React.js is a powerful client-side JavaScript library. Like any other JavaScript library, React.js gives you a smooth experience when building reactive and declarative user interfaces. However, there are a few negative aspects to it, like state management and data fetching.

Server state management is a little different from other libraries. In React.js, it is asynchronous and the data persists remotely with no direct control. This means that we have to update, cache or re-fetch the data to efficiently manage the state in React.js applications.

React Query is a pre-configured library that aims to solve these complexities.

Using React Query, we can fetch, cache, and update data in React-based applications in a simple and declarative manner without mutating the global state.

Goal

Nowadays, almost every web application works with remote data. Unfortunately, for developers, data fetching and handling server states in React applications are easier said, than done.

As developers, we need to rethink about:

• What to render while waiting for the remote data to load?
• What happens if an error occurs?
• How do we keep the client up to date with the server?
• How will these tradeoffs impact the users with a poor internet connection?

When dealing with asynchronous data that needs frequent updating, caching, and synchronization with the server, there is no better library than React-Query.

In this tutorial, we will go through how the React-Query library can improve the user experience in our react applications.

To demonstrate these concepts, we will use the JSON Placeholder as the third-party REST API.

Prerequisites

For this tutorial, we need to have:

• An intermediate level skills on React hooks and functional components are essential.
• A basic understanding of REST APIs and data fetching in JavaScript.
• Make sure to have Node.js runtime installed on your machine.

Understanding state management and server state

Every interactive client-side application will involve interactive events.

For example, when a user interacts with an application by clicking a button or closing a sidebar, the app must re-render the page accordingly to reflect these changes. We call this change, the state of the app.

In the context of React.js and Single Page Applications (SPA), state management is a way to share the data across different React components.

In other words, a state is simply a JavaScript object representing part of a component that can change based on user actions.

You can read more about this in the React.js documentation.

Benefits of using React-Query in react.js applications

Some of the features that React-Query provides include:

• Using window focus pre-fetching mechanism to pre-fetch the data depending on application tab activity.
• We can set the number of request retries for any request, in case of random errors.
• React-Query performs pre-fetching so that the application can update stale data in the background.
• Handling complex application caching so that the request operation is optimized.

Installation and setup

To demonstrate these concepts, we will implement a data fetching React Todo application that fetches posts from the JSON placeholder - a fake REST API.

First things first, let's set up a basic React.js application.

Use the command below to create a boilerplate React.js application in the react-query-demo directory:

npx create-react-app react-query-demo

Next, we need to add axios to make HTTP requests and react-query:

npm i react-query axios

For yarn, execute:

yarn add react-query axios

Next, start the application using this command:

npm start

This triggers our development server with hot-reload at http://localhost:3000.

To configure React-Query in a React.js application, we need to wrap the components that need data fetching, with the QueryClientProvider component.

The child components of the QueryClientProvider can now access the hooks provided by the React-Query library that provides us with a QueryClient instance. We will be using this instance to access the hooks provided by the React-Query library.

To start the React-Query, we will use a basic configuration in the root directory file index.js as:

import React from 'react'
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import { QueryClient, QueryClientProvider } from "react-query";
const queryClient = new QueryClient();
ReactDOM.render(
  <QueryClientProvider client={queryClient}>
    <App />
  </QueryClientProvider>,
    document.getElementById('root')
);

In the next steps, we will implement data fetching using the useQuery, useMutation, and the native React hooks.

Data fetching using React-Query

The difference between React-Query and the common data fetching patterns such as useEffect, is that React-Query will first return the previously fetched data and then re-fetch it again.

If the resource is the same as the first, React-Query will keep both the data as a reference without forcing the page to reload.

While useEffect fetches the data irrespective of the modified data and reloads the page.

To demonstrate these concepts, we will be using the Post and Posts components. The fetching data logic will be written in Posts.js.

Go ahead and create a components folder in the src application folder.

First, import React, useQuery hook, and the axios library:

import React from 'react'
import {useQuery} from 'react-query'
import axios from 'axios'

We need to define an asynchronous function fetchPosts() that will make network requests to the JSON placeholder API to fetch the posts.

async function fetchPosts(){
    const {data} = await axios.get('https://jsonplaceholder.typicode.com/posts')    
    return data
}

Finally, create a React functional component:

function Posts(){
    const {data, error, isError, isLoading } = useQuery('posts', fetchPosts) 
    // first argument is a string to cache and track the query result
    if(isLoading){
        return <div>Loading...</div>
    }
    if(isError){
        return <div>Error! {error.message}</div>
    }
    return(
        <div className='container'>
        <h1>Posts</h1>
        {
            data.map((post, index) => {
                return <li key={index}>{post.title}</li>
            })
        }
        </div>
    )
}
export default Posts

Let's briefly dissect the code above:

• We create a React functional component named Posts that is exported as default.
• The useQuery hook returns an application status that can either be isLoading or isError.
• useQuery hook takes two arguments where the first one is a string to cache and tracks the query result.
• The second argument is the function we define to make HTTP requests, fetchPosts. The react-query internally handles caching and the updating of data.
• Before the return statement of the component, we execute a basic logic to check if the application is in a loading state or if an error occured.
• Inside the JSX part, we return the posts by mapping through the posts array.

For an awesome developer experience, you can use react-query-devtools. react-query includes the in-built tool react-query-devtools that helps us view the state and the cache. To enable this in your application import { ReactQueryDevtools } from 'react-query/devtools'.

You can read more about this here.

Making a POST request using 'useMutation' hook

Unlike the useQuery hook, mutations are used to perform CREATE, UPDATE, or DELETE operations as the server-side operations inside our components.

In our case, we will perform a POST request using useMutation.

Inside the components folder, create a Post.js file.

In the Post.js component file, import axios, React and useMutation hook as shown below:

import React, {Fragment, useState} from 'react'
import { useMutation } from 'react-query'
import axios from 'axios'

Next, create a Post functional component:

export default function Post(){
    const [title, setTitle] = useState('')
    const [description, setDescription] = useState('')
    const [message, setMessage] = useState('')
    const {isLoading, isError, error, mutate} = useMutation(createPost, {retry: 3})
    async function createPost() {
        const response = await axios.post('https://jsonplaceholder.typicode.com/posts')
        setMessage(response.data)
    }
    return(
        <Fragment>
            <div className="post">
            <h1>Create a Post</h1>
                <label>Title:</label>
                <input type="text" value={title} onChange={e=>setTitle(e.target.value)}/>
                <label>Description:</label>
                <input type="text" value={description} onChange={e=>setDescription(e.target.value)}/>
                <button  onClick={() => {mutate({ id: Date.now(), title, description })}} >Create</button>
                <p> Created a new Post ID: {message && message.id}</p>
                <div style={{color: 'gray', background: '#234'}}>
        {isLoading
          ? "Saving...": ""
        }
        {
            isError
            ? error.message : ""
        }
      </div>
      </div>
        </Fragment>
    )
}

To briefly explain the code above:

• createPost is a function that makes HTTP POST requests using the axios library to the third-party API.
• The useMutation() hook returns isLoading, isError, error, and mutate function that will be used to wrap the values when making requests.
• It takes the createPost as an argument along with an option {retry:3} object. This comes in handy when prefetching queries after mutations and ensuring optimistic updates.
• Prefetching allows us to perform fetch operations to the data before it is needed. The cool thing about optimistic updates is that we have an option to roll back our updates if anything goes wrong.
• The useState() hook is used to create and update the title, and description state of the input elements.
• Below the create post functionality, we use the isLoading and isError to handle the mutation state accordingly. We can use isLoading to indicate that something is being asynchronous posted to the server. The isError and error will give us information in case of an error. This show that we can fetch and post the data from a third-party REST API using React-Query, without having to reload the screen.

Conclusion

Building front-end applications often start easy and turns out to be complicated as we continue adding features. For every API endpoint we add, we also need to deal with state management, synchronization, caching, and error handling.

React-Query library helps us manage data involved in the web service requests keeping our application maintainable as the complexity grows while improving user experience.

React-Query is often described as the missing piece in React ecosystem and this tutorial gave us an overlook of this awesome tool. Happy coding.