Walkthrough Mocking in a NodeJS Application

Hi folks, in this blog we would be looking how to use the power of javascript moreover the javascript way of mocking for testcases, recently got chance to work on a project on NodeJS me being a JavaScript lover was very exciting for it, it was fun learning it and working on that but when it came for me to write test cases, so as I came from a background of Java and scala where we have terms like mockito, Dependency Injection etc, I thought it must be the same for NodeJS, but conventions like module.export type of things were a little confusing for me at first, but slowly I began to get a grasp of those conventions, but still I wasn’t able to join threads from two backgrounds with context to unit testing, with little research and googling I came across with two amazing libs used for mocking/stubbing in NodeJS those were sinon and rewire

So let’s get started with some code, in this blog I would be using lab as a testing utility, now let’s look at a simple JS file dependency.js that has the following content

dependency.js

const add = (x, y) => x + y;
const sub = (x, y) => x >= y ? x - y : y - x;

module.exports = {
   add, sub
}

pretty simple right, that’s dependency.js and here is another file util.js

util.js

const dep = require('./dependency.js')
const addInt = (op1, op2) => dep.add(op1, op2);
const subInt = (op1, op2) => dep.sub(op1, op2);

module.exports = {
  addInt,
  subInt
}

here is our file that just requires it as a dependency, that’s a way how node applications require the modules, now in node applications we often require modules like this so let us say we want to mock some required module,  then we can simply use sinon it is a quite powerful tool for mocking, stubbing, spies, we kinda stub dependencies to get the flavor of mocking, here is how we can do it easily, here is my test.js

test.js

const {it} = exports.lab = require('lab').script();
const {expect} = require('code');
const sinon = require('sinon');

let util = require('../blog/util.js'); //location to my util file
it("should add two int", () => {
  let res = util.addInt(3, 4);
  console.log('when result isn\'t mocked');
  console.log(res);
  expect(res === 7);
})

it("should mock add", () => {
  sinon.stub(util, 'addInt').returns(10); //whatever value i give for args of addInt, i would always get 10
  let res = util.addInt(3, 4);
  console.log('when result is mocked for add');
  console.log(res);
  expect(res === 10);
})

so when you run the above code you would see, that the second testcase clearly revokes the actual implementation of addInt method and that’s the power of sinon.

Now, the above example lets you easily mock the functions that return something but, what if the function you wanna mock takes a call back, well sinon has also got that cover for you, here is the example let’s add a method in dependency.js that takes a call back,

const callBackDivide = (x, y, callback) => {
   y === 0 ? callback("Can't divide with zero", null) :
      callback(null, x / y)
};

and here is how it’s used in util.js

const divide = (op1, op2) => dep.callBackDivide(op1, op2, (err, res) => {
   err ? console.log("Error: "+err) : console.log("Result: "+ res)
});

and in our test file, we would have something like this

it("should mock divide methods", () => {
    sinon.stub(util, 'divide').callsFake((op1, op2, callback) => {
        console.log('Its mocked')
    })
    let res = util.divide(9, 3);
    console.log('when call back is mocked');
});

and it works like charm we can see that Its mocked gets printed on the console, that’s how javascript do it seamlessly,

So that’s all in this blog, in the next subsequent blogs we would be looking over how to use rewire and the concept of monkey patching along with some more utilities provided by sinon, here is the repo containing above code, keep coding :).