How to mock an external module's function with Jest

Question

Can Jest's mocking handle functions from modules I didn't write?

node-yelp-api-v3 has Yelp.searchBusiness(String) but my attempts to use Jest's mocking functionality are unsuccessful. The Jest examples seem to assume that I'm mocking a module I have in the project. From the documentation I'm also unclear how to mock a specific function in a module.

Neither of these are working:

jest.mock('Yelp.searchBusiness', () => {
  return jest.fn(() => [{<stubbed_json>}])
})

or

jest.mock('Yelp', () => {
  return jest.fn(() => [{<stubbed_json>}])
})

I'm currently using sinon but would like to use just Jest. This Sinon approach works:

var chai = require('chai')
var should = chai.should()
var agent = require('supertest').agent(require('../../app'))

const Yelp = require('node-yelp-api-v3')

var sinon = require('sinon')
var sandbox

describe('router', function(){
  beforeEach(function(){
    sandbox = sinon.sandbox.create()
    stub = sandbox.stub(Yelp.prototype, 'searchBusiness')
  })

  afterEach(function(){
    sandbox.restore()
  })

  it ('should render index at /', (done) => {
    /* this get invokes Yelp.searchBusiness */
    agent
      .get('/')
      .end(function(err, res) {
        res.status.should.equal(200)
        res.text.should.contain('open_gyro_outline_500.jpeg')

        done()
      })
  })
})

Answer 1

Mocking external modules is explained here.

If the module you are mocking is a Node module (e.g.: lodash), the mock should be placed in the __mocks__ directory adjacent to node_modules (unless you configured roots to point to a folder other than the project root) and will be automatically mocked. There's no need to explicitly call jest.mock('module_name').

For your exact case this would mean you need to create a folder __mocks__ with a file node-yelp-api-v3.js in it. In that file you create a mock object from the original module using genMockFromModule and override the method you want to mock.

// __mocks__/node-yelp-api-v3.js

const yelp = jest.genMockFromModule('node-yelp-api-v3')

function searchBusiness() {
    return [{<stubbed_json>}]
}

yelp.searchBusiness = searchBusiness

module.exports = yelp

Additionally you could also wrap the searchBusiness in jest.fn if you want to call assertions like searchBusiness.mock.calls.length for this method later.