Introduction to React Data Fetching Course

Course Introduction

Going through the problems with useEffect

What’s Wrong With useEffect?

Going through the problems with other libraries

Using Libraries or Frameworks for Data Fetching

Optimizing Your Dev Environment for this Course

Understand the problems with using the useEffect hook for data fetching, the pros and cons of using libraries and frameworks for data fetching.

Intro & Getting Started

We will learn how to fetch data before the component loads

Fetch Data Before you Component Loads

Here we'll learn how to create an efficient data cache

Creating an Efficient Data Cache

We will learn how to fetch data on an event

Fetching Data on an Event

We'll learn how to handle loading and error states in our application

Loading and Error Handling

Here we're going to recap what we've learned in Module 2

Module 2 Recap

Learn the benefits of prefetching data, how to create an efficient cache data to prevent refetching and also how to handle data fetching on an event.

Prefetching & Caching

Here we'll take a look at how to refetch data in a simple way

Simple Refetching

Here we'll learn how to fix race conditions

Fixing Race Conditions

Here we'll learn how to fix network waterfalls

Fixing Network Waterfalls

Here we'll recap what we've learned in Module 3

Module 3 Recap

Learn how to identify and fix two big issues with data fetching in React, Race Conditions and Network Waterfalls.

Race Conditions & Network Waterfalls

Here we'll learn how to use the useSyncExternalStore hook

The useSyncExternalStore Hook

Here we'll learn how to prefetch data based on the route

Prefetching Based on Page Route

This lesson, focus on saving and reusing prefetched data sources efficiently using context and a custom data-loading function.

Saving Data Sources

Stale While Revalidate

Module 4 Recap

Learn how the useSyncExternalStore hook can be used to improve the performance of our data fetching library. We'll also implement the stale-while-revalidate caching method.

Refactoring and SWR

Fetching on the Server

Final Words

Learn about how our data fetching library can be used in a server-side environment particularly with React Server Components.

Wrapping Up

Learn how to create a **small, efficient data fetching library** that will prefetch, cache your data as well as handle **error and loading states**, and even implement a caching technique called **stale-while-revalidate**.

### The Problem with Data Fetching in React

Data fetching is such a crucial part of building React apps and **most developers are doing it incorrectly**.

This is primarily because there are many videos and tutorials that show you how to fetch data using the useEffect hook, but also, because the [React documentation suggests it](https://react.dev/reference/react/useEffect#fetching-data-with-effects).

But then goes on to specify [all the problems with using it](https://react.dev/reference/react/useEffect#what-are-good-alternatives-to-data-fetching-in-effects) such as:

- Effects don’t run on the server

- Effects makes it easy to create “network waterfalls”

- Effects usually means you don’t preload or cache data

- It’s not very ergonomic

So for small projects, maybe the useEffect hook is fine, but for medium to larger projects it definitely should not be an option for data fetching.

You could go ahead and use one of the many popular frameworks and libraries for doing this, but they have **a lot of features** and a lot of **magic behind the scenes** that you may not need.

Also, because they are libraries, you may not be able to **customise them to your specific needs**.

### How Does _This Course_ Help?

In this course you will learn how to build a data fetching library **from scratch**.

That's right, you'll start with an **empty file**, understand every single line of code that you write. And end up with a **single-file library** that will be able to; **prefetch and cache your data** in a performant way, **handle loading and error states**, and also implement a caching technique called **stale-while-revalidate**, which will refetch new data in the background while still showing the old data.

But that's not all...

This library will also address two big issues with data fetching in React, [race conditions](https://maxrozen.com/race-conditions-fetching-data-react-with-useeffect) and [network waterfalls](https://blog.sentry.io/fetch-waterfall-in-react/).

If you haven't heard or encountered these issues before, you're lucky. But in case you do, this library will have you covered.

No matter if you want to get a better understanding of how data fetching in React works, if you want to build a custom data fetching solution for your next project, or if you want to build the next [TanStack Query](https://tanstack.com/query/latest), this course will help you do all those things and much more.

### Course Content Overview
Course Overview: Total Modules: **5** Total Lessons: **20** Total Video Runtime: **2 hours, 40 minutes**

Implement prefetching, caching and stale-while-revalidate with the useSyncExternalStore hook

This course will teach you how to fetch in a performant way for React without relying on a third-party library or the useEffect hook. We will build a custom data fetching library from scratch that will prefetch, cache, and address all the common issues with fetching data.

React Data Fetching: Beyond the Basics

A Solution to Network Waterfalls & Race Conditions

Implement the Stale While Revalidate technique

How React re-renders when data is fetched

What the problems are with using useEffect to fetch data

The pros and cons of using **libraries and frameworks for data fetching**

How to start fetching data **before your React component loads**

How to create a **data cache** to prevent data being refetched

How to identify and fix **Network Waterfalls and Race Conditions**

When and why to use the **useSyncExternalStore hook**

How tin implement the **stale-while-revalidate** caching method

How data fetching for **React Server Components** can be achieved

In this lesson we'll discusses a strategy to prefetching and caching data based on events. This lesson revolves around a simple application displaying Pokémon data. Initially, the data for a specific Pokémon which is prefetched before the component loads. However, we can optimize this process further by initiating a fetch on an event such as a click or a mouse hover, let's go through the process.

To accomplish this, a function will be added to the 

 that prefetches the data on a specific event. This function, which will be called 

, and will take two arguments: the key (which is used to check the cache and store data) and the function to be executed to fetch the data when necessary. Here's how it looks:

// data-loader.js

export function prefetchOnEvent(key, fn) {
  // Check if the data is in the cache
  if (dataCache.has(key)) {
    // If data is in cache, just return it
    return;
  }
  // If data isn't in cache, fetch it and add to cache
  prefetchData(key, fun);
}

We can put this code below our prefetch function on line 

Now, let's incorporate this into the application. In the 

 component, data will be prefetched when the user hovers over a Pokémon's name. By adding an 

 event to the link component, data fetching will start as soon as the user's mouse enters the area.

// Home.jsx

<Link
  className="details"
  to={`/details?name=${pokemon.name}`}
  onMouseEnter={() => prefetchOnEvent(pokemon.name, () => getPokemonDetails(pokemon.name))}
>
  View Details
</Link>

---
title: Fetching Data on an Event
description: We will learn how to fetch data on an event
privateVideoUrl: https://stream.mux.com/9gmb3jCZxnryAi2MUBI7R78gXkt2cFpBI1OA2M8gGOM.m3u8
code: https://gitlab.com/fullstackio/books/newline-course-apps/richard-bray-create-a-custom-usefecther-component-in-react-app/-/tree/main/02.03
---

# Fetching Data on an Event

In this lesson we'll discusses a strategy to prefetching and caching data based on events. This lesson revolves around a simple application displaying Pokémon data. Initially, the data for a specific Pokémon which is prefetched before the component loads. However, we can optimize this process further by initiating a fetch on an event such as a click or a mouse hover, let's go through the process.

## Prefetching Data on Event

To accomplish this, a function will be added to the **data-loader.js** that prefetches the data on a specific event. This function, which will be called `prefetchOnEvent`, and will take two arguments: the key (which is used to check the cache and store data) and the function to be executed to fetch the data when necessary. Here's how it looks:

```js
// data-loader.js

export function prefetchOnEvent(key, fn) {
  // Check if the data is in the cache
  if (dataCache.has(key)) {
    // If data is in cache, just return it
    return;
  }
  // If data isn't in cache, fetch it and add to cache
  prefetchData(key, fun);
}
```

:::note

We can put this code below our prefetch function on line **14**.

:::

## Triggering Fetch on Event

Now, let's incorporate this into the application. In the `home` component, data will be prefetched when the user hovers over a Pokémon's name. By adding an `onMouseEnter` event to the link component, data fetching will start as soon as the user's mouse enters the area.