utilities.md

Jest Utilities :

expect.extend() : is a utility allows you to add custom matchers and use them on any unit test in the file.test.js.

expect.extend({
  toBeJest(received) {
    const pass = received === `Jest`;
    if (pass) {
      return {
        message: () => 'The received is Jest',
        pass: true
      };
    } else {
      return {
        message: () => `Error: The received ${received} is not Jest`,
        pass: false
      };
    }
  }
});

it('Testing toBeJest Matcher...', () => {
  expect(sayJest()).toBeJest();
});

Any matcher in Jest returns an object has two fundamental properties message which is a function returns a string message, and pass which is a truth value indicates whether the test passed or failed.

Note: the expect().not modifier negates the pass property by returning its bang !pass.

Note: the received object doesn't belong to the matcher method explicitly but, belongs to the return of expect(something) as it's the return value of the something. you can add a parameter (ex: target) to your matcher method that mean your matcher accepts one parameter called target.

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