Testing Promises with Mocha

Question

I've been trying to test the following code using Mocha, but I always get the error.

Error: timeout of 2000ms exceeded. Ensure the done() callback is being called in this test

The code I want to test is as follows.

'use strict'
const Promise = require('bluebird');

const successResponse = {status: 'OK'};
const failResponse = {status: 'FAIL'};

function dbStatusSuccess () {
    return new Promise(function(resolve, reject) {
        setTimeout(() => {
            resolve(successResponse); 
        }, 2010);

    });
}

function dbStatusFail () {
    return new Promise(function(resolve, reject) {
        setTimeout(() => {
            reject(failResponse); 
        }, 2000);

    });
}

module.exports = {
    dbStatusSuccess,
    dbStatusFail
} 

and here are my tests.

'use strict'
const Promise = require('bluebird');
const chai = require('chai')
chai.use(require('chai-string'))
chai.use(require('chai-as-promised'));
const expect = chai.expect;
chai.should();


const healthyCheck = require('./healthyCheck');

const resp = {status:'OK'};
const resp2 ={status: 'FAIL'};

 describe('healthy-check end point', () => {

    it('should return successful response when connected to database', () => {

        return healthyCheck.dbStatusSuccess()
                            .then((res) => {
                                console.log(JSON.stringify(res, undefined, 2));
                                return expect(res).to.equal(resp);
                            }).catch( (err) => {
                                console.log(err);
                                return expect(err).to.deep.equal(resp2);
                            });


    });
 });

I also get an error { AssertionError: expected { status: 'OK' } to equal { status: 'OK' } in the console. Which I believe is from loggin the err from the .catch function.

EDIT 1. Removed the reject function from dbStatusSuccess function.

The issue is with the promises taking 2 seconds to complete/fail. If the time set in setTimeout is less than 2 seconds, the tests will pass.

Answer 1

The default timeout in your test seems to be 2000ms. Your code obviously takes longer to complete. Therefore, you have to up the timeout limit. As noted here you shouldn't use arrow functions so you can safely access this.

Then you can increase your timeout like so:

'use strict'
const Promise = require('bluebird');
const chai = require('chai')
chai.use(require('chai-string'))
chai.use(require('chai-as-promised'));
const expect = chai.expect;
chai.should();


const healthyCheck = require('./healthyCheck');

const resp = {status:'OK'};
const resp2 ={status: 'FAIL'};

 describe('healthy-check end point', () => {

    it('should return successful response when connected to database', function() {
        this.timeout(3000);
        return healthyCheck.dbStatusSuccess()
                            .then((res) => {
                                console.log(JSON.stringify(res, undefined, 2));
                                return expect(res).to.equal(resp);
                            }).catch( (err) => {
                                console.log(err);
                                return expect(err).to.deep.equal(resp2);
                            });


    });
 });

Then your test should run as expected.

Answer 2

  'use strict'
  const Promise = require('bluebird');
  const chai = require('chai');
  chai.use(require('chai-string'));
  chai.use(require('chai-as-promised'));
  const expect = chai.expect;
  chai.should();

  const healthyCheck = require('./healthyCheck');

  describe('healthy-check end point', function() {

    it('should return successful response when connected to database', function(done) {
      const resp = {status: 'OK'};

     healthyCheck.dbStatusSuccess()
        .then((res) => {
          console.log(JSON.stringify(res, undefined, 2));
          expect(res).to.equal(resp);
          done();
        }).catch(done);
    });
  });

1. In code example I don't return promise, so I need use callback. Usage callback in your async tests helps avoid Error: timeout of 2000ms exceeded
2. Don't use arrow function in descibe and it. More info

Answer 3

Well, I just found the problem, your test is tricky. You set the timeout timer to 2010ms but Mocha default execution time is 2000ms, so you will always get the error from Mocha.

I still think you shouldn't create .catch block in the returned promise chain, it will stop the promise chain to propagate.

describe('healthy-check end point', () => {

    it('should return successful response when connected to database', () => {

        return healthyCheck.dbStatusSuccess()
                            .then((res) => {
                                console.log(JSON.stringify(res, undefined, 2));
                                return expect(res).to.equal(resp);
                            });


    }).timeout(2500); //tell Mocha to wait for 2500ms
 });

Answer 4

You should use the done callback, for example:

it('reads some file', function(done) {
  fs.readFile('someFile.json', function(err, data) {
    if (err) return done(err);
    assert(data != null, "File should exist.");
    done();
  });
});

What's happening is the test ('it' function) returns before your promise resolves; using done means the test won't finish until you call done() when the promises resolves.

See http://tobyho.com/2015/12/16/mocha-with-promises/

and

https://mochajs.org/#working-with-promises