How to write business logic in a service as sinon in ExpressJS - express

Question

I want to use sinon to write test code for the business logic of a service using ExpressJS and Mongoose.
I wrote the following test code, but findOneService takes only id as an argument and returns a document with that id.
//service_test.js
const sinon = require('sinon');
// Service
const { findOneService } = require('../services/service');
// Schema
const Post = require('../models/mongoose/schemas/post');
describe('findOneService', () => {
let find;
beforeEach(() => {
find = sinon.stub(Post, 'findOne');
});
afterEach(() => {
find.restore();
});
it('should findOne', async () => {
const id = ???;
...?
});
})
//service.js
exports.findOneDocument = async (id) => {
const result = await Post.findOne({_id: id});
if (!result) {
throw new Error('404');
}
return result;
};
How can I define the result of this to pass the test code?

Answer 1

To test this kind of behaviour, I strongly suggest an integration test (with an embedded/dockerized MongoDB, for example). This would allow to test-drive more things than just the service, such as schema, migration, db config.
However, if you just want to test-drive the if (!result)... logic, you can stick with sinon. What you're missing is stubbing the return value:
it('returns the document if found', async () => {
find.returns('a post');
expect(await findOneService.findOneDocument('id')).toReturn('a post');
});
it('throw error when document does not exist', async () => {
find.returns(null);
expect(() => await findOneService.findOneDocument('non-existent id')).toThrow(Error);
});