In this exercise you'll be figuring out how to separate the test cases into a separate module and make testing functions globally available.

Learn how to gracefully move tests to a separate file, export functions, make test functions global, and run tests using Node.js.

Modular Tests and Global Helpers

[00:00] So the first thing I would do is take our existing tests, creating a file called read.test.yes, and paste them there. Once I do, there's a bunch of errors going on, so let's address them one by one. First, we need to have our actual code available in these tests. To do that, let's go back to read.yes and export these functions publicly.

[00:20] With this, we can now import them in the test file from the source module. Great, now the problem is with test and expect. Those don't really exist, but we don't wanna import them explicitly because we will be using them a lot in all of our tests. So how about we make them global?

[00:39] Let's start by creating a file called setup.yes. And first, before we do anything else, let's approach this type first. So let's describe this global functions by writing declare, global, then var, expect, and var, test. Let's provide the type annotations for them.

[00:59] So we know that expect accepts actual, which is a known, and returns an assertion object. And test accepts a title, which is a string, and a callback, which is a function. Now, assertion is missing, so let's describe it too. I'll create an interface for assertion

[01:17] that has just one property called to be, which is expected, and it doesn't return anything. So with this types in place, we can see that our greet.test.yes module is fine. But if we try to run it, we'll still get an error, because test is not defined. Here's the thing.

[01:35] What we did just now is told TypeScript how to treat this globals, but the actual runtime implementation is still missing. Node.js doesn't know how those functions work, because they're not defined. So how about we define them? I will go to the greet.yes and copy their functionality.

[01:50] So there's expect function, and also the test function. To expose these functions globally, I will assign them on the global this object, using the key of this object as the function name. So global this.expect will be the global expect function,

[02:09] and the same way global this.test will be the global test function. Because we described those global functions here, we can leverage the type inference in TypeScript and drop this explicit type annotations, because types will be inferred from our global definitions. And the same for the test function.

[02:26] Now, if we run tests, we will see them both passing. So let's recap a little what we did here. We moved the test from our source code into a separate module called greetTest.yes. We exported the function that we test publicly,

[02:45] so we can use them in tests. But functions like test and expect, we introduced on a global test. And test and expect, we introduced on a global level. First, by describing them on a type level and letting TypeScript know how to treat them, but then providing the actual implementation for these functions by assigning them on the global this object,

[03:04] so Node.js would know how they behave on runtime. And with this, we use the import flag in Node.js and provided the setup.ts module before running tests. This means that this whole file will be imported before the greetTest.ts file is even run. provided the test to run, which printed us the test result just gracefully.

Move Tests into Modules

[00:00] Right now, we keep the test cases for our functions collocated next to the source code. This is called an in-source testing and can be really handy when testing small unit functions like this. But it's a far more resilient approach to move the test cases into a separate module. This will allow you to keep the source code clean, but also help you maintain, change, edit,

[00:19] and delete the test files if you need to. So in this exercise, your task would be to take the existing tests and move them into a separate module called greet.test.ts. Once you do that, you would also want to make the functions expect and test available globally, both type-wise and runtime-wise, and put them

[00:38] in a file called setup.ts. So then when running tests, you can point to that setup file by using the --import flag, point to the setup, and then the greet.test.ts file to run. I know this is going to be a little tricky, but you can manage it. So give it your best, and see you on the other side.

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.

In the Testing Fundamentals workshop, you will learn testing principles, framework structure, async handling, and more by building a mini testing-framework!

Testing Fundamentals

Artem Zakharchenko

This workshop covers testing framework structure, asynchronous code, and migrating to Vitest.

Testing Fundamentals Workshop Introduction

Introduction

In this first section, you'll learn by example the core principle of having intentions when testing.

Overview of Testing Principles

Learn how writing an automated test captures your intentions, while manually calling console.log() and viewing the result only really lives in your head.

Learn how you can explicitly define your intentions and automatically test your code without any testing frameworks.

Implement the Automated Greeting Test

Write an Intentional Test

Learn how to create an expect function that provides a readable API for writing tests and refactor existing tests to use it.

Trusting Your Test

Debugging Intent vs. Implementation

You've completed the principles exercise!

Testing Principles Outro

Principles

In this exercise, you will learn about the setup, action, and assertion phases of automated testing and how they applied to testing our simple greet function.

Structure Your Tests

Learn how to write declarative and readable assertions in your TypeScript tests using a custom expect function.

Implement a Declarative API

Imperative vs. Declarative Assertions

In this exercise, you will figure out how to create a test function to isolate and report test results.

Learn how to create a simple testing function in JavaScript to wrap existing tests, provide meaningful test names, and display visual indicators.

Create Test Blocks for Better Organization

Isolating Tests with Test Blocks

Learn how to handle side effects introduced by the Date constructor in your JavaScript tests.

Create global hooks for setup and tear-down, mock the Date constructor using a Proxy, and then restore it after your tests are ran.

Implement Hooks for Test Setup & Cleanup

Handle Test Side Effects

You learned how to write declarative assertions, provide test isolation, put your tests in separate files, and deal with testing setup.

Test Structure Recap

Test Structure

In this exercise, you will expand your testing library to support testing asynchronous code, and then write tests for an asynchronous greet function.

Overview of Testing Async Code

In this problem, you'll need to figure out how to adjust the test function to support asynchronous callbacks.

Learn how to handle asynchronous functions in tests by using the `await` keyword and marking test functions as asynchronous. 

Awaiting Async Tests

Handling Promises in Tests

In this problem, you will need to figure out how to handle a pending promise and then chain .rejects.toThrow on expect.

In this solution, you'll learn how to use the object chaining syntax to extend the `expect` function with a new assertion called `rejects`.

Handling Promise Rejections

Error Assertions for Rejected Promises

In this problem, you'll create a waitFor method that can await state changes to a notification manager.

Learn how to use a waitFor method to retry a callback until it no longer throws an error or until the maximum number of retries is reached.

Reliably Wait for Async Operations

Testing with Async Side Effects

You've learned about asynchronicity that you can and can't control, and testing strategies for dealing with both.

Recap of Testing Asynchronous Code

Testing Asynchronous Code

In this final section, you'll be migrating your tests to Vitest.

Overview of Vitest

Your task is to follow the outlined steps to migrate your project from your custom framework to Vitest.

Congrats! You are now ready to apply everything you've learned today about testing in practice!

Installing, Configuring, and Testing in Vitest

Migrate Tests to Vitest

Outro to the Moving to Vitest section of the Testing Fundamentals workshop.

Vitest Review

Moving to Vitest

After completing this workshop you are now equipped to approach any test. There is a lot more to learn about testing! Be sure to check out the linked materials.

Testing Fundamentals

Testing Fundamentals

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Move Tests into Modules

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