Learn how to create a custom wrapper function in React Router to provide the missing routing context to your isolated component tests.

Discover how to use React Router's Memory Router to achieve better test boundaries and maintain component-level test isolation.

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[00:00] Since our component now depends on routing, it can no longer render in isolation. So what I have to do is prepare this component tree to wrap around our tested form to make sure that it can render correctly. To do that, I will create a variable called Wrapper, and assign it a function. This will be a React component.

[00:18] It will access Children, and it will return a component tree. So in this case, let me just wrap it in a fragment of Children. The Children prop here will be the component that we are currently testing, the one we are providing to the render function. And the returned component will be this component tree that we prepared in order for this tested

[00:38] component to render. This is exactly where we include things like Provider. In our case, we need a routing provider. Normally, in your application, you use a browser router that keeps its state in the URL, in the actual browser. But because our component tests render in a controlled environment, the actual browser

[00:55] location becomes an implementation detail of the testing framework. We don't want to meddle with that. So instead, it would be nice to use it in memory routing. And this is where React Router has an exact component to achieve that, and it's called the Memory Router. So let me import Memory Router and wrap our tested component in this provider.

[01:14] This Memory Router functions identically to the browser router we used in the actual application, but instead it keeps all the routing state in memory. And it will still provide that state to all the dependent components or hooks, like our link component, so our discount form can render as we expect.

[01:30] Now that we have this wrapper, it's time for us to apply it to individual render functions. So for example, in this test case, add a second argument here, which is an object, and provide this wrapper function here as the value. It will reference our prepared wrapper. And let's do the same for the other test cases, whenever we render this discount form, basically.

[01:49] This one, this one, and this one. And yeah, this is it. So this prepares our component to be rendered, and all the existing tests shall now pass. But we have a new test case, the one around that back to the cart page link. So let's write this one.

[02:07] We need to render it correctly, so let's provide the wrapper. Next, let's create this back to cart link. We will grab it by its role, because links have accessible roles, link, and its name that will be back to cart. So once we have this component, let's quickly assert that it's visible.

[02:25] So we expect element back to cart link to be visible. And now to the interesting part. We don't actually want to interact with this link. I know what you're thinking. We should model our tests after what our users do with this UI. But here's perhaps one slight exception. The users will indeed click on this link, and it will take them to the cart page.

[02:45] But this navigation has two parts. There is an initiator part, our form component, and also the destination part, the actual cart page. And we want to make sure to provide clear test boundaries when testing this navigation on a component level.

[03:01] So our discount code form is only responsible for rendering this link, and making sure that this link is accessible and leads to the correct page. And that's exactly what I'm going to reflect in the expectations. So let's add another assertion.

[03:14] We will say await expect element are back to cart link to have attributes href that leads to the cart page. And this is where assertions for this particular scenario will end, because we don't want to

[03:31] bring everything that the cart page renders to the context of this page. For example, you may write an assertion that makes sure that there's a certain title element on the cart page. But what if you remove that element tomorrow, or maybe rename the text inside that element? That will be an issue, but not an issue for this particular test case.

[03:48] Because on the discount code form level, all it cares about are these things, including the link, making sure that it's accessible, and that it leads to the correct page. Now you still would want to test this navigation on an end-to-end level. And this is where you would actually click the link like your user does, and assert that it lands on the correct page in the browser.

[04:08] But when it comes to component level testing, this is where you draw the line to achieve reliable and isolated tests.

[00:00] Components you create don't exist in a vacuum. They're part of a larger component tree that eventually comprises a page. And whether you're building single-page or multi-page applications, there will still be a way to navigate from one page to another, which means you also need to know how to test the components that initiate that navigation, like our discount code form.

[00:19] I went in and refactored this application to have client-side routing using React Router. Our form now has a link that allows the user to return to the cart page by clicking on it. But you see, in order for this link to render, it needs a routing context. It gets that context because somewhere up the component tree, it has this browser provider,

[00:38] this browser router component. But here we have a problem similar to the shared assets we had before. In our isolated component test for this form, there is no such component, no such provider. So the routing context will be missing and rendering that link will fail. This is where you come in. In this exercise, you will create a custom wrapper function

[00:57] that will provide this missing context and make sure that the existing tests are passing. But then write a new test case for the back to the cart link. And while you're doing that, make sure to ask yourself a question. How much of this navigation is the responsibility of this form? And how much is not? I know you're gonna nail this one, so see you once you're done.

The closer your tests resemble how your software is being used, the more confidence they will give you. There’s so many great patterns stemming from this simple rule, patterns I’m sure you’re already using to write better tests.

Then, there’s the elephant in the room. **Your test environment.**

For decades, web developers have been testing their UIs in make-believe environments that are neither the browser nor Node.js. It made them come up with a frankly ridiculous amount of test setup, polyfills, and workarounds just to make it work. Remember when you had to mock `location` in tests? `window.matchMedia`? Yeah…

It’s time to abandon such practices. The tools have improved and there’s no excuse for you to be sleeping on it. It’s time for you to learn how to use Vitest Browser Mode for in-browser component testing!

## What You’ll Learn

In this workshop, you will learn a new approach to component testing that’s both different and the same. It is different because you will write and run your tests in the actual browser, taking advantage of the standard APIs and powerful browser automation. It is the same because it’s still the component level test you know and love, with rendering, querying, and asserting. 

Your users don’t interact with the UIs you build in JSDOM. They do that in the real browser, and now your tests will too!

### **Sunsetting JSDOM**

The first step in our journey is understanding why browser-like environments fall short. JSDOM has served us well, but it's time to recognize its limitations. You'll experience firsthand how browser-like environments like JSDOM are letting you down.

We'll explore examples of valid JavaScript code that works perfectly in browsers and Node.js but fails in JSDOM, undermining the value of the tests you work so hard to write. You'll discover why these inconsistencies happen and why moving to real browser testing is the solution.

### **Vitest Browser Mode**

Vitest Browser Mode allows you to run isolated component-level tests in the actual browser while providing the convenience and flexibility of integration-level testing. It's not a full end-to-end testing framework like Playwright or Cypress, but rather the perfect middle ground for component testing.

In this module, you'll learn:

- How to install and configure Vitest Browser Mode in your project;
- The technical differences between how JSDOM and Browser Mode run your tests;
- Steps needed to migrate existing JSDOM tests to Browser Mode;
- How to set up Playwright as the browser automation provider for your component tests;
- Common setup scenarios like applying global assets in tests and configuring Vitest for both unit and component-level testing within the same project.

### **Best Practices**

Choosing the right tools is important, but knowledge that transcends any particular tool is even more valuable. In this section, we'll focus on five key areas of best practices when it comes to testing your React components:

- **Queries**: Find elements on the page the same way your users or their assistive technology would;
- **User events**: Interact with the tested elements from the user’s perspective;
- **API mocking**: Use Mock Service Worker to intercept requests and handle their responses as well as discover how to efficiently use MSW in Vitest Browser Mode by creating a custom test context;
- **Element presence**: Test elements that should (or shouldn't) appear on the page reliably without false positives or flaky results.
- **Page navigation**: Learn where to draw the line when testing page transitions and routing in the actual browser.

### **Debugging Tests**

The true value of tests comes when they fail. But dealing with failing tests can be quite frustrating without the right techniques.

In this block, you'll learn some of my favorite debugging techniques that go well beyond console logging:

- How to inspect the DOM state at any point in test;
- Using the debugger statement to stop JavaScript execution entirely;
- Wielding conditional breakpoints to step through the problematic code.

These techniques will transform your approach to test failures from dread to confidence so the next time you see a failing test you will know what to do.

## Who Is This Workshop For

This workshop is for all React developers who want to write reliable and valuable tests for their components. Whether you're new to component testing or have been writing tests for years, you will inevitably learn new tools, APIs, and techniques to gain more value out of your test suite. That’s my sole motivation behind this material.

Test React components in real browsers with Vitest! Say goodbye JSDOM’s limits and catch bugs users actually face. Real tests, real results—start now!

React Component Testing with Vitest

Artem Zakharchenko

Writing tests for their own sake is a pointless chore:

```tsx
describe('add numbers', () => {
  it('should add two numbers', () => {
    expect(add(1, 2)).toBe(3)
  })

  it('should add two other numbers', () => {
    expect(add(3, 4)).toBe(7)
  })
});
```

Another annoying testing problem is when tests are too brittle, and paint your logs red even though the application seems to work just fine.

When your tests lack real purpose or seem to fail for no good reason, it's no wonder you start to question their value.

Tests are supposed to give you confidence in your code, and save you from shipping broken stuff to prod. So why does writing them feel like such a burden?

The problem is that tests like these are focused on implementation details instead of the underlying behavior. Everyone talks about the importance of testing, but there's a lack of understanding about how to write tests that are actually useful.

How would it feel to have 100% trust in your tests, knowing that any failures are not a false alarm? What if you could seamlessly translate the underlying behavior of your application into tests that are easy (and even fun) to write?

It's possible, but it requires you to take a step back from just syntax.

In order to have the confidence you're testing the right things in the right ways, you need to dig into the underlying patterns behind the scenes of a testing framework.

This is exactly what you will do in Testing Fundamentals.

In this workshop, Artem Zakharchenko, testing expert and creator of the Mock Service Worker API mocking library, will guide you through creating your own version of the Vitest testing framework.

Building a testing framework from scratch will give you a deep understanding of how the pieces fit together, and change the way you think about testing for the better.

## What You'll Learn

The workshop is broken out into four parts, each with its own challenges and exercises that build upon the last:

### Principles

First, you'll explore the principles and mental models that underlie testing strategies. This is where you will begin to distinguish between the intention of your code and its implementation details. When you focus the intention of what your code should accomplish, you can write tests that are more resilient to change.

### Test Structure

Next, you'll do a deep dive into the structure of the testing framework. You'll create your own `expect` and `test` functions, configure your test environment, and use hooks to manage side effects in your tests.

### Testing Asynchronous Code

Testing asynchronous code requires a slightly different approach when compared to testing synchronous code. Here you'll practice with testing promises and async functions, as well as handling errors and rejections.

### Moving to Vitest

Once you've learned the fundamentals of testing and built your own mini-framework, you will migrate your tests to the Vitest framework. This process will cement your understanding of not just the "hows", but also the "whys" of testing.

## Who This Workshop Is For

This workshop is for developers who have some testing experience, but feel like they're missing the bigger picture. If you're tired of writing tests that feel like a chore, and want to understand how to write tests that are actually useful, this workshop is for you.

Testing Fundamentals

Let’s face it, mocking can be really confusing.

First, you need to know what to mock. Then, find the right way to mock that thing so it doesn’t compromise your test suite. Over-mock, and your tests become pointless. Under-mock, and they get brittle instead.

In either case, you put in so much effort, and you don’t feel like you’re getting much in return.

This is where Artem comes in. He’s been proving developer misconceptions around mocking wrong for nearly a decade, and he’s here to make you a mocking pro.

## What You’ll Learn

In this hands-on workshop, you’ll learn:

- The purpose of mocking;
- What to mock and when;
- How to write mocks that bring value instead of being a maintenance chore;
- How to separate your mocks from implementation details;
- Type safety strategies for your mocks;
- And **all the mocking techniques** you will need to test your code!

By solving targeted, real-world problems using the Vitest framework, you’ll gain a comprehensive understanding of mocking and finally unlock it as a useful tool in your testing arsenal.

## Workshop Structure

This workshop contains 6 exercise blocks dedicated to various techniques on mocking different data types.

### Boundaries

Understand what mocking really is, and learn how it helps you create test boundaries. See what each mock you introduce does to your tested code using visual metaphors.

### Mocking Functions

Functions are at the heart of JavaScript logic. Learn how to mock functions for effective input-output testing. Create mock functions, spy on function calls, and mock function implementation to tap into the right behaviors of the tested code.

### Mocking Date and Time

Eliminate the side effects of ever-changing date and time in your tests. Learn how to freeze time and mock date/time-dependent code to ensure consistent test results across different runs and time zones.

### Mocking Globals

Explore how to mock implicit dependencies, in a non-intrusive way. Learn to mock global values, methods, and environment variables to create the perfect test setup without meddling with things you don't own.

### Mocking Network

Gain control over network communication and test your client-side code reliably. Intercept requests and mock responses using Mock Service Worker. Reproduce various network scenarios, from success and error responses to offline behavior and response timing.

### Mocking Modules

Understand when to use module mocking and explore alternative techniques before you do. Learn how to mock modules in a type-safe and efficient way using built-in APIs in Vitest.

## Who This Workshop Is For

This workshop is meant for all developers, both those who are new to automated testing and those who’ve been writing tests for years. If you are unsure when to introduce mocks into your test suite, or how to do that so you don’t have to refactor it the next day, this workshop was made specifically for you.

Mocking Techniques in Vitest

Epic Testing Bundle

Learn about the world of React component testing using Vitest and discover best practices that will help you write reliable and maintainable tests for your components.

Introduction to React Component Testing with Vitest Browser Mode

Workshop Introduction

Explore the history of browser-like environments, their advantages and disadvantages, and reasons to switch to actual browser testing.

Sunsetting JSDOM

Break JSDOM

Learn about the challenges of browser-like environments such as GSDOM, how it impacts component testing, and how to utilize Vitest browser mode as a solution.

Recap Of Sunsetting JSDOM

Vitest new browser mode promotes a proper component testing strategy, allowing you to run tests in actual browsers rather than simulated Node.js environments.

Vitest Browser Mode

Learn how to install Vitas Browser Mode dependencies and configure your React project for seamless browser-based testing of your components.

Streamline your testing process by updating your tasks to utilize Vitest's powerful browser mode capabilities.

Installation and Setup

Learn how to transform your existing test cases for React components by switching from React test library to Vitest.

Get started with Vitest Browser Mode and experience improved React component testing through well-structured concepts like locator objects and streamlined element selectors.

Migrate The Test

Learn how to make your browser testing more reliable and explicit by switching from the default preview provider to PlayWrite in Vitest.

Learn how to install and configure Playwright as a browser automation tool for Vitest, ensuring more reliable component testing for your web applications.

Playwright

Make sure your component tests include the necessary styles for both visual consistency and accessibility, by updating your testing setup.

Learn how to apply styles consistently to all component tests and use a global browser setup to handle unique situations.

Shared Assets

Learn how to configure Vitest to handle unit tests, integration tests, and component tests within the same codebase for efficient testing.

Understand how to distinguish between unit and browser tests with Vitest workspaces for better organization, smooth execution, and easier debugging.

Multiple Workspaces

Dive into the versatile world of Vitest Browser Remote, exploring installation, migration, custom asset loading, and best practices for component-level testing.

Outro Vitest Browser Mode

Improve your component testing knowledge through a guided exploration of accessibility selectors, user events, API mocking, element presence, and page navigation to ensure high-quality web applications.

Best Practices

Learn how to test React components by focusing on user interactions and selecting elements in an accessible way.

Queries

Move beyond basic component tasks to interact with UI elements, just like real users do, for more effective browser automation testing.

Writing test cases to simulate user interactions for applying discount codes using Playwright and Vitest.

User Events

Network Mocking

Learn how to build reliable tests for element presence and disappearance in discount form applications, while avoiding false positives.

Explore techniques to test React components using Vitest, including waiting for elements to appear or disappear efficiently and accurately without inversing assertions.

Element Presence

Gain in-depth knowledge of web testing best practices and lay the groundwork for successfully debugging test failures.

Recap Of Best Practices

Learn to conquer failing tests with advanced debugging techniques in this exercise focused on component-level tests in the browser.

Debugging Tests

Learn helpful strategies for debugging component tests by examining the DOM state at different points during the test execution process.

Learn how to efficiently debug your DOM issues using the debug method from Vitest, locating specific elements and fixing layout problems.

DOM Snapshots

Uncover the power of the debugger statement in JavaScript and explore how to set up Visual Studio Code for debugging and attaching to browser tests.

Troubleshoot and debug failing tests more effectively with breakpoints and debug mode, making test execution stop so that you can examine the problematic steps.

Debugger

Learn debugging features such as in-line breakpoints and conditional breakpoints, and see how they can help you tackle the root cause of complex issues in your JavaScript codebase.

Learn how to use conditional breakpoints to effectively debug issues with React components and understand their implications in your tests.

Breakpoints

Find out how to quickly deal with failed tests while increasing productivity and embracing CI pipelines using a variety of JavaScript debugging techniques.

Recap Of Debugging

Improve your web development skills by learning advanced component testing methodologies, effective debugging techniques, and how to put your newfound knowledge into practice.

React Component Testing with Vitest

React Component Testing with Vitest

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