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);
  });
});

Take a look at this question: Javascript: Mocking Constructor using Sinon. — sdgluck
– sdgluck, Commented Sep 13, 2015 at 13:48
@sdgluck I did, but I wasn't able to extract the solution for my case — krl
– krl, Commented Sep 13, 2015 at 13:51
Possible duplicate of Javascript: Mocking Constructor using Sinon — Jon Schneider
– Jon Schneider, Commented Jan 13, 2018 at 0:41

sdgluck · Accepted Answer · 2015-09-13 17:57:04Z

27

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 {}
  });
});

edited Sep 13, 2015 at 17:57

sdgluck

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answered Sep 13, 2015 at 15:41

victorkt

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5 Comments

krl Over a year ago

thanks! and how do I add a mock of this.internalService.getEvents returning a string?

sdgluck Over a year ago

In this answer stub will have each method from internalService as a sinon.stub. So you can do this: stub.getEvents.returns("some string")

krl Over a year ago

@sdgluck I figured it out. This is what I needed

let stub = sinon.createStubInstance(Service); stub.internalService = { getDataFromEvents: function() { return 'test'; } }; expect(stub.getData()).to.equal(...);

Bas Slagter Over a year ago

@krl I don't think you are doing things the right way now. You are now just overriding the property 'internalService' after the instance is created. This is not stubbing InternalService with sinon as you probably would want to do.

Jose Quijada Over a year ago

The Creating a namespace: suggestion in answer provided above worked like a charm for me. I only had to make sure that in the code being tested as well that I imported via the name space import pattern suggested above: Changed const { FuncName } = require('..') to const nameSpace = require('..'), and in the rest of the code prefixed FuncName with nameSpace.FuncName wherever it gets invoked with the new keyword when creating objects: new nameSpace.FuncName(...) instead of new FuncName(...). In my test JS I create spy like this: `sinon.spy(nameSpace, "FuncName").

Daniel · Accepted Answer · 2023-05-09 21:04:22Z

0

You could also use proxyquire:

const proxyquire = require('proxyquire');
const sinon = require('sinon');

describe('Service', function() {
    it('getData', function() {
        const internalServiceStub = {
            getEvents: sinon.stub() // mock however you wish
        };
        const InternalServiceStub = sinon.stub().callsFake(() => internalServiceStub);
        const ServiceStub = proxyquire('./service', {
            "./internal-service": InternalServiceStub
        });

        const serviceStub = new ServiceStub();
        // call serviceStub.getData(...) with mocked internalService
    });
});

Notice that this solution will not invoke the constructor.

edited May 9, 2023 at 21:04

answered May 9, 2023 at 15:05

Daniel

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