How can I stub an internal function in my route with sinon - node.js

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. E.g.
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
----------|---------|----------|---------|---------|-------------------