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.

Install, Configure, and Script Cypress for JavaScript Web Applications

Simulate HTTP Errors in Cypress Tests

Instructor: 00:01 I love that Cypress allows us to test the happy path. Normally, my end to end test are going to mostly just test the happy path and I'll leave the sad path or the bad path for the integration or unit style test that I have in my app, which I'm writing with Jest and React testing library. Sometimes, it can be useful to test the sad path.

00:19 Let's see what happens if I go to this login page and I fill in some value for the username and password. I hit some int and the server has some sort of back end error, we get a 500 result back. What actually happens in our app is we have coded up, "There was an error. Please try again." We want to make an assertion for that. I want to show you how we're going to do that.

00:37 I'm going to add another test in `register.js`, it should show an error message if there's an error registering. Then, what we'll do is we'll `cy.visit` the registration page and we're going to go straight to that `register` page because we know users can get there by clicking on the register link.

00:55 We don't need to over test that. We'll go straight to the registration page, save ourselves a little bit of time. We're going to `findByText`, `submit` and then just `click` on the submit button because we don't really need to send a valid data because we're going to be mocking this out anyway.

01:08 We'll `click` on that submit button and we'll `findByText`, `error`, `try again`. We don't need to be too specific here, so I'll just make it a little generous with that rain checks.

#### register.js
```js
it(`should show an error message if there's an error registering`, () => {
  cy.visit('/register')
    .findByText(/submit/i)
    .click()
    .findByText(/error.*try again/i)
})
```

We'll save this, we'll come back here, our test re-run.

01:21 This test is going to fail because it can't find that error message. The reason that it can't find that error message is because it doesn't show up because this post request actually passes and we don't want that to happen.

![cant find error message](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574727282/transcript-images/cypress-simulate-http-errors-in-cypress-tests-cant-find-error-message.png)

01:32 Let's go ahead and we'll pin that, open up our dev tools and we'll see a bunch of information about this request that we made. In fact, if we go to our network and refresh this, we'll actually see all of those request right here in our network tab as well.

01:45 If I click on that register line here, this is the request that we made for the register endpoint on our API server. What I'm going to do is I want to say, "Hey, Cypress, instead of allowing this request to actually go to the server, I want you to handle it and this is what I want you to do with it."

01:59 What we're going to do is we'll say `cy.server`, `route`. When there's a method of post as this request is, a `url` going to the register endpoint of our server, we want to respond with a `status: 500` and `response` of what our server would typically send back. We'll just do an empty object here, but you could put whatever you want that response to be.

#### register.js
```js
it(`should show an error message if there's an error registering`, () => {
  cy.server()
  cy.route({
    method: 'POST',
    url: 'http://localhost:3000/register',
    status: 500,
    response: {},
  })
  cy.visit('/register')
    .findByText(/submit/i)
    .click()
    .findByText(/error.*try again/i)
})

```

02:20 Now, we can save that, come back here, our test re-run and our test is passing because now we are saying, "There was an error. Please try again."

![passing tests with error](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574727281/transcript-images/cypress-simulate-http-errors-in-cypress-tests-passing-tests-with-error.png)

As you're developing this, it might be useful to separate this two, so we're going to add a `.only` so that you can only run that one test that you're actually working on.

```js
it.only(`should show an error message if there's an error registering`, () => {
  ...
}
```

02:35 In review to make all of this magic happen, we use the cy.server command. Then we changed the route command off of that to say, "Hey, when there's a method post at this API URL then we want to respond with the status of 500. Here's the response we want to send back."

02:49 Then we go ahead with our cy.visit. Any request that matches this method in that URL Cypress will intercept and send back the status that we blocked out. That request actually does go through. If we look at this register that gets us to 500, Cypress actually intercepts it and sends it to a URL that it can handle.

03:08 As far as the browser is concerned we're getting a 500 error back and so all of our code works can handle this error as we expect.


 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

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)

Use a Custom Cypress Command to Login as a User

Combine Custom Cypress Commands into a Single Custom Command

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.

Simulate HTTP Errors in Cypress Tests

Transcript

Simulate HTTP Errors in Cypress Tests