test-async-function-with-jest.md

Testing asynchronous functions

In previous sections, we have learned that some of the functions can be async (e.g. a function that tries to retrieve some data from another web server on the Internet). If we need to write test cases for those functions, we need to be careful. We need to wait for the asynchronous function to return before a test case finishes.

Here are some sample codes on how to test asynchronous functions.

In the codes below, the fetchData function is asynchronous and it returns a promise. So we can wait for the tasks to finish with async/await.

test('the data is peanut butter', async () => {
  expect.assertions(1);
  const data = await fetchData();
  expect(data).toBe('peanut butter');
});

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

To make the tests easier to read, jest provides some helper functions like .resolves and .rejects. The same test cases can be re-written as below:

test('the data is peanut butter', async () => {
  expect.assertions(1);
  await expect(fetchData()).resolves.toBe('peanut butter');
});

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

Check out jest documentation on how to test asynchronous functions and you can also checkout a tutorial here.