How to add custom message to Jest expect?

Image following test case:

it('valid emails checks', () => {
  ['abc@y.com', 'a@b.nz'/*, ...*/].map(mail => {
    expect(isValid(mail)).toBe(true);
  });
});

I would like to add auto-generated message for each email like Email 'f@f.com' should be valid so that it's easy to find failing test cases.

Something like:

// .map(email =>
expect(isValid(email), `Email ${email} should be valid`).toBe(true);

Is it possible in Jest ?

In Chai it was possible to do with second parameter like expect(value, 'custom fail message').to.be... and in Jasmine seems like it's done with .because clause. But cannot find solution in Jest.

I don't think it's possible to provide a message like that. But you could define your own matcher.

For example you could create a toBeValid(validator) matcher:

expect.extend({
  toBeValid(received, validator) {
    if (validator(received)) {
      return {
        message: () => `Email ${received} should NOT be valid`,
        pass: true
      };
    } else {
      return {
        message: () => `Email ${received} should be valid`,
        pass: false
      };
    }
  }
});

And then you use it like this:

expect(mail).toBeValid(isValid);

Note: toBeValid returns a message for both cases (success and failure), because it allows you to use .not. The test will fail with the corresponding message depending on whether you want it to pass the validation.

expect(mail).toBeValid(isValid);
// pass === true: Test passes
// pass === false: Failure: Email ... should be valid

expect(mail).not.toBeValid(isValid);
// pass === true: Failure: Email ... should NOT be valid
// pass === false: Test passes