Tests de código asíncrono

Es común en JavaScript ejecutar código de forma asíncrona. Si tienes código que se ejecuta de forma asíncrona, Jest debe saber cuándo ha terminado de ejecutarse el código de prueba, antes de que puede pasar a otro test. Jest tiene varias formas de manejar esto.

Promesas

Devolver una promesa desde el test, y Jest esperará a que esa promesa resuelva. Si la promesa se rechaza, la prueba fallará.

For example, let's say that fetchData returns a promise that is supposed to resolve to the string 'peanut butter'. Podríamos testearlo con:

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.

caution

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. De lo contrario, una promesa cumplida no hará que el test falle.

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 defecto, Jest da por completos los tests una vez que llegan al final de su ejecución. 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).

caution

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. Si se rechaza la promesa, la prueba fallará automáticamente.

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. Si se cumple la promesa, el test fallará automáticamente.

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

Ninguna de estas formas es particularmente superior a las demás, y se pueden mezclar y combinar a través de una base de código o incluso en un solo archivo. Depende del estilo que sientas que hace tus tests más sencillos.