I'm running into problems with typescript during unit testing. I try to unit test an module like the following code example:

import {Express} from 'express'

export interface BootClassInterface {
  setup(): Express
}

export class BootClass implements BootClassInterface {

  constructor(private express: () => Express ){

  }

  public test(){
      const app = this.express()
      app.get("/test", (req, res) => {
        res.send("Hello World")
      })

      return app;
  }
}

For testing propose I want to know if express().get() was called and if the first parameter was '/test'. Before I switched to typescript I always used the module sinonJS to spy or stub the functionality so that I could test of a certain dependency was used correctly. Now with Typescript I run into problems with the strict types that I've set in the module. By example:

import * as chai from 'chai'
import 'mocha'
import * as sinon from 'sinon'
import * as express from 'express'

import * as Boot from '../../src/Core/Boot'

const assert = chai.assert

suite('[CORE] unit test /Core/Boot.ts', () => {
  let BootClass: Boot.BootClassInterface

  setup(() => {
    const expressApp = () => {
      const app = express()
      app.get = sinon.spy()
      return app
    }

    const Services: Boot.BootServicesInterface = { express: expressApp }
    BootClass = new Boot.BootClass(Services)

  })

  test('Do first test', () => {
    const app = BootClass.setup()
    chai.assert.equal(app.get.called, 1)
  })
})

The code example above results in the following Typescript compiling error:

error TS2339: Property 'called' does not exist on type 'ApplicationRequestHandler<Express>'.

I can see why typescript returns this error and somehow it is even obvious. I even know a possible solution where I accept Express as any in the module.

But I'm looking for a more elegant way to be able to spy/stub/mock my dependencies for testing propose, but at the same time have the advantages of strict typing.