #5364 is merged

[tur-nr]

It's also reports false positives. I think there is something wrong the resolves api.

it('should fail', () => {
    expect(Promise.resolve(false)).resolves.toBeTruthy();
});

This test case reports back as passed.

However if I write it another way;

it('should fail', async () => {
    const res = await Promise.resolve(false);
    expect(res).toBeTruthy();
});

This test case fails, as expected.

---

$ node --version
v8.9.2

$ jest --version
v22.1.4

EDIT

I do get the following in the console output

(node:9596) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: expect(received).toBeTruthy()

[incleaf]

I think return expect(Promise.resolve(false)).resolves.toBeTruthy(); would be correct in the first case as doc says. But we can also give a synchronous error in resolves as rejects will do.

[SimenB]

@tur-nr see https://github.com/facebook/jest/issues/5361#issuecomment-359475752