How to add custom message to Jest expect?

Question

Image following test case:

it('valid emails checks', () => {
  ['[e-mail]', '[e-mail]'/*, ...*/].map(mail => {
    expect(isValid(mail)).toBe(true);
  });
});

I would like to add auto-generated message for each email like Email '[e-mail]' 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.

Answer 1

Although it's not a general solution, for the common case of wanting a custom exception message to distinguish items in a loop, you can instead use Jest's test.each.

For example, your sample code:

it('valid emails checks', () => {
  ['[e-mail]', '[e-mail]'/*, ...*/].map(mail => {
    expect(isValid(mail)).toBe(true);
  });
});

Could instead become

test.each(
    ['[e-mail]', '[e-mail]'/*, ...*/],
    'checks that email %s is valid',
    mail => {
        expect(isValid(mail)).toBe(true);
    }
);

Answer 2

You try this one: https://github.com/mattphillips/jest-expect-message

test('returns 2 when adding 1 and 1', () => {
  expect(1 + 1, 'Woah this should be 2!').toBe(3);
});

Answer 3

Just had to deal with this myself I think I'll make a PR to it possibly: But this could work with whatever you'd like. Basically, you make a custom method that allows the curried function to have a custom message as a third parameter.

It's important to remember that expect will set your first parameter (the one that goes into expect(akaThisThing) as the first parameter of your custom function.

import diff from 'jest-diff'

expect.extend({
toBeMessage (received, expected, msg) {
  const pass = expected === received
  const message = pass
? () => `${this.utils.matcherHint('.not.toBe')}\n\n` +
        `Expected value to not be (using ===):\n` +
        `  ${this.utils.printExpected(expected)}\n` +
        `Received:\n` +
        `  ${this.utils.printReceived(received)}`
      : () => {
        const diffString = diff(expected, received, {
          expand: this.expand
        })
        return `${this.utils.matcherHint('.toBe')}\n\n` +
        `Expected value to be (using ===):\n` +
        `  ${this.utils.printExpected(expected)}\n` +
        `Received:\n` +
        `  ${this.utils.printReceived(received)}` +
        `${(diffString ? `\n\nDifference:\n\n${diffString}` : '')}\n` +
        `${(msg ? `Custom:\n  ${msg}` : '')}`
      }

    return { actual: received, message, pass }
  }
})

// usage:
expect(myThing).toBeMessage(expectedArray, ' was not actually the expected array :(')

Answer 4

You can rewrite the expect assertion to use toThrow() or not.toThrow(). Then throw an Error with your custom text. jest will include the custom text in the output.

// Closure which returns function which may throw
function isValid (email) {
  return () => {
     // replace with a real test!
     if (email !== '[e-mail]') {
       throw new Error(`Email ${email} not valid`)
     }
  }
}

expect(isValid(email)).not.toThrow()

Answer 5

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

Answer 6

You can use try-catch:

try {
    expect(methodThatReturnsBoolean(inputValue)).toBeTruthy();
}
catch (e) {
    throw new Error(`Something went wrong with value ${JSON.stringify(inputValue)}`, e);
}