How to jest.spyOn mock implementation only for the first call then use default implementation? - unit-testing

Question

I would like to use jest.spyOn to mock the implementation of a method only for the first call. On 2nd 3rd ...nth call want to call it's actual previous implementation.
I tried the below in ts and it is not working:
import handler from '../src/handler';
import * as cRedis from '../src/redis';
jest.spyOn(cRedis, 'rGetAsync');
describe('handle cart quantity validation', () => {
test('test 1 blabla', async () => {
(cRedis.rGetAsync as jest.Mock).mockImplementationOnce(
() =>
new Promise(resolve =>
setTimeout(() => {
resolve('{}');
}, 1000),
),
);
const response = await handler();
});
});
the handler function calls the method rGetAsync of cRedis two times.
just for illustrative example:
handler.ts
import { rGetAsync } from './redis';
export default function () {
const a = await rGetAsync('a');
const b = await rGetAsync('b');
console.log(a, b);
}
My problem is that the mockedImplementation is used in both calls!
So mockImplementationOnce is not really mocking it once.
I expect that for first call to use the mock implementation and second one the real one.
How can I achieve this with jest?

Answer 1

I have used your code and the following test:
import * as cRedis from '../redis';
import handler from '../handler';
describe('handle cart quantity validation', () => {
it('test 1 blabla', async () => {
const spy = jest.spyOn(cRedis, 'rGetAsync');
spy.mockResolvedValueOnce('Not Original')
await handler();
});
});
My redis looks like this:
export const rGetAsync = async () => Promise.resolve('Original implementation');
And my output is:
console.log
Not Original Original implementation
So it works properly.
You can check my post about clear/reset/restore of spies and mocks.