How can you test asynchronous code in JavaScript?

TL;DR

To test asynchronous code in JavaScript, you can use testing frameworks like Jest or Mocha. These frameworks provide built-in support for handling asynchronous operations. You can use async/await or return promises in your test functions. For example, in Jest, you can write:

test('fetches data successfully', async () => {
  const data = await fetchData();
  expect(data).toBeDefined();
});

Alternatively, you can use callbacks and the done function to signal the end of an asynchronous test.

Testing asynchronous code in JavaScript can be challenging, but modern testing frameworks like Jest and Mocha provide robust support for handling asynchronous operations. Here are some common methods to test asynchronous code:

Using `async`/`await`

One of the most straightforward ways to test asynchronous code is by using async/await. This approach makes your test code look synchronous, which can be easier to read and write.

Example with Jest

// fetchData.js
export const fetchData = async () => {
  const response = await fetch('https://api.example.com/data');
  return response.json();
};

// fetchData.test.js
import { fetchData } from './fetchData';

test('fetches data successfully', async () => {
  const data = await fetchData();
  expect(data).toBeDefined();
});

Returning a promise

Another way to handle asynchronous tests is by returning a promise from your test function. Jest and Mocha will wait for the promise to resolve before finishing the test.

Example with Jest

// fetchData.js
export const fetchData = () => {
  return fetch('https://api.example.com/data').then((response) =>
    response.json(),
  );
};

// fetchData.test.js
import { fetchData } from './fetchData';

test('fetches data successfully', () => {
  return fetchData().then((data) => {
    expect(data).toBeDefined();
  });
});

Using callbacks

For older codebases or specific scenarios, you might need to use callbacks. In Jest, you can use the done function to signal the end of an asynchronous test.

Example with Jest

// fetchData.js
export const fetchData = (callback) => {
  fetch('https://api.example.com/data')
    .then((response) => response.json())
    .then((data) => callback(null, data))
    .catch((error) => callback(error));
};

// fetchData.test.js
import { fetchData } from './fetchData';

test('fetches data successfully', (done) => {
  fetchData((error, data) => {
    expect(error).toBeNull();
    expect(data).toBeDefined();
    done();
  });
});

Using Mocha

Mocha also supports async/await, returning promises, and using callbacks. Here is an example using async/await with Mocha:

// fetchData.js
export const fetchData = async () => {
  const response = await fetch('https://api.example.com/data');
  return response.json();
};

// fetchData.test.js
import { fetchData } from './fetchData';
import { expect } from 'chai';

describe('fetchData', () => {
  it('fetches data successfully', async () => {
    const data = await fetchData();
    expect(data).to.be.an('object');
  });
});

How can you test asynchronous code in JavaScript?

TL;DR