useSyncExternalStore Hook: A Powerful New Addition to the React Ecosystem

React, a popular JavaScript library for building user interfaces has introduced a new experimental hook, useSyncExternalStore, in its React 18 release. This hook is designed to work with external stores and provides a more direct API for synchronizing an external store with a React component.

The useSyncExternalStore hook is part of React's concurrent rendering features, which allow React components to render more smoothly by breaking the rendering process into smaller tasks and allowing the browser to interrupt this process to handle user input and other high-priority tasks.

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What is useSyncExternalStore?

useSyncExternalStore is a custom hook that allows React components to read from an external store and re-render whenever the store changes. It is a replacement for the useSubscription hook and provides better support for concurrent rendering and server rendering.

The useSyncExternalStore hook takes three arguments: a subscribe function, a getSnapshot function, and an optional getServerSnapshot function.

The subscribe function is a callback function that is called whenever the store changes. It allows the component to re-subscribe to the store updates whenever the store changes.

The getSnapshot function is used to read the current value from the store. This function is called during the render phase, so it should be a pure function without side effects.

The getServerSnapshot function is used for server side rendering. It is similar to the getSnapshot function, but it is used to get the initial snapshot of the store on the server.

Here is a basic example of how to use the useSyncExternalStore hook:

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     import {useSyncExternalStore} from 'react';
    
    function subscribe(callback) {
      // Subscribe to the store updates
    }
    
    function getSnapshot() {
      // Get the current value from the store
    }
    
    function App() {
      const value = useSyncExternalStore(subscribe, getSnapshot);
      // Render the value
    }
     

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How Does useSyncExternalStore Work?

The useSyncExternalStore hook works by subscribing to the external store in the useEffect hook and forcing a re-render whenever the store changes. The getSnapshot function is used to get the current value from the store during the render phase.

When the component re-renders, React compares the current value from the getSnapshot function with the previous value. If the values are the same, React bails out of the re-render to avoid unnecessary updates.

This hook also supports server rendering and selective hydration. When rendering on the server, React calls the getServerSnapshot function to get the initial snapshot of the store. This allows React to render the component with the correct data from the store on the server, avoiding hydration mismatches and visual inconsistency when the component is hydrated on the client.

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Advantages of useSyncExternalStore

The useSyncExternalStore hook has several advantages over the useSubscription hook and other methods of synchronizing an external store with a React component.

First, it provides automatic support for concurrent rendering features in React 18, such as time slicing and selective hydration. This allows the component to render more smoothly and respond to user input more quickly.

Second, it provides better support for server rendering. The getServerSnapshot function allows React to render the component with the correct data from the store on the server, avoiding hydration mismatches and visual inconsistency when the component is hydrated on the client.

Third, it provides a more direct API for synchronizing an external store with a React component. The subscribe function and getSnapshot function provide a clear and straightforward way to subscribe to store updates and read data from the store.

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useSyncExternalStore in Practice

Let's take a closer look at how to use the useSyncExternalStore hook in a React component. In this example, we'll create a simple todo app that reads data from an external store.

First, we'll create our external store. For simplicity, we'll use a simple JavaScript object as our store, but in a real application, this could be a more complex data structure or an external data source.

Next, we'll create our App component. This component uses the useSyncExternalStore hook to subscribe to the store updates and read the current value from the store.

In this example, the App component will re-render whenever the todos in the store changes. The useSyncExternalStore hook ensures that the component always renders with the latest data from the store, even when concurrent updates are happening in the React component tree.

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Answering Common Questions

Now that we have a good understanding of what useSyncExternalStore is and how it works, let's answer some common questions related to React and its hooks.

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How do you get the previous state in React hooks?

In React hooks, you can get the previous state by using a ref. A ref is a mutable value that can hold any value and does not cause a re-render when it changes. Here's an example:

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What is the useLayoutEffect?

The useLayoutEffect hook is similar to useEffect, but it runs synchronously after all DOM mutations, before the browser has a chance to paint. This makes it suitable for reading layout from the DOM and making synchronous updates, but it can also cause blocking and slow down your app if used excessively.

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What is the second argument in useEffect?

The second argument to useEffect is an array of dependencies. When any of the dependencies change, the effect function will run again. If the array is empty, the effect will only run once after the initial render.

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What is the difference between useState and useContext?

useState is a hook that lets you add state to your function components, while useContext is a hook that lets you access the value of a context. The main difference is that useState is local and scoped to the component, while useContext allows you to share state between multiple components.

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What is React used for?

React is used for building user interfaces in web applications. It allows developers to create reusable UI components, manage component state, and handle user events, among other things.

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What are the advantages of ReactJS?

React has several advantages, including a virtual DOM for efficient updates and rendering, the ability to create reusable components, a unidirectional data flow for easy reasoning about the state of your application, and a vibrant ecosystem with a wealth of libraries and tools.

Is React still being used?

Yes, React is still widely used and actively maintained by Facebook and the open-source community. It is used by many large companies and is one of the most popular libraries for building web applications.

Why use React over other frameworks?

React has a number of advantages over other frameworks, including a simple programming model, a virtual DOM for efficient updates, the ability to create reusable components, and a vibrant ecosystem. It also has strong support for server-side rendering and concurrent rendering with React 18.

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What are stores in React?

In React, a store is an object that holds the application state. It is not a built-in concept in React, but it is commonly used in combination with state management libraries like Redux or MobX.

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Does React have stores?

React itself does not have a built-in store concept, but it provides hooks like useState and useReducer that can be used to manage state in a component. For more complex state management, libraries like Redux or MobX can be used.

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How do I create a React store?

You can create a store in React using state management libraries like Redux or MobX, or you can use the built-in hooks like useState or useReducer for simpler state management. With the introduction of useSyncExternalStore in React 18, you can also create a store that can be synchronized with a React component.

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How do I access a React store?

You can access a store in React using the useSelector hook in Redux, the useStore hook in MobX, or the useSyncExternalStore hook in React 18. These hooks allow you to read data from the store and cause your component to re-render when the data in the store changes.

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Wrapping Up!

The useSyncExternalStore hook is a powerful new addition to the React ecosystem. It provides a more direct API for synchronizing an external store with a React component, and it provides automatic support for the concurrent rendering features in React 18.

Whether you're working with an external data source, managing complex state in a large application, or just looking for a more efficient way to synchronize your store with your React components, useSyncExternalStore is a tool worth exploring.

Remember, useSyncExternalStore is still experimental and its API could change in future releases of React. Always check the official React documentation for the most up-to-date information.

React continues to evolve and improve, and the introduction of useSyncExternalStore is a testament to the library's commitment to performance, efficiency, and developer experience. Whether you're a seasoned React developer or just starting out, there's always something new to learn and explore in the world of React. Happy coding!

     const store = {
      todos: [],
      subscribe(callback) {
        // Subscribe to the store updates
      },
      getSnapshot() {
        // Get the current value from the store
      },
    };
     

     import {useSyncExternalStore} from 'react';
    
    export default function App() {
      const todos = useSyncExternalStore(store.subscribe, store.getSnapshot);
      // Render the todos
    }
     

     import {useEffect, useRef, useState} from 'react';
    
    function App() {
      const [count, setCount] = useState(0);
      const prevCountRef = useRef();
    
      useEffect(() => {
        prevCountRef.current = count;
      });
    
      const prevCount = prevCountRef.current;
    
      // Now you can use prevCount to get the previous state
    }