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

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

Implement a Declarative API

[00:00] Let's start from taking the API we want and just copy it for reference. So we can see that expect is a function, and let's declare it like that. Function expect, it accepts one argument which is the actual value of the type unknown, because we don't know what the test is going to pass here. And to have this nice little chaining of to be, it has to return an object

[00:19] that has a key to be, which is a function. That function accepts the expected value, which is also the type unknown. And inside does the same comparison that we did here with our manual approach. So I'm just going to copy it here and compare the actual value to expected.

[00:39] And then we want to adjust the error message to include both of these. Let's just copy a nice little placeholder from here. So this error. This is a very basic expect function that accepts the actual result, returns an object to have this to be chaining, and then the expected result, and does the comparison.

[00:59] If those are not the same, we will throw a meaningful error message to let us know. As the next step, let's refactor the existing tests. So for John, let's just use expect greetJohn to be helloJohn. And for Sarah, we will actually drop this need to have the additional message variable,

[01:17] because now the actual result will be encapsulated in the expect call. So I'll drop this, and I will wrap this in the expect call. To be congratsSarah. Notice how smaller and more readable our assertions became. But first, let's try to run them. Let's open a terminal and run the test.

[01:36] We can see that the test is passing. The whole point of doing this exercise and introducing this expect function is to have a more declarative API, which is more readable, but also more manageable. So in the future, when we will be comparing different data structures, not just strings, we can introduce different methods in addition to this to be method,

[01:53] like to equal, to throw, or maybe to be truthy, to help us with writing better tests.

Imperative vs. Declarative Assertions

[00:00] Let's start with the assertions. I'm talking about this little guy. Assertions are easily the most useful part of any test. In fact, all the other test phases exist just so you can write the assertion the way you want. Remember how you learned that tests are about describing the intention behind the code? Well, assertions are precisely the means of how you can do that.

[00:18] The way we have written them as of now, though, has a few problems. First of all, they're imperative. This means that they concern themselves too much with how they work. All this if statements, comparison, throwing an error, these have a place to be, but they don't have to surface as much in the test. Second problem, take a look at this, another test we added for the congratulate function.

[00:37] Here's another simple assertion, but it takes way too much space. Another four lines of code. Imagine writing dozens, hundreds of this. The test file will grow out of scope very quickly. Wouldn't it be nice if we could write the same assertion in a more declarative way like this? So we say that we expect the result of the greet function to be hello, John.

[00:56] Notice how this reads and feels just much nicer. But you may be wondering, where does this expect function comes from? You guessed it. You will be implementing it as a part of this exercise. So have fun and see you when you're done.

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

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

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

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

Move Tests into Modules

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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Imperative vs. Declarative Assertions

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