In this section, you'll be creating your own useEffect hooks!

Introduction to useEffect

[00:00] So now that we have the ability to have multiple use state hooks, I think it would be good for us to try and implement another hook, useEffect. This one has a pretty simple API. It doesn't return anything. It has a callback. That callback can return a cleanup function.

[00:15] And then it also has optional dependencies, which when those change is when the callback will be called. If there are no dependencies provided, then the callback will simply be called every single time. And one interesting thing about the callback is that it can return that cleanup function.

[00:33] Unfortunately, we don't have an API in React to know when a component is getting unmounted. So we can't implement the cleanup function ourselves. Maybe some of you watching are creative and can figure out a way to make that work. But I couldn't, and so we're not going to go that far.

[00:48] But we can ensure that our effects are run after React has finished rendering. And we do that by making React render synchronously. We're going to use the FlushSync API to tell React, hey, render this synchronously so that I have all my callbacks that I can then go through and call.

[01:07] So we're going to implement this API. We're going to start out with no dependencies, and then we'll add dependencies. It's going to be a lot of fun, so let's get into it.

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With a poor understanding of hooks, you get this lingering feeling that you're missing out on some potential. 

If you could just deepen your understanding of your tools, you'd gain another level of productivity. 

You could be using a variety of hooks and have the `eslint-plugin-react-hooks` plugin installed that keeps you on track. 

But, there's something missing. How is this all working?

`useState` and `useEffect` are your *most* fundamental tools in the hooks tool-belt. 

And like with all hooks, they follow these two rules:

1. Only Call Hooks at the Top Level
2. Only Call Hooks from React Functions

Why do these rules exist?

Learning the answer to this question is how you'll deepen your understanding. 

And the best way to do that is to jump in and build your own hooks!

In this tutorial, you'll be recreating `useState` and `useEffect` from scratch. 

And by the end, your brain will be its own `eslint-plugin-react-hooks`. Because you'll know exactly why the rules can't be violated. 

## What to expect:

**Recreate useState:** You'll be recreating your own version of useState. In the process you'll learn about how hooks preserve state, and manage component renders. 

**Support multiple hooks:** Learn exactly why hooks *must* be in a deterministic order and how multiple hooks are managed and persisted.

**Recreate useEffect:** Finally you'll apply what you learned to useEffect and learn how React stores the callbacks and diffs the dependency array. 

In this tutorial, you'll recreate useState and useEffect from scratch, learn essential rules, and understand how hooks manage state and renders.

Build React Hooks

In this workshop you'll be building your own versions of React Hooks without using any existing hooks!

Build React Hooks Introduction

Introduction

Introduction to useState

The goal of this challenge is to replace the useState the comes from React with your own. Then, just render the initial state.

In this solution, you'll learn how to setup a useState generic function and some of the type nuances that need to be considered. 

Create Your Own useState Generic Function

Render an Initial State

The goal in this problem is to make the setState function from your custom useState hook work. 

In this solution you'll work towards making your own version of the useState hook's setState function.

Reassign state to a New Value

Make the setState Function Work

In this problem, your task is to figure out how to manually re-render a component whenever state is updated.

Learn how to create your own render function that you can trigger whenever you need.

Manually Trigger Re-Renders

Component Doesn't Re-Render when State Updates

In this problem, you will need to to figure out how to preserve state between renders.

Learn the steps you need to take to ensure that your state doesn't get overwritten by the initial state on each render.

Prevent State From Being Reset Each Render

State Is Reset to initialState on Each Render

Time to take a break before moving on to the next section!

Dad Joke and Break

useState

In this section you'll be breaking renders down into phases and supporting multiple hooks at once.

Introduction to Multiple Hooks

Explicitly define render phases so that it's clear what phase you are in on each render. 

In this solution, we go over the steps we need to take to allow multiple useState hooks to initialize their own state.

Explicitly Defined Phases

Multiple Calls to useState Breaks initialState

The initial state is working, but state is still being shared across useState functions.

We can use an index that is incremented on each call to useState to keep track of each instance's location in a states array. 

Track useState Hooks with a Hooks Index

All useState Hooks Are Overwriting the Same State

Refresh yourself and get ready for the next section!

Multiple Hooks

We have a useEffect in place that we are importing from react. But, we want to get rid of that and create our own!

Time to lay down your first iteration of useEffect without the dependency array. 

Implementing Your Own useEffect

Replace React's useEffect With Your Own

Now it's time to tackle the dependency array. When an array is provided, the callback should only be called when the values change.

In this solution, you'll learn why you need to track and compare in order to implement the dependency array.

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