Jest run async function ONCE before all tests

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07-10-2024

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Running Async Functions Once Before All Tests with Jest

Testing asynchronous code in JavaScript can be tricky, especially when you need to set up data or resources before running your tests. Jest, the popular testing framework, provides a flexible way to handle this through its beforeAll hook. However, when dealing with asynchronous operations within beforeAll, it's crucial to ensure the setup is completed before any tests begin.

The Problem: Asynchronous Setup in beforeAll

Let's consider a scenario where we have a function fetchUserData that retrieves user data from an API:

async function fetchUserData() {
  const response = await fetch('https://api.example.com/users');
  return response.json();
}

We want to run this function once before all our tests, to populate the test environment with user data. However, the following code will not work as expected:

beforeAll(async () => {
  global.userData = await fetchUserData();
});

test('should check user name', () => {
  expect(global.userData.name).toBe('John Doe');
});

Why? Because the beforeAll hook executes immediately, even before the await keyword resolves the promise returned by fetchUserData. This leads to the tests running before the userData is available, resulting in errors or inconsistent test results.

The Solution: Using Promise.resolve

The key is to ensure the beforeAll hook doesn't proceed until the async operation is complete. This can be achieved by wrapping the async function call in a Promise.resolve:

beforeAll(() => {
  return Promise.resolve(fetchUserData()).then(data => {
    global.userData = data;
  });
});

test('should check user name', () => {
  expect(global.userData.name).toBe('John Doe');
});

Here's how this code works:

1. Promise.resolve(fetchUserData()): This line creates a new promise that resolves to the promise returned by fetchUserData. This ensures that the promise is resolved before the then callback is executed.
2. .then(data => {...}): This callback is executed after fetchUserData completes successfully. Inside the callback, we store the fetched user data in the global userData variable.

Now, the beforeAll hook will wait for the fetchUserData to finish before proceeding with the tests, guaranteeing that the userData is accessible and ready to be used in your test cases.

Additional Tips and Best Practices

• Avoid Global Variables: While the example uses a global variable for simplicity, it's generally recommended to avoid relying on global scope within your tests. Consider passing the fetched data directly to your test functions or using a testing library that provides better data management.
• Error Handling: Remember to handle potential errors during your asynchronous setup. You can add a catch block to the Promise.resolve chain to handle any exceptions.
• Mocking and Stubbing: For complex scenarios, consider using Jest's mocking and stubbing capabilities to simulate API responses and isolate your tests from external dependencies.

By understanding the intricacies of asynchronous operations within beforeAll, you can reliably set up your testing environment and write robust, consistent tests for your asynchronous JavaScript code.