mockFromInterface.ts

export type MockedInterface<T> = {
    [K in keyof T]: T[K] extends (...args: infer A) => infer B
        ? jest.Mock<B, A>
        : MockedInterface<T[K]>;
};

export function mockInterface<T>(
    initialValue: { [K in keyof T]?: MockedInterface<T[K]> } = {}
): MockedInterface<T> {
    const source = {
        toString: jest.fn(),
        // Initialise then and catch to null to avoid recursive resolution if ever awaited or promise chained
        then: null,
        catch: null,
        ...initialValue,
    };
    const proxy = new Proxy(source, {
        get(target: Record<string | number, unknown>, prop: string | number) {
            if (!target.hasOwnProperty(prop)) {
                target[prop] = jest.fn();
            }
            return target[prop];
        }
    });
    return proxy as MockedInterface<T>;
}