Mocking JavaScript constructor with Sinon.JS

Question

I'd like to unit test the following ES6 class:

// service.js const InternalService = require('internal-service');  class Service {   constructor(args) {     this.internalService = new InternalService(args);   }    getData(args) {        let events = this.internalService.getEvents(args);     let data = getDataFromEvents(events);     return data;   } }  function getDataFromEvents(events) {...}  module.exports = Service; 

How do I mock constructor with Sinon.JS in order to mock getEvents of internalService to test getData?

I looked at Javascript: Mocking Constructor using Sinon but wasn't able to extract a solution.

// test.js const chai = require('chai'); const sinon = require('sinon'); const should = chai.should();  let Service = require('service');  describe('Service', function() {   it('getData', function() {     // throws: TypeError: Attempted to wrap undefined property Service as function     sinon.stub(Service, 'Service').returns(0);   }); }); 

Answer 1

You can either create a namespace or create a stub instance using sinon.createStubInstance (this will not invoke the constructor).

Creating a namespace:

const namespace = {   Service: require('./service') };  describe('Service', function() {   it('getData', function() {     sinon.stub(namespace, 'Service').returns(0);     console.log(new namespace.Service()); // Service {}   }); }); 

Creating a stub instance:

let Service = require('./service');  describe('Service', function() {   it('getData', function() {     let stub = sinon.createStubInstance(Service);     console.log(stub); // Service {}   }); });