How to strongly type jest mocks

#typescript, #jestjs, #typescript-typings, #ts-jest, 

To strongly type my jest mocks. To a certain extent, I can make it work.

When I use mocks (the way I currently do) the return type is of the original type but when I have to access any method added by Jest I have to typecast it so jest.Mock to access a method. 

 I've tried to work with jest.Mock, jest.Mocked, jest.MockInstance.

class MyTest {
    constructor(private readonly msg: string) {}

    public foo(): string {
        return this.msg;
    }
}

const myTestMock: jest.Mock<MyTest, [string]> = jest.fn<MyTest, [string]>(() => ({
    msg: 'private',
    foo: jest.fn().mockReturnValue('aaa'),
}));
// Results in error:
// Type '{ msg: string; foo: Mock<any, any>; }' is not assignable to type 'MyTest'.
// Property 'msg' is private in type 'MyTest' but not in type '{ msg: string; foo: Mock<any, any>; }'

const myTestMockInstance: MyTest = new myTestMock('a');
console.log(myTestMockInstance.foo()); // --> aaa

// Accessing jest mock methods:
(<jest.Mock>myTestMockInstance).mockClear(); // <-- can this be done without type casting

Dirty workaround:

const myTestMock: jest.Mock<MyTest, [string]> = jest.fn<MyTest, [string]>(
    // Cast to any to satisfy TS
    (): any => ({
        msg: 'private',
        foo: jest.fn().mockReturnValue('aaa'),
    })
);

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1. There is a helper which makes types: look ts-jest

2. usually you should test public interface, not an implementation. Instead of testing private methods you could extract that logic to a public interface (maybe to another class that provides the interface)