Jest class mock

mock.ts

type JestMock<T extends (...args: any[]) => any> = jest.Mock<
  ReturnType<T>,
  Parameters<T>
>;

export type InstanceMock<T> = T &
  {
    [K in keyof T]: T[K] extends (...args: any[]) => any ? JestMock<T[K]> : T[K]
  };

type Class<T> = new (...args: any[]) => T;

const initializeMethod = (prop: any, instance?: any): any => {
  if (!instance || !instance[prop]) {
    return jest.fn(() => {
      throw new Error(`Implementation for ${prop} is not provided`);
    });
  }

  const orig = instance[prop];

  return typeof orig === "function" ? jest.fn(orig.bind(instance)) : orig;
};

/**
 * Mock gives class or interface using jest mocks
 */
export function mock<T>(instance?: Partial<T>): InstanceMock<T> {
  return new Proxy(
    {},
    {
      get: (target: Partial<InstanceMock<T>>, prop: keyof T) => {
        if (!target[prop]) {
          target[prop] = initializeMethod(prop, instance);
        }

        return target[prop];
      }
    }
  ) as InstanceMock<T>;
}