Typescript stub/mock with type check

instance-method-stub.ts

// at b.ts
export class B {
  public iAm() {
    return 'B'
  }
}

// at a.ts
import { B } from './b'

const b = new B()

export class A {
  public doIt() {
    return 'doIt ' + b.iAm()
  }
}

// at a.spec.ts

jest.mock('../src/b')

import { A } from '../src/a'
import * as bModule from '../src/b'

const a = new A()
const { B } = bModule as jest.Mocked<typeof bModule>
B.prototype.iAm.mockImplementation(() => 'C')

// GREEN
describe('a', () => {
  it('do', () => {
    expect(a.doIt()).toEqual('doIt C')
  })
})

// Typecheck will fail
B.prototype.iAm.mockImplementation(() => 11)

// Type 'number' is not assignable to type 'string'.ts(2322)
// index.d.ts(1183, 33): The expected type comes from the return type of this signature.