Making 'expect()' mandatory for all test cases

user_6133559 0
Viewed 2507

Is there a way to make the expect() mandatory for all test cases? If the programmer doesn't put expect it should throw some warning/error. I am using Jest and @testing-library/react.

For example:

  • Should work fine:

    test('Should work file',()=>{
    ...code...
    expect(value).toBe(expectedValue)
})

  • Should throw error/warning:

    test('Should fail',()=>{
    ...code...
})
jestjs reactjs javascript
2 Answers

There are two ways I'm aware of to automatically do this:

  1. The ESLint Jest plugin's jest/expect-expect rule, which will ensure that every test calls expect at least once. This can also be configured to allow other assertion functions/methods, like Cypress's "cy.**.should" or Supertest's "request.**.assert".

  2. Use expect.assertions/expect.hasAssertions inside the tests, so that they will fail if the required number of expectations isn't met.

    This is a bit less automated, so may not solve the problem of people forgetting to include things, but will also catch cases where assertions exist but aren't reached (e.g. due to asynchronous code or no error being thrown). There's another rule in the ESLint plugin, jest/prefer-expect-assertions, which would check that you always use it, giving you the best of both worlds.

There's also a process-based approach: test-driven development. If you're always running the tests before you expect them to pass, you make sure they do indeed fail (and give useful feedback when they do so).

user_30017610

Linting rules such as jest/expect-expect have been very useful, but I've seen expect.hasAssertions, placed in the shared Jest setup file, catch a few things the linter hasn't:

beforeEach(() => {
   expect.hasAssertions();
});

The linter does a static analysis while hasAssertions tracks the statements executed during a test:

it('should detect whether an expectation is actually run', () => {

    // Oops, we forgot the await!

    waitFor(() => {
        expect(someElement).toBeVisible();
    });

    // The waitFor essentially becomes a no-op from the point of view of
    // the test.  The test moves on and quietly passes without running  
    // an expectation.

});

Using expect.hasAssertions() also allows jest/expect-expect to be reconfigured slightly to allow more flexibility when abstracting expectations into helper functions, with no loss of rigour, if that's something you're interested in.

user_2948550
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