Jest test for async promise with async/await

Question

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?

If you await mockAsync(), the rejected promise throws as an error. That's nothing to do with Jest, it's how the syntax works. — jonrsharpe
– jonrsharpe, Commented Jan 13, 2022 at 21:04

Tal Joffe · Accepted Answer · 2022-01-16 13:10:40Z

1

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')
    })
})

If you want to use the Try/Catch approach simply catch the error and make sure the error is the expected one:

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

        try{
            await mockAsync()
        }
        catch(err){
            expect(err.message).toEqual('Error')
        }
    })
})

edited Jan 16, 2022 at 13:10

answered Jan 13, 2022 at 21:06

Tal Joffe

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8 Comments

Emille C. Over a year ago

Now I get it, as it throws we need to be withing the expect context, or within a throw/catch statement

Tal Joffe Over a year ago

correct. in the try/catch scenario you will not use the expect().rejects way, you will simply expect the error you caught in the catch to equal the error you are expecting

Emille C. Over a year ago

May you add a try/catch example? Just for reference

jonrsharpe Over a year ago

@EmilleC. read the docs: jestjs.io/docs/asynchronous

Emille C. Over a year ago

@jonrsharpe great advice, except it isn't.

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