Global methods

[00:00] To test that the print server URL is actually printing something to the console, I would have to spy on the console.log method. To do that, I would go to the B4ALL hook and call v.spyOn. This is the same utility that you've used to spy on your own classes and their methods previously. As a target, I will provide a global console object.

[00:19] It is the method name, the log method, the one I want to spy on. What this is going to do, it will introduce the mock, it will patch console.log, and VTest will start recording any calls to this method during the tests. Next, in the afterEach hook, I will call v.resetAllMocks. This utility will do two things.

[00:38] First, it will throw away any recorded calls between the tests so they don't persist. And second, it will also throw away any mocked implementations that we might have added in individual tests. And lastly, in the afterAll hook, I will call v.restoreAllMocks. And this one is a little different. It will basically restore any mocks that we introduced before,

[00:58] effectively unpatching, unmodifying the console.log method so it behaves as usual. With this spy in place, I can finally start to certain on the console. So, in this first test, we're printing the URL for a server that only has a host. So, I expect the console.log to be called with the respective message.

[01:15] The server is listening and providing the address and a trailing slash. Now, in the second scenario, we also include an optional port. So, let me reflect that in the URL, 5639. Now, if I run tests, I will see them passing, but I will also see that this message is being printed in the console during the test run.

[01:36] And sometimes this is not what you want because it may clutter the test output. Here's what you can do about it. In the beforeAll hook, you can mock the implementation for the console, providing an empty function as the implementation. This will effectively force console.log to do nothing in tests. But be mindful here because once you mock this implementation,

[01:55] you can no longer use resetAllMocks because this utility will throw away this mock after the first test. Instead, you should use clearAllMocks. This is a similar utility. It will clear any recorded calls, but it will preserve any mock implementations that you introduce in the test setup. So, with this one, we can see the test passing

[02:15] and no console logs actually being printed in the test output.

[00:00] Here you have a printServerUrl function that accepts a server and then constructs a URL from server's host and attaches an optional port to that URL, so that it could print to the console that the server is listening at a particular address. So the main purpose of this function is precisely this console.log call.

[00:18] In this exercise you will be testing that functionality, but since the console is a global object, you would have to find some way to spy when its log method is being called and whether it's being called with the right arguments. That sounds awfully familiar, doesn't it? I will leave it up to you to figure out the exact details, and see you on the other side.

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

Global methods

Transcript