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An Async Example

En primer lugar, habilita el soporte de Babel en Jest como se indica en la guía.

Let's implement a module that fetches user data from an API and returns the user name.

// user.js
import request from './request';

export function getUserName(userID) {
  return request('/users/' + userID).then(user => user.name);
}

In the above implementation, we expect the request.js module to return a promise. We chain a call to then to receive the user name.

Now imagine an implementation of request.js that goes to the network and fetches some user data:

// request.js
const http = require('http');

export default function request(url) {
  return new Promise(resolve => {
    // Esto es un ejemplo de  http request, para descargar
    // datos de usuario de una API.
    // This module is being mocked in __mocks__/request.js
    http.get({path: url}, response => {
      let data = '';
      response.on('data', _data => (data += _data));
      response.on('end', () => resolve(data));
    });
  });
}

Because we don't want to go to the network in our test, we are going to create a manual mock for our request.js module in the __mocks__ folder (the folder is case-sensitive, __MOCKS__ will not work). Podría ser algo como esto:

// __mocks__/request.js
const users = {
  4: {name: 'Mark'},
  5: {name: 'Paul'},
};

export default function request(url) {
  return new Promise((resolve, reject) => {
    const userID = parseInt(url.substr('/users/'.length), 10);
    process.nextTick(() =>
      users[userID]
        ? resolve(users[userID])
        : reject({
            error: 'User with ' + userID + ' not found.',
          }),
    );
  });
}

Ahora vamos a escribir un test para nuestra funcionalidad asíncrona.

// __tests__/user-test.js
jest.mock('../request');

import * as user from '../user';

// The assertion for a promise must be returned.
it('works with promises', () => {
  expect.assertions(1);
  return user.getUserName(4).then(data => expect(data).toEqual('Mark'));
});

Llamamos jest.mock('.. /request ') a Jest a utilizar nuestro mock manual. Se espera it que el valor devuelto a una promise que va a resolverse. You can chain as many Promises as you like and call expect at any time, as long as you return a Promise at the end.

.resolves

There is a less verbose way using resolves to unwrap the value of a fulfilled promise together with any other matcher. If the promise is rejected, the assertion will fail.

it('works with resolves', () => {
  expect.assertions(1);
  return expect(user.getUserName(5)).resolves.toEqual('Paul');
});

async/await

Writing tests using the async/await syntax is also possible. Here is how you'd write the same examples from before:

// async/await can be used.
it('works with async/await', async () => {
  expect.assertions(1);
  const data = await user.getUserName(4);
  expect(data).toEqual('Mark');
});

// async/await can also be used with `.resolves`.
it('works with async/await and resolves', async () => {
  expect.assertions(1);
  await expect(user.getUserName(5)).resolves.toEqual('Paul');
});

To enable async/await in your project, install @babel/preset-env and enable the feature in your babel.config.js file.

Manejo de errores

Los errores pueden ser gestionados usando el método .catch. Asegúrate de añadir expect.assertions para verificar que un cierto número de afirmaciones están siendo llamadas. Otherwise a fulfilled promise would not fail the test:

// Testing for async errors using Promise.catch.
it('tests error with promises', () => {
  expect.assertions(1);
  return user.getUserName(2).catch(e =>
    expect(e).toEqual({
      error: 'User with 2 not found.',
    }),
  );
});

// Or using async/await.
it('tests error with async/await', async () => {
  expect.assertions(1);
  try {
    await user.getUserName(1);
  } catch (e) {
    expect(e).toEqual({
      error: 'User with 1 not found.',
    });
  }
});

.rejects

The.rejects helper works like the .resolves helper. Si se cumple la promesa, el test fallará automáticamente. expect.assertions(number) is not required but recommended to verify that a certain number of assertions are called during a test. It is otherwise easy to forget to return/await the .resolves assertions.

// Testing for async errors using `.rejects`.
it('tests error with rejects', () => {
  expect.assertions(1);
  return expect(user.getUserName(3)).rejects.toEqual({
    error: 'User with 3 not found.',
  });
});

// Or using async/await with `.rejects`.
it('tests error with async/await and rejects', async () => {
  expect.assertions(1);
  await expect(user.getUserName(3)).rejects.toEqual({
    error: 'User with 3 not found.',
  });
});

The code for this example is available at examples/async.

If you'd like to test timers, like setTimeout, take a look at the Timer mocks documentation.