Because Cypress runs in a real Chrome browser, we can install extensions, like React DevTools. The tricky bit will be to make our application hook up to the extension properly.

Install, Configure, and Script Cypress for JavaScript Web Applications

Install React DevTools with Cypress

Instructor: 00:01 Considering the fact that this is a React application and we want to work Cypress into our workflow and maybe replace the browser with Cypress as part of our workflow, it'd be really nice if we had access to the React developer tools in this browser.

00:14 As a matter of fact, we actually can. I'm going to go ahead and open a new tab here. We'll search for [React dev tools Chrome](https://chrome.google.com/webstore/detail/react-developer-tools/fmkadmapgofadopljbjfkapdkoienihi?hl=en). We'll pop that open, and then we'll click add to Chrome. That will get that extension installed in our Chrome browser for Cypress.

00:29 With that installed, I'm going to go ahead and close this out. We're going to refresh this whole page. We'll see that React is active on the page. Now I can open up my dev tools again and I've got my components right there. Awesome.

![devtools](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574727282/transcript-images/cypress-install-react-devtools-with-cypress-devtools.png)

00:41 Now we've got...What in the world. You know what, it turns out that Cypress is actually built with React as well. That's what we're seeing right here is Cypress is getting loaded up in our React DevTools, but our app is not getting loaded up in our React DevTools.

![cypress loading in devtools](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574727283/transcript-images/cypress-install-react-devtools-with-cypress-cypress-loading-in-devtools.png)

00:56 We need to register our app with the DevTools, so that all the components that our app is creating are getting registered with the DevTools. That's actually pretty easy to do. Let's go to our `public/index.html` and this is the file that loads up when Cypress goes to our app. This is the file that loads our bundle. I want to make a `script` tag in `head`.

01:14 The reason that I'm doing this is because this needs to happens before React loads onto the page. Putting it into `index.html` is the best and easiest way to do that, so we're going to do `script`.

01:25 Basically, what we want is if we're running with Cypress, so `window.Cypress`, then we want to set `window`. This really long variable name, it's `__REACT_DEVTOOLS_GLOBAL_HOOK__`.

#### public/index.html
```html
<head>
  ...
  <script>
    if(window.Cypress) {
      window.__REACT_DEVTOOLS_GLOBAL_HOOK__ =
    }
  </script>
</head>
```

01:44 What we're doing is we're saying, "Hey, this variable, I want to assign it to the `window.parent` version of that variable so that when React loads up, it sees this variable and starts registering things with the parents version of this variable."

```html
<head>
  ...
  <script>
    if(window.Cypress) {
      window.__REACT_DEVTOOLS_GLOBAL_HOOK__ =
        window.parent.__REACT_DEVTOOLS_GLOBAL_HOOK__
    }
  </script>
</head>
```

01:57 With that all set up, now if we close out our DevTools, we'll do a full refresh of this and then open up our DevTools again. Go to the components here, we'll see our container still for our app, but then we'll also see our own app loading up right in here.

![app loading in devtools](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574727283/transcript-images/cypress-install-react-devtools-with-cypress-app-loading-in-devtools.png)

02:13 We can access some of these values. We could go here to the theme provider, update our theme, this can be blue and now our text is blue, which is amazing. We can use the React DevTools to help us develop our Cypress test.

02:26 Again, the key thing here is not that this happens inside your `index.html`, just that this happens before you import React. One thing that we could do is take all this code, get rid of that, go into our `src` directory, and create a new file here, `react-devtools-hook.js`. We'll paste that in there.


#### rect-devtools-hooks.js
```js
if(window.Cypress) {
  window.__REACT_DEVTOOLS_GLOBAL_HOOK__ =
    window.parent.__REACT_DEVTOOLS_GLOBAL_HOOK__
}
```

02:46 Then in our `index.html`, we'll just make sure this is the first thing that we import.

#### index.js
```js
import './react-devtools-hook'
import './global.css'
...
```

That would work just as well. I'm going to go ahead and get rid of that file. We'll just leave that in our `index.html` because it's less likely that somebody will delete it by accident or accidentally import React first in that file. This just ensures that we get that hook set up before React is loaded onto the page.


 is an incredibly powerful web testing tool. It’s capable of testing any web application. Its architecture places it a cut above similar end-to-end testing tools. Its developer experience is best-in-class. And because Cypress runs your tests in the same context as the rest of your application, you’re able to get speed, reliability, and debuggability that are just a long - distant dream with similar tools. The catch? There is no catch. Cypress is exceptional.

In this course, we’ll go over how you can install, configure, and script Cypress to test modern, real - world JavaScript web applications.

What's changed in Install, Configure, and Script Cypress for JavaScript Web Applications

Intro to Install, Configure, and Script Cypress for JavaScript Web Applications 

Despite the fact that Cypress is an application that runs natively on your machine, you can install it and add it as a dependency just like all other dependencies in your package.json file. So let's install Cypress and get our project ready to start end-to-end testing with Cypress.

Install and Run Cypress

Cypress starts out your project with a ton of tests, let’s remove those and add our first test that verifies our calculator can add two numbers together using the Cypress selector playground and Cypress commands.

Write the First Cypress Test

Cypress is fairly simple to configure, but it’s important to optimize our experience in testing Cypress. We’ll configure the baseUrl to make it easier to visit our app and we’ll also move our test files to a folder called e2e to make it more clear what types of tests we want to write with Cypress. We’ll use the cypress.json file, but Cypress can also be configured via command line flags and environment variables.

Configure Cypress in cypress.json

Our selectors are pretty ridiculous, mostly because we’re using CSS-in-JS and css modules for our react application. But even if we weren’t, using class names as selectors for our application testing is pretty brittle and not really testing in the way a user would. So instead let’s use cypress-testing-library to improve our selectors and make it easier to read our tests.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Installing Cypress Testing Library

We want to make starting cypress as simple as possible for local development and we need to be able to run a single script that exits when the tests are complete for continuous integration. So let’s add a few scripts to get ourselves setup for this development experience.

Script Cypress for Local Development and Continuous Integration

One of the greatest features of Cypress is how awesome the experience of debugging tests is. You do have to get used to the asynchronous nature of E2E testing, but once you get the hang of that debugging is pretty straightforward, and the ability to debug your own source code is awesome. The fact that your test and source code are executed in the same context also gives you some powerful advantages as well. You can potentially even expose some internal state to your Cypress app.

Debug a Test with Cypress

Let’s write a test to fill out our registration form. Because we’ll be running this against a live backend, we need to generate the user’s information to avoid re-runs from trying to create new users that already exist. There are trade-offs with this approach. You should probably also clean out the application database before all of your tests start (how you accomplish this is pretty application-specific). Also, if your application requires email confirmation, I recommend you mock that on the backend and automatically set the user as confirmed during tests.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

 Test User Registration with Cypress

Because Cypress allows you to use all the developer tools you’re used to from Google Chrome, you can actually use Cypress as your main application development workflow. If you’ve ever tried to develop a feature that required you to be in a certain state you’ve probably felt the pain of repeatedly refreshing the page and clicking around to get into that state. Instead, you can use cypress to do that and developer your application entirely in Cypress.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Cypress Driven Development

Normally I prefer to test error states using integration or unit tests, but there are some situations where it can be really useful to mock out a response to test a specific scenario in an E2E test. Let’s use the cypress server and route commands to mock a response from our registration request to test the error state.

Simulate HTTP Errors in Cypress Tests

To test user login we need to have a user to login with. We could seed the database with a user and that may be the right choice for your application. In our case though we’ll just go through the registration process again and then login as the user and make the same assertions we made for registration.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Test User Login with Cypress

We’re duplicating a lot of logic between our registration and login tests and not getting any additional confidence, so lets reduce the duplicate logic and time in our tests using cy.request to get a user registered rather than clicking through the application to register a new user.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Create a User with cy.request from Cypress

We’re going to need a newly created user for several tests so let’s move our cy.request command to register a new user into a custom Cypress command so we can use that wherever we need a new user.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Keep Tests Isolated and Focused with Custom Cypress Commands

Use Custom Cypress Command for Reusable Assertions

For most applications you’re going to need to be logged in as a user to interact with the application. Let’s see how we can register as a new user and login as that user to test using the application as a logged in user.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Run Tests as an Authenticated User with Cypress

We’re duplicating a bunch of commands between our new test and the login test and it’s not giving us any more confidence that the application is working properly. So let’s do the same thing we did with registration and make a cy.request command to login with the newly created user to speed up our test and make it more focused.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Use cy.request from Cypress to Authenticate as a New User

It’s pretty likely that we’ll need to login as a particular user for many tests, let’s take our cy.request command and turn it into a custom command to make it easier for the places we’ll be needing a logged in user.

There's been a small breaking change to Cypress Testing Library that affects the code in this video. [Learn more here.](https://github.com/testing-library/cypress-testing-library/releases/tag/v6.0.0)

Install React DevTools with Cypress

Transcript

Install React DevTools with Cypress