How can I stub an internal function in my route with sinon

Question

I have my internal function

//in greatRoute.ts
async function _secretString(param: string): Promise<string> {
   ...
}

router
  .route('/foo/bar/:secret')
  .get(
    async (...) => {
      ...
      const secret = _secretString(res.params.secret);
      ...
    },
  );

export default {
  ...
  _secretString
};

and now I'm trying to mock the call with sinon.stub like this:

sinon.stub(greatRoute, '_secretString').resolves('abc');

But that doesn't work the way I want it to. When i call the route in my test it still goes into the _secretString function. Am I missing something here? I already tried to put the export in front of the function header like this: export async function _secretString(param: string): Promise<string> instead of doing the export default {...} but that didn't help.

Answer 1

You can use rewire package to stub _secretString function. Eg

index.ts :

async function _secretString(param: string): Promise<string> {
  return 'real secret';
}

async function route(req, res) {
  const secret = await _secretString(req.params.secret);
  console.log(secret);
}

export default {
  _secretString,
  route,
};

index.test.ts :

import sinon from 'sinon';
import rewire from 'rewire';

describe('61274112', () => {
  it('should pass', async () => {
    const greatRoute = rewire('./');
    const secretStringStub = sinon.stub().resolves('fake secret');
    greatRoute.__set__('_secretString', secretStringStub);
    const logSpy = sinon.spy(console, 'log');
    const mReq = { params: { secret: '123' } };
    const mRes = {};
    await greatRoute.default.route(mReq, mRes);
    sinon.assert.calledWithExactly(logSpy, 'fake secret');
    sinon.assert.calledWith(secretStringStub, '123');
  });
});

unit test results with coverage report:

fake secret
    ✓ should pass (1383ms)


  1 passing (1s)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |      75 |      100 |      50 |      75 |                   
 index.ts |      75 |      100 |      50 |      75 | 2                 
----------|---------|----------|---------|---------|-------------------