I have quite some experience with TDD in Java and Kotlin and currently try to learn testing with Javascript.

I am not sure if this is really a question about weak vs. strong typing or about general design.

I always was under the impression that mocking/stubbing code you don’t own is a bad idea. In Kotlin I would create and interface for the library and implement that interface with a wrapper.

Then inject a mock of my interface into the tests.

In one of the books I am reading the suggestions to test routes
of an express app is to stub the express.Router() class:

const { expect } = require('chai'); const express = require('express'); const sinon = require('sinon');  describe('user routes', () => {   var sandbox;   var router;    beforeEach(() => {     sandbox = sinon.sandbox.create();     sandbox.stub(express, 'Router').returns({       get: sandbox.spy()     });      router = require('../src/routes/user');   });    afterEach(() => {     sandbox.restore();   });    it('should register GET / route', () => {     expect(router.get.calledWith('/', sandbox.match.any)).to.be.true;   }); }); 

The SUT is:

const express = require('express');  const router = express.Router();  router.get('/', (req, res) => {   res.send(""); });  module.exports = router; 

Is this ok, or is there a better way of doing this?