test-rejection.js

describe('When the promise is rejected', () => {
    beforeEach(async () => {
        fetch.mockRejectedValueOnce(new Error("I'm a teapot"));
        averagePrice = await getPricesLastDays(7);
    });

    it('Then the fuction should return null', () => {
        expect(averagePrice).toEqual(null);
    });
});