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

[00:00] So I'm back at this problematic test, and what I want to do to debug it is insert breakpoints after each interaction with the game. So I will add this debugger statement after the first interaction, then the second, and the third. Now since I have this debugger statement, I want my test execution to stop whenever

[00:19] it reaches those statements. To make that happen, I need to run my test in a special debug mode. That's where that launch.json configuration comes in. So I go to this Run & Debug panel, select the configuration, Debug Vitas Browser, and click Run. I select the test I want to debug, I just have one here, click OK.

[00:39] It will spawn the automation, and you can see, let me clean this up, you can see that the test freezes at this render call over here. This is because we used inspect break flag. So what I can do now is use this control panel here for debugger to help me debug this test. For now, I just wanted to continue to the first breakpoint, and I will click Resume Execution.

[00:58] You can see that the code stops at this debugger after this particular action with the game, and I can see the result in the UI. I can continue executing the code, jumping between debuggers, until I reach the third debugger, and here I can see the problem. And if I keep running it after this one, I can see that the test failed now over here.

[01:17] Keep in mind that your tests isn't the only place where you can use breakpoints. You can also add them to your code, and this will allow you to see why certain actions are happening, and also stop the time and inspect the state of your UI when they do. Let's explore what else you can do with debugger and breakpoints in the next exercise.

[00:00] Being able to print the state of the DOM in tests is really powerful, but you know what else is powerful? Stopping JavaScript execution altogether, taking a look around, and stepping through the code. In JavaScript, you can do that using a debugger statement. So here I have a compute function that consists of two steps, and then a debugger statement in between. So when this

[00:18] function runs, it will do the first step, and then meet the debugger statement, and the execution will stop. The debugger itself inserts a breakpoint, basically an instruction for JavaScript Engine to stop running, and allowing you to inspect the state of the code. You're able to see the values in the current moment of time, and also step through the execution, basically resume the

[00:37] execution or jump to the next breakpoint, if any. And it works the same whether you're debugging Node.js tests in Vitas or browser tests, but there is just one catch when it comes to the browser tests. Because your tests run in the browser, the debugger statement will also run there. So the browser execution will stop, and you'll be able to debug it, but your test and your source files

[00:56] still live in your IDE. It would be really nice if you could make the two talk to each other, basically connect them. You can do that by configuring your IDE, and in this exercise, we will go through the configuration for Visual Studio Code. So let's do it together. First, make sure that at the root of your application, you have a folder called .vscode, and inside this folder, a special

[01:16] file called launch.json. You can copy the content of this file from the description of this exercise. If this file looks intimidating to you, don't worry, we're going to go through what it does step by step. The purpose of this file is to configure VS Code so it can run and debug certain things, in our case, browser tests. And we do that by listing two configurations, the first one to

[01:36] run Vitas browser mode, and the second to attach a debugger instance to the running browser process. Let's go through them in more detail. So the first configuration spawns a terminal that launches a command, and in this case, it's our npm test script. It's the same script we use to run tests, but for this debug process, we need to add a few special options to it. Let's go through them. So the first

[01:55] one is inspect break, or BRK. This flag tells Vitas to automatically insert the debugger statement whenever you render a component in your test. This is just a good quality of life improvement, and it's nice to have it here. The dash dash browser option tells Vitas to run only browser tests. This is

[02:11] handy if you have multi-test setups. And finally, the no file parallelism option disables parallel test runs, so we have predictable and reliable debugging experience in the browser. And finally, this configuration sets an environment variable called debug to the value of true. And I use this

[02:27] variable in the Vitas configuration to conditionally trigger headless mode in the browser. Basically, whenever I'm debugging my browser tests, I want for this automation window, the browser to appear, so I'm able to see what's going on. And now the last thing I haven't covered is the special input

[02:42] token. Without it, we will run all browser tests, and this may not be what we want. So instead, I'm using a special extension for VS Code called common variable that allows me to create a custom picker for files that I want to debug. I describe it here in the inputs, give it an ID, and this way

[03:00] I'm able to pick a test suite that matches the pattern. And I can pick multiple test suits if I want to debug multiple things at the same time, which is really handy. Now the second and last configuration here attaches debugger instance to the running browser, and it does that at this specified port, which is the port that Vitas uses to spawn that browser automation. And once we have

[03:19] this configuration, we create a compound task called debug Vitas browser that will run both of those configs together. It will run Vitas and attach the debugger. And now to use this launch JSON, you will head to the run and debug panel over here, click on it, and select the correct task

[03:35] from this drop down, debug Vitas browser, and click on this play or run icon. Now having this all configured, your task is to debug the same issue from the previous exercise, but now using a debugger statement. And don't forget to run your tests using that run and debug panel using our

[03:51] task that we just configured. And once you're done, let's discuss using debuggers and breakpoints in more detail together.

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

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.

Page Navigation

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

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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Debugger

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