When working with asynchronous functions that throw errors, you need to handle promises and rejected errors. In Vitest, you can test asynchronous error handling using async/await or the .rejects matcher for promises.

Using async/await to Test Rejected Errors

// Asynchronous function to be tested
async function fetchData() {
  throw new Error('Network error');
}

describe('fetchData', () => {
  it('should throw an error when the promise is rejected', async () => {
    // Use try/catch to verify the error thrown by the asynchronous function
    await expect(fetchData()).rejects.toThrow('Network error');
  });
});

In this example:

• The fetchData function throws an error asynchronously (in a rejected promise).
• The test uses await expect(fetchData()).rejects.toThrow('Network error') to verify that the error is correctly thrown and matches the expected message.

Using .rejects for Promise-Based Error Handling

Vitest provides the .rejects matcher to test whether a promise is rejected with an error. This is a concise way to check asynchronous errors without needing try/catch blocks.

// Asynchronous function to be tested
async function fetchUserData() {
  return Promise.reject(new Error('User not found'));
}

describe('fetchUserData', () => {
  it('should reject with an error', () => {
    // Test that the promise rejects with the correct error message
    return expect(fetchUserData()).rejects.toThrow('User not found');
  });
});

In this example:

• fetchUserData() returns a rejected promise.
• The test checks that the promise is rejected and the error message matches 'User not found'.