Response delay

[00:00] I will start by creating a test case for this scenario. So a new test, and I will use handles the request timeout as the title. In this test, I will add a new runtime handler, server.use for the same request, but here instead of resolving it in any way, I will force it to pend forever.

[00:19] To do that, I will import a special function from MSW called delay. I will await it in this runtime request handler, right here, await delay. I will mark the resolver as a sync, and what this will do, delay will simply return me a timeout promise.

[00:38] So instead of providing a specific duration of how much to pend, I will provide a special keyword called infinite. This will return the infinitely pending promise from this delay function. With this functionality, I will create a new variable called token promise and assign it the result of calling this auth token function.

[00:57] I will grab the credentials from right here and have this promise ready. Notice that I will not await this promise just yet because before I do, I need to trigger that timeout logic. And for that, I need to advance time. So I'm actually marking date and time again here. So as I know, I will head to the test setup

[01:17] and introduce the before all hook and call view use fake timers. And after all hook, where I will be use real timers to make the date and time marking possible. And now here, once I performed the token request, I will use the advanced timers by time and I will provide three seconds.

[01:36] This is the same duration as this timeout promise duration right over here. And now finally, I'll be able to write an assertion that I expect this token promise to reject with an error that says request timed out. I will fix this typo, expect, and I will mark the whole test as asynchronous.

[01:57] And now let's verify this by running NPM test and see our timeout test passing. So what's happening here is that whenever this request happens, MSW will cause it to pend forever. It will never resolve. And this allows our timeout promise to kick in because as the next step,

[02:19] I'm advancing the timers, the mock date by three seconds, which is the exact duration of the timeout logic. And then I'm able to assert that this promise rejects with a timeout error.

[00:00] By now, we've handled a bunch of network-related scenarios, from successful responses to error responses, and even network errors, and still there's more to the network. What happens if the request we're making never receives a response? It doesn't error, it simply hands forever.

[00:18] Well, in that case, it'd be nice to abort it, to time that out. For that, I've refactored our function to introduce a timeout promise that simply rejects with a timeout error, up to three seconds, and then I introduce a promise race condition between the actual response promise and the timeout promise.

[00:37] So whichever rejects or resolves first, wins. Your task in this exercise is to complete a new test case for the timeout scenario. Basically, to make sure that if the request takes more than three seconds to complete, it will be rejected with this particular error message. And keep in mind, you're working with time,

[00:56] which you know how to mock and advance.

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.

Learn to mock functions, time, globals, network, and modules. Practice real-world scenarios using Vitest to write effective and maintainable tests.

Mocking Techniques in 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.

Mocking Techniques in Vitest

Mocking Techniques in Vitest

Transcript

Response delay

Transcript