Big Testing in React

Jun 13, 2018 · Wil Wilsman

Does my application work in real life?

This is a big question, and big questions need big answers. How do we answer that question, though? With big tests. When we write and develop our applications, it’s with the intent that somebody will use it. We write tests so we can be assured that everything in our app works as expected and is bug free. But how confident are you in your tests?

A real person using your app is going to use a real browser; do your tests test your app in a real browser? And not everybody uses the same browser; do your tests test your app across multiple, different browsers and devices? What about the network? Chances are your app talks to the network, and your tests should account for this too, right? Not to mention a person isn’t going to interact with your app on a component level. They’re consuming the entire app , using all of it together. All of your components working with each other to create an experience.

If your tests don’t test the app in the same way a person would use it, how confident can you really be in them?

Where do we even begin to test our apps like this? There are tools like Jest, but those tests don’t run in a real browser. There’s also Cypress, but as of this writing, you currently can’t use it outside of Chrome. There’s also a looming thought that “user tests are slow.” Do they have to be?

Anatomy of a Big Test

When we talk about testing the entire app in a way that a person would interact with it, we’re of course talking about acceptance tests, or user tests, or end-to-end tests. These types of tests have many names, but it all refers to the same process of testing your app as if you were an actual user.

The life of an acceptance test is pretty simple. First, our app is mounted and we wait for it to load. Then, some interaction is performed, such as filling out a form, or clicking a button; something a person would actually do in our app. Finally, we make some expectations about the state of our app after the interaction is performed. Maybe this is a success message, or some other confirmation. If a user does something in our app, they probably received some feedback. We want to test this interaction and ensure that it works as they would expect it too.

When a person is interacting with your app, they’re using their mouse and keyboard which is translating to browser events that your app responds to. They’re not calling methods and expecting them to return certain values, they’re actually clicking things and expecting to achieve results. This is what our app’s tests should do too: send browser events and assert that there was feedback.

But how do we actually do all of this?

The Big Setup

Luckily, it’s not nearly as hard as it sounds. How we interact with our app should be so identical to how our tests interact with our app that the only difference is the entry point.

In development, our entry point imports our app, mounts it, and that’s it! The app is ready for us to start interacting with. If we’re working on a certain feature, we navigate directly to the part of our app containing it.

In testing, our entry point imports all of our tests and runs them. It’s each test that mounts and interacts with our app. But from our app’s point of view, nothing is any different than if it were being interacted with by a person. Only, in our tests, we can reset our app when necessary to make sure we’re working with a clean state.

Let’s get to some actual code! For example’s sake, let’s say our app is built using React and Webpack with babel-loader, style-loader, and html-webpack-plugin. We’ll also be using Mocha and Chai to write our tests. The app in our example is a simple countdown app that accepts a date input through some custom spinner inputs and displays the time until or elapsed since the specified date.

Here’s a quick look at our example app in action (direct link):

Our application’s entry point looks like this:

// src/index.js
import React from 'react';
import { render } from 'react-dom';
import App from './app';

render(<App />, document.getElementById('root'));

If you have other necessary app code in your entry point, consider moving it into a reusable module. You could even have a Root component with the logic written in lifecycle hooks. The point is: during development, or in production, our app is only mounted once; for our tests, we need to be able to remount our app before each scenario.

Our testing entry point will look something like this:

// tests/index.js

// mocha doesn't support es modules, so we import the pre-compiled assets
import 'mocha/mocha.js';
import 'mocha/mocha.css';

// other setup could be done here too, such as registering chai helpers

// we import our tests using webpack's require.context
// if we just use `import`, it will be hoisted above mocha's bdd setup
const requireTest = require.context('.', true, /-test/);

// run mocha;

It’s each test’s job to mount and interact with the app, so all we need to do here is import all of our tests and run them. We’ll go over what our tests will actually look like in just a minute. For now, let’s pretend that our tests just do their aforementioned jobs. You might be able to figure out where I’m going since I’ve been mentioning entry points. We’re going to use our existing Webpack config to bundle our tests.

// webpack.config.js
module.exports = (env) => ({
  entry: env.testing ? './tests/index.js' : './src/index.js',
  // ... other webpack config

We’re using Webpack v4, which allows us to export our config as a function. The first argument of this function, env, is populated by the CLI argument of the same name. This way, when we start our app, we can just call yarn start --env.testing to switch to our testing index. This new bundle will run all of our tests for us, and we can open this bundle in any browser on any device to run our tests there as well!

Note: in our specific case, Mocha’s HTML reported requires a #mocha container in the DOM to output it’s report. We’re using html-webpack-plugin along with html-webpack-template, so we can just change the appMountId of the template when we’re in a testing environment.

Okay, so we know how to bundle our tests, but how do we actually write them?

Writing Big Tests

If we reflect back on our anatomy lesson, our tests do three important things:

  1. Mount/Remount our app
  2. Interact with our app in a meaningful way
  3. Assert that something happened

To successfully do all of these things, we’ll be using a set of libraries developed specifically for testing big. The BigTest suite of libraries and framework extensions help to answer the big question: does my application work in real life? We’ll touch on a few of those libraries in the rest of this article, but there are some we won’t get to, and more planned for development.

Mounting Our App

First, to mount our app we can just use render from react-dom. But according to the docs: “If the React element was previously rendered into the container, this will perform an update on it and only mutate the DOM as necessary to reflect the latest React element.” However, this is not exactly what we want. While this does mount our app, we actually want to mount a fresh instance of our app every time. To do this we can combine render and unmountComponentAtNode, which will completely remove a component from the DOM after calling the appropriate lifecycle hooks.

This is pretty simple to do ourselves in our own mount helper, but to spare us from even more DIY architecture, we can just utilize the helpers from @bigtest/react which already do this. It also provides some other useful things, such as setting up an in-memory history object for our app to provide to React Router, and binding this history object to other helpers that allow us to navigate our app from within our tests.

This is how we would use @bigtest/react to mount our app and visit a route:

import { setupAppForTesting, visit } from '@bigtest/react';
import { expect } from 'chai';

import App from '../src/app';

describe('mounting our app and visiting a route', () => {
  beforeEach(async () => {
    await setupAppForTesting(App);

  it('shows the date', () => {
    let $date = document.querySelector('[data-test-countdown-target]');
    expect($date.textContent).to.equal('January 1st, 2019');

We can reduce some boilerplate for all of our tests by creating a helper file and combining the React helpers with Mocha hooks. This way we also won’t have to import our app into every test file.

// tests/helpers.js
import { setupAppForTesting } from '@bigtest/react';
import App from '../src/app';

export { visit, location } from '@bigtest/react';

export function setupApplicationForTesting() {
  beforeEach(async function() { = await setupAppForTesting(App);

We could have other helpers in this file, or do further setup such as creating a mock server. Once we have an easy way for our tests to mount our app on demand, we can interact with it.

Interacting With Our App

Again, we’re not testing that some function returns some value or calls some other function. We want to interact with our app like a real person would.

To do this, we’re going to use @bigtest/interactor to send real browser events to our app. I won’t go into too much detail here, but interactors are a very powerful, composable, way to interact with the DOM. Interactors will wait for elements to exist before interacting with them, so we don’t have to worry about timing our tests correctly to sync up with any run loops.

If you’re familiar with the page object pattern, you’ll know that they can abstract the internal structure of a page away from the user-facing functionality. They also reduce a lot of boilerplate around the UI in our tests, so that if changes are necessary, only the page objects need to be updated. You can think of interactors as composable page objects for modern components.

Let’s create an interactor for the custom spinner input in our app:

// tests/interactors/spinner.js
import { interactor, clickable, value } from '@bigtest/interactor';

export default @interactor class SpinnerInteractor {
  // common helpers provide easy to define interactions
  prev = clickable('[data-test-spinner-prev]');
  next = clickable('[data-test-spinner-next]');
  value = value('input');

  // complex interactions can be expressed as methods
  select(option) {
    let self = this;

    // for each letter in `option`, send a keypress event
    // that will trigger the spinner's typeahead feature
    for (let key of option) {
      self = self.trigger('keypress', {
        charCode: key.charCodeAt()

    // methods that return a new instance of itself are chainable
    return self;

And we can use this interactor like so:

// interactors are lazy, so we can initialize them without anything
// actually existing in the DOM yet
const year = new SpinnerInteractor('[data-test-year-field]');

// when accessing properties, an error might be thrown if the element does
// not exist; this can be more useful than typical expectation failures
//=> Error: unable to find "[data-test-year-field]"

// methods return new interactors that can then be chained from one another
let selectYear ='2019')
let selectYearThenPrev = selectYear.prev();

// interactions do not start until `` is called, or when
// used with the async/await syntax; they will resolve once the element
// exists and has been interacted with, or when the timeout ellapses
await selectYearThenPrev;

We can then take these smaller interactors and compose them to create more complex interactions.

// tests/interactors/form.js
import { interactor, scoped } from '@bigtest/interactor';
import SpinnerInteractor from './spinner';

export default @interactor class DateFormInteractor {
  // the default scope allows us to omit the selector when calling `new`
  static defaultScope = '[data-test-date-form]';

  // interactors are composable and nested methods return an instance
  // of the top-most interactor for chaining
  year = scoped('[data-test-year-field]', SpinnerInteractor);
  month = scoped('[data-test-month-field]', SpinnerInteractor);
  day = scoped('[data-test-day-field]', SpinnerInteractor);
  hour = scoped('[data-test-hour-field]', SpinnerInteractor);
  minute = scoped('[data-test-minute-field]', SpinnerInteractor);
  submit = scoped('button[type="submit"]');

  // getters can also be used for computed properties
  get value() {
    let date = `${this.year.value}/${this.month.value}/${}`;
    let time = `${this.hour.value}:${this.minute.value}`;
    return `${date}@${time}`;

You’ll notice our use of data-test-* attributes. This is because, if using element IDs or CSS selectors, updates to the app’s design or markup mean your tests will most likely break. By using data attributes, we can make our tests and interactors resilient to future changes. We can also use a babel transform like babel-plugin-remove-jsx-attributes to remove our testing attributes in production.

Now that we’ve successfully set up some interactors, we can use them to start interacting with our app in our tests.

Making Assertions About Our App

Making assertions sounds pretty straightforward, but since real apps typically have some async operations associated with them, making assertions at the correct time can be cumbersome and headache-inducing.

For example, let’s say we need to test that when a user clicks a button they are shown a random message. When does the message appear in the DOM so that we can make an assertion about it? After the user clicks, sure, but how long after the user clicks? In React, it’s whenever the render loop gets around to rendering that specific message. What if the app makes a network request for this message first? Well, now we need to also wait for a response before making our assertion.

Interactors wait for elements to exist in the DOM before interacting with them, but can we do the same thing with our assertions? Well, yes we can! Interactors use @bigtest/convergence under the hood, and we can utilize convergences for assertions too.

What’s a convergence? To put it simply, it’s a pure assertion that can run repeatedly until either it passes, or a timeout has been exceeded. And pure assertions are expectations that do not cause side effects.

import { when } from '@bigtest/convergence';

// will only continue once the expectation is true
await when(() => !app.state.isLoading);

// works when throwing errors as well
await when(() => {

Convergent assertions will be called several, maybe even hundreds of times, depending on the timeout and if the assertion ever passes. So having side effects in our assertions could cause unintended behavior that may also result in some serious slowdown.

In regard to our tests, if we make sure all of our assertions are all pure by keeping our interactions inside of hooks, we can use @bigtest/mocha to automatically turn all of our it statements into convergences for us.

import { describe, beforeEach, it } from '@bigtest/mocha';
import { expect } from 'chai';

import { setupApplicationForTesting, location } from './helpers';
import DateFormInteractor from './interactors/form';

describe('Date Picker', () => {
  const form = new DateFormInteractor();

  // for our example app, this function also stubs the date to a constant value

  it('defaults to the current date', () => {

  describe('submitting a date and time', () => {
    beforeEach(async () => {
      await form'2019')'12')'30');

    // if this fails, @bigtest/mocha's `it` will keep rerunning until it
    // passes or a timeout has been exceeded
    it('goes to the date countdown', () => {

And there we go! We’ve officially written some big tests that properly interact with our app how a user would.

The Big Finale

Alright, so we’ve written our tests and we can run them just by running our app in a testing environment. Our tests interact with our app just how a user would, and all of our tests pass with flying colors. But we have real projects with CI/CD pipelines and these tests need to be automatically run, reported, and tracked for us.

Thankfully again, we don’t have to do this ourselves. Karma is a framework agnostic test runner that can automatically launch browsers and generate reports for us. It has plenty of plugins to work with the browsers and frameworks we’re already using. For our test suite, we’re going to be using karma-webpack, karma-mocha, and karma-mocha-reporter. To launch our browser, we’ll use karma-chrome-launcher.

There are a couple things we need to change first to get our Karma plugins working smoothly:

  1. karma-webpack takes a Webpack config, but will disregard our entry and use the file defined in our Karma config instead. This means some of the testing setup we did in our Webpack config won’t be necessary anymore.
  2. karma-mocha will automatically set up Mocha and run our tests for us, so we’ll need to remove the Mocha imports and initialization we manually did in our testing index.

Our new testing index will now look something like this:

// tests/index.js
// other setup could be done here too, such as registering chai helpers
const requireTest = require.context('.', true, /-test/);

And our Karma config will look like this:

module.exports = (config) => {
    frameworks: ['mocha'], // sets up the Mocha framework automatically
    reporters: ['mocha'], // reports back to the CLI formatted like Mocha
    browsers: ['Chrome'], // automatically launches chrome to run our tests

    files: [
      // karma-webpack will watch our files
      { pattern: 'tests/index.js', watched: false }

    preprocessors: {
      // tells Karma that we'll be using Webpack to process this file
      'tests/index.js': ['webpack']

    // Mocha reporter options
    mochaReporter: {
      showDiff: true

    // our webpack config exports a function
    webpack: require('./webpack.config')(),

    // webpack dev middleware options
    webpackMiddleware: {
      stats: 'minimal'

    // for more Karma config options, check out the documentation

With all of that set up, we can run karma start to launch our browser and automatically start our tests! For our CI/CD integration, we can add the flag --single-run to exit after running our tests in the specified browsers. To add other browsers, we simple just have to install other launchers like karma-firefox-launcher or karma-safari-launcher. We could even use any of the browsers and devices available on BrowserStack via karma-browserstack-launcher!

With a little bit of manual setup, and through testing big, we’ve answered the question: Does my application work in real life? However, we’ve only scratched the surface in this article. We glossed over some things like convergences and interactors, and we didn’t even get to touch on server mocking with @bigtest/mirage.

Our countdown app lives in the BigTest examples repository where you can check out the rest of our tests and the entirety of our configuration files. There will be more examples to come, including other frameworks and more complicated test setups. Keep an eye out in the future for more BigTest blog posts and guides, and feel free to follow and contribute to all of the projects over at the BigTest GitHub organization.