Mock Interface

Zero dependency library that deeply mocks typescript interfaces via jest.fn() allowing you to adhere to easily SOLID principals.

Type safety

Full type safety is preserved, type errors are thrown if a mock return/resolve type is incorrect or if method called incorrectly.

Natively Jest

Mock interfaces arbitrarily deeply, and they will behave as jest mocks.
All jest assertions will work as if it was a jest mock e.g. toHaveBeenCalledTimes, toHaveBeenCalledWith, ...
See full list from jest docs https://jestjs.io/docs/mock-function-api

Examples

describe('Mock Interface', () => {

  it('Should test person interface', () => {
    interface Person {
      speak: Speaker
      getName(): string
    }

    interface Speaker {
      say(phrase: string): string
    }

    const person = mockInterface<Person>()

    person.getName.mockReturnValue('Navi')
    person.speak.say.mockReturnValue('Hey, listen!')

    const name = person.getName()
    const say = person.speak.say('Hey, listen!')

    expect(name).toEqual('Navi')
    expect(person.getName).toHaveBeenCalledTimes(1)

    expect(say).toEqual('Hey, listen!')
    expect(person.speak.say).toHaveBeenNthCalledWith(1, 'Hey, listen!')
  })

  // This is typically an edge case of similar interface mockers that aren't caught
  it('Should mock toString function of interface', ()=>{
    interface JsonWriter{
      toString(): string
    }
    const mockJsonWriter = mockInterface<JsonWriter>()
    mockJsonWriter.toString.mockReturnValue('{"nice": "json"}')
    const w = mockJsonWriter.toString()
    expect(w).toBe('{"nice": "json"}')
    expect(mockJsonWriter.toString).toHaveBeenCalledTimes(1)
    expect(mockJsonWriter.toString).toHaveBeenCalledWith()
  })
  
  it('Should test arbitrarily deep interface', ()=>{
    interface l1 {
      l2: l2;
    }
    interface l2 {
      l3: l3;
    }
    interface l3 {
      l4: l4;
    }
    interface l4 {
      dig(): string;
    }

    const levels = mockInterface<l1>();
    levels.l2.l3.l4.dig.mockReturnValue('end of the line');

    const d = levels.l2.l3.l4.dig();
    expect(d).toBe('end of the line');
    expect(levels.l2.l3.l4.dig).toHaveBeenCalledTimes(1);
  })
})