[Bug]: `constructorA.mock.instances` not correct when using `jest.fn().mockImplementation(() => ({ ... }))` about jest HOT 1 OPEN

@dipunm
The difference between the two test cases lies in how Jest manages mock functions' behavior regarding their results and instances.

In the first test case, jest.fn() is used to define a mock function, and mockImplementation is used to create a mock object when new customMock() is called:

const customMock = jest.fn().mockImplementation(() => ({ mock: 'mock' }));
const mock3 = new customMock();

Here, customMock.mock.instances is used to track instances created when the mock function is called. This array collects instances each time the mock function is invoked. However, accessing customMock.mock.instances[0] immediately after the function call might fail because the instance may not have been added to the array yet.

In the second test case, again jest.fn() is used to define a mock function, and mockImplementation creates a mock object when new customMock() is called:

const customMock = jest.fn().mockImplementation(() => ({ mock: 'mock' }));
const mock3 = new customMock();

Here, customMock.mock.results is used to track the results of each invocation of the mock function. Each element in customMock.mock.results contains information about a specific call, including the value property which holds the return value of that call.

console.log(mock3);
console.log(customMock.mock.results[0].value);
expect(customMock.mock.results[0].value).toStrictEqual(mock3);

This test case checks whether customMock.mock.results[0].value (the return value of the first call) is equal to mock3, the instance created by new customMock(). This approach is more reliable because it directly accesses the specific return value of the first call rather than relying on the instance being added to an array.

Therefore, the second test case is more likely to pass because it verifies the return value explicitly associated with the first function call using customMock.mock.results.

from jest.