Stub function does not executed when executing test using sinon and mocha

Question

I'm trying to mock a function in order to pass tests using sinon and mocha.

app.js:

const express = require('express');
const { isValid } = require('./utils/index');

const config = require('./config.json')[process.env.NODE_ENV || 'development']


const app = express();

app.get('/', (req, res)=> {


    const url = config.url;
   try {
    const validUrl = isValid(url)
    .then(() => {
        return res.redirect(`https://${url}`);
    })
    .catch(() => {
        return res.status(400).send('Unable to redirect to the given url');
    })
   } catch(error) {
      return res.send('Internal Server Error') 
   }




})

const port = process.env.port || 3000;

const server = app.listen(port, ()=> {
    console.log('server listens on 127.0.0.1:3000');
})

module.exports = {server, app};

config.json:

{
"development": {
    "url": "www.stackoverflow.com"
}, 
"test": {
    "url": "www.stackoverflow.com"
}

}

utils/index.js:

const http = require('http');


module.exports.isValid = (url) => {
    const options = {
        method: 'HEAD',
        host: url
    }
const promise = new Promise((resolve, reject) => {
    const req = http.request(options, () => {
        return resolve(true)
    })
    req.on('error', () => {
        return reject(new Error('Not valid'))
    })

    req.end();
})

return promise;
}

test/index.js:

const request = require('supertest');
const chai = require('chai');
const sinon = require('sinon');
const index = require('../utils/index')
const { expect } = chai;

const { server } = require('../app');
const {url} = require('../config.json')['test'];



describe('isValid Test', () => {

    it('Should redirects an error when the url is not valid', async() => {

        const stub = sinon.stub(index, 'isValid');
        stub.withArgs(url).returns(Promise.reject(new Error('Not Valid')));
        const { status } = await request(server).get('/');

        expect(status).to.equal(400);

    })
})

When I execute the test I got this error:

    (node:23622) UnhandledPromiseRejectionWarning: Error: Not Valid
    at Context.it (/home/hs/perso/mockTests/chaiMock/test/index.js:17:52)
    at callFn (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runnable.js:387:21)
    at Test.Runnable.run (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runnable.js:379:7)
    at Runner.runTest (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:535:10)
    at /home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:653:12
    at next (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:447:14)
    at /home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:457:7
    at next (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:362:14)
    at Immediate.<anonymous> (/home/hs/perso/mockTests/chaiMock/node_modules/mocha/lib/runner.js:425:5)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)
(node:23622) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:23622) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
url:  www.stackoverflow.com
    1) Should redirects an error when the url is not valid


  0 passing (122ms)
  1 failing

  1) isValid Test
       Should redirects an error when the url is not valid:

      AssertionError: expected 302 to equal 400
      + expected - actual

      -302
      +400

      at Context.it (test/index.js:22:27)
      at process._tickCallback (internal/process/next_tick.js:68:7)

Answer 1

The problem here is that your app module is required and in turn pulls in the utils/index file before sinon stubs the function you're interested in, I believe the module loader then caches it so sinon trying to stub it has no effect.

To see your test pass successfully, you need to stub your isValid function before you require your app i.e.

const request = require('supertest');
const chai = require('chai');
const sinon = require('sinon');
const { expect } = chai;

const index = require('../utils/index');

/* Stub here before the server is required */
const stub = sinon.stub(index, 'isValid');

const { server } = require('../app');
const { url } = require('../config.json')['test'];

describe('isValid Test', () => {
  it('Should redirects an error when the url is not valid', async () => {
    stub.withArgs(url).rejects('Not Valid');

    const { status } = await request(server).get('/');

    expect(status).to.equal(400);
  });
});

Also, to stop seeing the unhandled promise rejection when your tests run, you can use the .rejects() function rather than creating your own rejected promise.