Testando Código Assíncrono

É comum em JavaScript executar código de forma assíncrona. Quando você tiver o código que é executado de forma assíncrona, Jest precisa saber quando o código que está testando for concluído, antes que possa passar para outro teste. Jest tem várias maneiras de lidar com isso.

Promises

Retorne uma promise do seu teste, e o Jest vai esperar essa promise ser resolvida. Se a promessa for rejeitada, o teste irá falhar.

For example, let's say that fetchData returns a promise that is supposed to resolve to the string 'peanut butter'. Podemos fazer um teste com:

test('the data is peanut butter', () => {
  return fetchData().then(data => {
    expect(data).toBe('peanut butter');
  });
});

Async/Await

Alternatively, you can use async and await in your tests. To write an async test, use the async keyword in front of the function passed to test. For example, the same fetchData scenario can be tested with:

test('the data is peanut butter', async () => {
  const data = await fetchData();
  expect(data).toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
  expect.assertions(1);
  try {
    await fetchData();
  } catch (error) {
    expect(error).toMatch('error');
  }
});

You can combine async and await with .resolves or .rejects.

test('the data is peanut butter', async () => {
  await expect(fetchData()).resolves.toBe('peanut butter');
});

test('the fetch fails with an error', async () => {
  await expect(fetchData()).rejects.toMatch('error');
});

In these cases, async and await are effectively syntactic sugar for the same logic as the promises example uses.

atenção

Be sure to return (or await) the promise - if you omit the return/await statement, your test will complete before the promise returned from fetchData resolves or rejects.

If you expect a promise to be rejected, use the .catch method. Make sure to add expect.assertions to verify that a certain number of assertions are called. Otherwise, a fulfilled promise would not fail the test.

test('the fetch fails with an error', () => {
  expect.assertions(1);
  return fetchData().catch(error => expect(error).toMatch('error'));
});

Callbacks

If you don't use promises, you can use callbacks. For example, let's say that fetchData, instead of returning a promise, expects a callback, i.e. fetches some data and calls callback(null, data) when it is complete. You want to test that this returned data is the string 'peanut butter'.

Por padrão, testes de Jest completam uma vez que eles chegam ao fim da sua execução. That means this test will not work as intended:

// Don't do this!
test('the data is peanut butter', () => {
  function callback(error, data) {
    if (error) {
      throw error;
    }
    expect(data).toBe('peanut butter');
  }

  fetchData(callback);
});

The problem is that the test will complete as soon as fetchData completes, before ever calling the callback.

There is an alternate form of test that fixes this. Instead of putting the test in a function with an empty argument, use a single argument called done. Jest will wait until the done callback is called before finishing the test.

test('the data is peanut butter', done => {
  function callback(error, data) {
    if (error) {
      done(error);
      return;
    }
    try {
      expect(data).toBe('peanut butter');
      done();
    } catch (error) {
      done(error);
    }
  }

  fetchData(callback);
});

If done() is never called, the test will fail (with timeout error), which is what you want to happen.

If the expect statement fails, it throws an error and done() is not called. If we want to see in the test log why it failed, we have to wrap expect in a try block and pass the error in the catch block to done. Otherwise, we end up with an opaque timeout error that doesn't show what value was received by expect(data).

atenção

Jest will throw an error, if the same test function is passed a done() callback and returns a promise. This is done as a precaution to avoid memory leaks in your tests.

.resolves / .rejects

You can also use the .resolves matcher in your expect statement, and Jest will wait for that promise to resolve. Se a promessa for rejeitada, o teste automaticamente irá falhar.

test('the data is peanut butter', () => {
  return expect(fetchData()).resolves.toBe('peanut butter');
});

Be sure to return the assertion—if you omit this return statement, your test will complete before the promise returned from fetchData is resolved and then() has a chance to execute the callback.

If you expect a promise to be rejected, use the .rejects matcher. It works analogically to the .resolves matcher. Se a promessa é cumprida, o teste automaticamente irá falhar.

test('the fetch fails with an error', () => {
  return expect(fetchData()).rejects.toMatch('error');
});

Nenhuma dessas formas é particularmente superior às outras, e você pode misturar e combiná-las através de uma base de código, ou até mesmo em um único arquivo. Apenas vai depender de qual estilo torna os testes mais simples. Só depende do estilo que você sente que torna seus testes mais simples.