I am attempting to mock a class Mailer using jest and I can't figure out how to do it. The docs don't give many examples of how this works. The process is the I will have a node event password-reset that is fired and when that event is fired, I want to send an email using Mailer.send(to, subject, body). Here is my directory structure:
project_root-- __test__---- server------ services-------- emails---------- mailer.test.js-- server---- services------ emails-------- mailer.js-------- __mocks__---------- mailer.jsHere is my mock file __mocks__/mailer.js:
const Mailer = jest.genMockFromModule('Mailer');function send(to, subject, body) {return { to, subject, body };}module.exports = Mailer;and my mailer.test.js
const EventEmitter = require('events');const Mailer = jest.mock('../../../../server/services/emails/mailer');test('sends an email when the password-reset event is fired', () => {const send = Mailer.send();const event = new EventEmitter();event.emit('password-reset');expect(send).toHaveBeenCalled();});and finally my mailer.js class:
class Mailer {constructor() {this.mailgun = require('mailgun-js')({apiKey: process.env.MAILGUN_API_KEY,domain: process.env.MAILGUN_DOMAIN,});}send(to, subject, body) {return new Promise((reject, resolve) => {this.mailgun.messages().send({from: 'Securely App <[email protected]>',to,subject: subject,html: body,}, (error, body) => {if (error) {return reject(error);}return resolve('The email was sent successfully!');});});}}module.exports = new Mailer();So, how do I successfully mock and test this class, using Jest? Many thanks for helping!
Best Answer
You don't have to mock your mailer class but the mailgun-js module. So mailgun is a function that returns the function messages that return the function send. So the mock will look like this.
for the happy path
const happyPath = () => ({messages: () => ({send: (args, callback) => callback()})})for the error case
const errorCase = () => ({messages: () => ({send: (args, callback) => callback('someError')})})as you have this 2 cases it make sense to mock the module inside your test. First you have to mock it with a simple spy where we later can set the implementation for our cases and then we have to import the module.
jest.mock('mailgun-js', jest.fn())import mailgun from 'mailgun-js'import Mailer from '../../../../server/services/emails/mailer'As your module uses promises we have 2 options either return the promise from the test or use async/await. I use the later one for more info have a look here.
test('test the happy path', async() => {//mock the mailgun so it returns our happy path mockmailgun.mockImplementation(() => happyPath)//we need to use async/awit here to let jest recognize the promiseconst send = await Mailer.send();expect(send).toBe('The email was sent successfully!')});If you would like to test that the mailgun send method was called with the correct parameter you need to adapt the mock like this:
const send = jest.fn((args, callback) => callback())const happyPath = () => ({messages: () => ({send: send})})Now you could check that the first parameter for send was correct:
expect(send.mock.calls[0][0]).toMatchSnapshot()Just for Googlers and future visitors, here's how I've setup jest mocking for ES6 classes.I also have a working example at github, with babel-jest for transpiling the ES module syntax so that jest can mock them properly.
__mocks__/MockedClass.js
const stub = {someMethod: jest.fn(),someAttribute: true}module.exports = () => stub;Your code can call this with new, and in your tests you can call the function and overwrite any default implementation.
example.spec.js
const mockedClass = require("path/to/MockedClass")(); const AnotherClass = require("path/to/AnotherClass");let anotherClass;jest.mock("path/to/MockedClass");describe("AnotherClass", () => {beforeEach(() => {mockedClass.someMethod.mockImplementation(() => {return { "foo": "bar" };});anotherClass = new AnotherClass();});describe("on init", () => {beforeEach(() => { anotherClass.init(); });it("uses a mock", () => {expect(mockedClass.someMethod.toHaveBeenCalled();expect(anotherClass.settings).toEqual(expect.objectContaining({ "foo": "bar" }));});});});