How to unit test console output with mocha on nodejs?

Question

Take into account the following example Javascript code below:

function privateFunction (time) {   if (time < 12) { console.log('Good morning'); }   if (time >= 12 && time <19) { console.log('Good afternoon'); }   else { console.log('Good night!'); } }; 

How should I unit test that on nodejs using mocha (and possibly sinonjs), noticing that this is a private function called inside a module? I need to pass in the argument and check if the function is logging the right thing to the console.

Can I do the same with console.warn and console.error?

Answer 1

I prefer mocha-sinon over "plain" sinon because it integrates nicely with Mocha.

Example:

var expect = require('chai').expect; require('mocha-sinon');  // Function to test, can also be in another file and as long as it's // being called through some public interface it should be testable. // If it's not in any way exposed/exported, testing will be problematic. function privateFunction (time) {   if (time < 12) { console.log('Good morning'); }   if (time >= 12 && time <19) { console.log('Good afternoon'); }   else { console.log('Good night!'); } }  describe('privateFunction()', function() {    beforeEach(function() {     this.sinon.stub(console, 'log');   });    it('should log "Good morning" for hours < 12', function() {     privateFunction(5);     expect( console.log.calledOnce ).to.be.true;     expect( console.log.calledWith('Good morning') ).to.be.true;   });    it('should log "Good afternoon" for hours >= 12 and < 19', function() {     privateFunction(15);     expect( console.log.calledOnce ).to.be.true;     expect( console.log.calledWith('Good afternoon') ).to.be.true;   });    it('should log "Good night!" for hours >= 19', function() {     privateFunction(20);     expect( console.log.calledOnce ).to.be.true;     expect( console.log.calledWith('Good night!') ).to.be.true;   });  }); 

One potential issue: some Mocha reporters use console.log as well, so the tests that stub it may not yield any output.

There's a workaround, but it's not ideal either because it will intersperse Mocha output with the output from privateFunction(). If that's not a problem, replace beforeEach() with this:

beforeEach(function() {   var log = console.log;   this.sinon.stub(console, 'log', function() {     return log.apply(log, arguments);   }); }); 

Answer 2

ignoring the fact that it's a private function, i would take a couple of steps; refactor my code for better separation of concerns, and utilise this separation with test doubles.

• take all the side effects outside to their own modules (the side effect here is writing to the console):

  out.js

  function log (message) {   console.log(message); };  module.exports = {log}; 

  app.js

  const {log} = require('out');  function greeter (time) {   if (time < 12) {     log('Good morning');   }   if (time >= 12 && time < 19) {     log('Good afternoon');   } else {     log('Good night!');   } };  module.exports = {greeter}; 

• use some module proxy/spy, like proxyquire to replace the whole out writer when testing:

  app.spec.js

  describe('output writers', function(){    const fakeOut = {     log: sinon.spy(),   };    const app = proxyquire('./app', {     'out': fakeOut   });    it('should log to the fake out', function(){     app.greeter(15);     assert(fakeOut.log.calledOnce);   }); });