ReferenceError: MutationObserver is not defined

I’m creating a test for a controller. But got this error when I do Application.start(). I’m using jest for testing.

Here’s the code

import {Application} from 'stimulus'
import SomeController from '../controllers/some_controller'

describe('SomeController', () => {
  beforeEach(() => {
    const stimulusApp = Application.start()
    stimulusApp.register('some', SomeController)
  })
})

[damonbauer]

For anyone who comes across this:

Add mutationobserver-shim to your project:

yarn add mutationobserver-shim --dev

Include it in Jest setup. In your jest.config.js, update setupFilesAfterEnv array to include a setup file, like so:

// jest.config.js

setupFilesAfterEnv: [
  '<rootDir>/setup-jest.js'
],

Then, in the setup file, import the shim:

// setup-jest.js

import 'mutationobserver-shim';

[Matt-Yorkley]

In case anyone lands here looking for test setup tips with StimulusJS and Jest; you don’t need that MutationObserver shim anymore, newer versions of JSDOM support it by default.