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How do you write an effective test for asynchronous code?

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Answer

Return or await the Promise inside the test function so the test runner waits for it to resolve before checking assertions; forgetting to await causes the test to pass falsely before the async work even finishes.

test('fetches user data', async () => {
  const user = await fetchUser(1);
  expect(user.name).toBe('Sam');
});

Real-world example Testing that an async API wrapper correctly parses and returns the expected shape of data.

Common follow-ups: What happens if you forget the 'async' keyword on a test function that awaits something?

Promises & async/await

All 10 Debugging, Testing & Tooling questions