Using Sinon to mock a constant/variable?

Question

I'm fairly new to testing and even newer to Sinon.

Here I have an express route set up:

import context = require("aws-lambda-mock-context");

this.router.post('/', this.entryPoint);

public entryPoint(req: Request, res: Response, next: NextFunction) {
    const ctx = context();
    alexaService.execute(req.body, ctx);
    ctx.Promise
        .then((resp: Response) => res.status(200).json(resp))
        .catch((err: Error) => res.status(500));
}

My aim is to test that the post call to / runs appropriately. My test script is:

describe('/POST /', () => {
    it('should post', () => {
        chai.request(app)
            .post('/v2')
            .end((err, res) => {
                expect(res).to.be.ok;
            });
    });
});

Though my test passes it returns status: 500 due to the const ctx = context() not being recognized. Is there an appropriate/correct way to spy on the variable ctx and return a mock variable within my test using Sinon? I've been spinning my wheels here for so long.

Answer 1

This is a common problem which I've come accross myself. I've tested multiple solutions, the one I've found to work best is Mockery.

It works like this: before you require your module under test, you tell Mockery to substitute modules the module under test requires with mocks.

For your code, it would look something like this:

const mockery = require('mockery');
const { spy } = require('sinon');

describe('/POST /', () => {
    let ctxSpy;
    beforeEach(() => {
        mockery.enable({
            useCleanCache: true,
            warnOnUnregistered: false
        });
        ctxSpy = spy();
        mockery.registerMock('"aws-lambda-mock-context"', ctxSpy);

        // change this to require the module under test
        const myRouterModule = require('my-router-module'); 

        myRouterModule.entryPoint({}, {}, () => {});
        return ctxSpy;
    });

    it('should call ctx', () => {
        expect(ctxSpy).called.to.be.ok;
    });

    afterEach(() => {
        mockery.deregisterAll();
        mockery.disable();
    });
});