Jest test for async promise with async/await

January 13, 2022

I have this function that returns a promise and fails when is called:

export const mockAsync = () => {
  return new Promise((_, reject) => {
    setTimeout(() => reject(new Error('Error')), 50);
  });
};

And I have this test, which is passing:

describe('mockAsync', () => {
  test('it should throw if we ask for it', async () => {
    const result = mockAsync();

    await expect(result).rejects.toThrow('Error');
  });
});

But I find it weird, I would expect it to call await before the function call:

describe('mockAsync', () => {
  test('it should throw if we ask for it', async () => {
    const result = await mockAsync();

    expect(result).rejects.toThrow('Error');
  });
});

This last one is not passing, and I cant understand why. Is not the correct syntax?

>Solution :

For the test the work you want the promise to be rejected within the context of the expect.
for that reason we usually write it like this:

await expect(/*sync function call*/).rejects.toThrow('someError')

And in your particular example:

describe('mockAsync', () => {
    test('it should throw if we ask for it', async () => {

        await expect(mockAsync()).rejects.toThrow('Error')
    })
})