Testing file read wtih fs using JEST

Question

I am trying to test the following function:

async function read () {
  return new Promise(function(resolve, reject) {
    fs.readFile("INVALID_PATH", (err, contents) => {
      if (err) {
        reject(new Error('ERROR'));
      }
      resolve(contents);
    });
  });
}

through the test:

try {
  await read();
} catch (e) {
  expect(e).toMatch('ERROR');
}

However, I can't catch the rejection on catch, it gives me timeout.

Any suggestions?

EDIT:

I managed to get it working by using a mock, this way:

try {
  fs.readFile.mockReset()
  fs.readFile.mockImplementation((path, options, cb) => {
    cb('ERROR')
  })

  await readFile()

} catch (err) {
  expect(err).toBe('ERROR')
}

However, I still don't understand why this is necessary...

Answer 1

Have you tried declaring the test function as async in Jest? await cannot be used outside async functions (or in top level code), you have to do something like this:

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');
});

More info for testing async/await in jest https://jestjs.io/docs/en/tutorial-async.html