Cannot set mock return value on `toString`

[spalladino]

The following fails with a TypeError: foo.toString.mockReturnValue is not a function:

interface Foo {
  toString(): string;
}

const foo = mock<Foo>();
foo.toString.mockReturnValue('bar');

My guess for the issue is that mock uses an empty object as a default first argument, which has the toString default js method, and that causes jest-mock-extended not to generate a jest.fn for it.

FWIW, it can be worked around by just providing a different toString to the default implementation:

const foo = mock<Foo>({
  toString: () => 'bar',
});

[IhsanMujdeci]

Check out my library https://www.npmjs.com/package/mock-interface#mock-interface
I solve for this exact case