• Refactor existing React code into custom hooks

One of the most powerful features of React hooks is that they give us the ability to share logic and state between multiple components by writing our own custom hooks. You've already encountered some custom hooks: the useParams and useHistory hooks from React Router are hooks that let us access the params and history objects from React Router in any component we want. In this lesson, you'll learn how to create your own custom hooks by extracting hooks-related logic out of components and into a reusable hook function.

This lesson has some starter code for a blog site using React Router. The data for the blog is saved in a db.json file, which we'll serve up using json-server. To get started, run npm install. Then, run npm run server to run our json-server backend in one terminal tab. Open another terminal tab and run npm start to run our React frontend.

We'll be focusing on two components: the HomePage and ArticlePage components. Make sure to run the app and familiarize yourself with the code before moving on.

In both the HomePage and ArticlePage components, you'll notice that we are using the useEffect hook in order to set the document title. The document title is an important part of any website, because it:

• is displayed in the browser tab
• is also displayed in the browser history
• helps with accessibility and search engine optimization (SEO)

Since we have similar logic for updating the title in both of our components (and we might want this functionality in other components as our app grows), this logic is a good candidate for a custom hook! Let's start by creating a new file for our custom hook: /src/hooks/useDocumentTitle.js. Let's take the useEffect code from the HomePage component and place it in that file, inside a function called useDocumentTitle:

// src/hooks/useDocumentTitle.js
import { useEffect } from "react";

function useDocumentTitle() {
  useEffect(() => {
    document.title = "Underreacted | Home";
  }, []);
}

export default useDocumentTitle;

There are a couple important things going on with this custom hook already, so let's review.

So far, any time we've wanted to use a React hook (like useState or useEffect), we've only been able to do so inside of our React components (not inside of any other JavaScript functions).

Custom hooks also allow us to call React hooks, so long as we call our custom hook from a React component.

Another important convention to note here: the name of our custom hook starts with the word use. This is a signal to React (and ESLint) that our hook should follow the Rules of Hooks, and also a signal to other developers that this code is meant to be used as a React hook.

Now that we've extracted this custom hook to its own file, we can import it and use it in our HomePage component:

import React, { useEffect, useState } from "react";
import About from "./About";
import ArticleList from "./ArticleList";
import useDocumentTitle from "../hooks/useDocumentTitle";

function HomePage() {
  // fetch data for posts

  // set the document title
  useDocumentTitle();

  // return ...
}

export default HomePage;

After that update, our component should work the same. But now our HomePage component doesn't have to worry about the logic for updating the document title: it just needs to call the useDocumentTitle hook, which will handle that work.

Updating our ArticlePage component won't quite work with our new custom hook just yet, since the title is dynamic in this component:

// src/components/ArticlePage.js
function ArticlePage() {
  //...

  const pageTitle = post ? `Underreacted | ${post.title}` : "Underreacted";
  useEffect(() => {
    // depends on the page title
    document.title = pageTitle;
  }, [pageTitle]);

  //...
}

To solve this, we can update our useDocumentTitle hook to accept an argument of the page title:

function useDocumentTitle(pageTitle) {
  useEffect(() => {
    document.title = pageTitle;
  }, [pageTitle]);
}

Now, both our components can use this custom hook by passing in a page title when calling the hook:

// src/components/ArticlePage.js
function ArticlePage() {
  //...

  const pageTitle = post ? `Underreacted | ${post.title}` : "Underreacted";
  useDocumentTitle(pageTitle);

  //...
}

// src/components/HomePage.js
function HomePage() {
  //...

  useDocumentTitle("Underreacted | Home");

  //...
}

One other common piece of logic that is shared between our components is fetching data from our API. Both the ArticlePage and HomePage share some similarities with regards to data fetching:

• They both have a couple state variables related to data fetching (isLoaded, posts, posts)
• They both use the useEffect hook to fetch data as a side effect of rendering the component

By noticing these similarities, we can recognize what logic is coupled together and what we'd need to extract in order to build out our custom hook.

Let's start with the HomePage component once again. Here's all of the logic that is related to working with our API:

// src/components/HomePage.js
function HomePage() {
  // fetch data for posts
  const [isLoaded, setIsLoaded] = useState(false);
  const [posts, setPosts] = useState([]);

  useEffect(() => {
    setIsLoaded(false);
    fetch("http://localhost:4000/posts")
      .then((r) => r.json())
      .then((posts) => {
        setPosts(posts);
        setIsLoaded(true);
      });
  }, []);

  // return ...
}

To start off with, let's take all this code out from our HomePage component and create a new custom hook called useQuery, for querying data from our API:

// src/hooks/useQuery.js
import { useState, useEffect } from "react";

function useQuery() {
  const [isLoaded, setIsLoaded] = useState(false);
  const [posts, setPosts] = useState([]);

  useEffect(() => {
    setIsLoaded(false);
    fetch("http://localhost:4000/posts")
      .then((r) => r.json())
      .then((posts) => {
        setPosts(posts);
        setIsLoaded(true);
      });
  }, []);
}

export default useQuery;

Just like before, we created a new file for our custom hook, and imported the React hooks that our custom hook will use. We also gave our custom hook a name that starts with use.

Unlike our previous custom hook, however, we're going to need to get some data back out of this component. Specifically, when we're using this component, we'll need access to two things:

• the data returned by the fetch request (posts)
• the isLoaded state

But how can we get this data out of the custom hook? Well, since a custom hook is just a function, all we need to do is have our hook return whatever data we need!

// src/hooks/useQuery.js
import { useState, useEffect } from "react";

function useQuery() {
  const [isLoaded, setIsLoaded] = useState(false);
  const [posts, setPosts] = useState([]);

  useEffect(() => {
    setIsLoaded(false);
    fetch("http://localhost:4000/posts")
      .then((r) => r.json())
      .then((posts) => {
        setPosts(posts);
        setIsLoaded(true);
      });
  }, []);

  // return an *object* with the data and isLoaded state
  return {
    posts: posts,
    isLoaded: isLoaded,
  };
}

export default useQuery;

Now, in order to use this custom hook, we can call it from our HomePage component, and destructure the return value to get the posts and isLoaded state out:

import useQuery from "../hooks/useQuery";

function HomePage() {
  // fetch data for posts
  const { posts, isLoaded } = useQuery();

  // ...
}

Our HomePage component is now significantly cleaner, because it no longer has to worry about all the logic related to handling the fetch request and setting state based on the response — all of that logic is now nicely encapsulated in our useQuery hook!

In order to get this hook to work with the ArticlePage component as well, we need to refactor it a bit and abstract away the logic that is specific to the HomePage component's needs.

// take in the url
function useQuery(url) {
  const [isLoaded, setIsLoaded] = useState(false);
  // rename `posts` to a more generic `data`
  const [data, setData] = useState(null);

  useEffect(() => {
    setIsLoaded(false);
    fetch(url)
      .then((r) => r.json())
      .then((data) => {
        setData(data);
        setIsLoaded(true);
      });
  }, [url]);
  // the url is now a dependency
  // we want to use the side effect whenever the url changes

  // return an *object* with the data and isLoaded state
  return { data, isLoaded };
}

Now, to use our more generic version of this hook in the HomePage component, we just need to make a couple small changes:

function HomePage() {
  const { data: posts, isLoaded } = useQuery("http://localhost:4000/posts");

  // ...
}

Since useQuery now accepts a URL for the fetch request, we must pass that URL in when we call the hook. It now also returns an object with a more generic name (data), so we can re-name that variable to posts when destructuring.

The useQuery hook should now also work with our ArticlePage component:

function ArticlePage() {
  const { id } = useParams();
  const { data: post, isLoaded } = useQuery(
    `http://localhost:4000/posts/${id}`
  );

  // ...
}

With our custom hooks in place, the completed versions of the HomePage and ArticlePage components are now both significantly shorter. Also, adding new components to our application that need access to similar functionality is now significantly easier, since we don't have to rewrite that functionality from scratch in each new component.

Note: While our useQuery hook works nicely in this example, there are some optimizations we could make to improve it, such as:

• Handling errors with .catch and adding an error state
• Using one state variable instead of multiple state variables, so that it doesn't re-render more than necessary
• Using the useReducer hook instead of useState to manage state transitions
• Caching our fetched data to prevent unnecessary network requests
• Cancel the fetch if the component un-mounts before the fetch is complete

You're encouraged to try adding a few of these optimizations to this hook yourself! There's also a version of the useQuery hook in the solution branch called useQueryAdvanced that handles some of these optimizations. However, there are also pre-built solutions out there, such as React Query, that handle this logic (and more) with a pre-built custom hook.

Creating custom hooks allows us to share stateful logic across multiple components. The ability to use custom hooks lets us create more concise components that are focused more on the UI logic.

The React community has also embraced custom hooks in a big way — major libraries like React Redux and React Router use custom hooks to provide a lot of their functionality, and there are lots of community generated custom hooks out there to explore and add to your projects!