React 17 and React 18 introduced several differences, particularly with regard to features, performance improvements, and changes to the way React works under the hood. Here’s a comparison between the two:

1. Concurrent Rendering (React 18)

• React 17: React 17 did not have support for concurrent rendering. The rendering process was synchronous, meaning React rendered the entire component tree in one go, blocking the UI thread until the render process was completed.

• React 18: React 18 introduces concurrent rendering as an opt-in feature. This allows React to work on multiple tasks simultaneously, making the UI more responsive and improving the user experience. With concurrent rendering, React can interrupt rendering to keep the UI interactive while it continues to update in the background.

  • ReactDOM.createRoot: In React 18, you now need to use ReactDOM.createRoot instead of ReactDOM.render to enable concurrent features.

  import ReactDOM from 'react-dom/client';
  const root = ReactDOM.createRoot(document.getElementById('root'));
  root.render(<App />);

  


2. Automatic Batching (React 18)

• React 17: React only batched state updates in event handlers (like clicks or user input), but not in asynchronous code (like setTimeout or promises).
• React 18: React 18 introduces automatic batching for all updates, including asynchronous ones (such as inside setTimeout, Promises, etc.), which means React will group updates together and minimize unnecessary re-renders, improving performance.

3. Suspense and Suspense List (React 18)

• React 17: React 17 supported Suspense, but it was limited to code-splitting (i.e., handling the loading of components).
• React 18: Suspense is enhanced in React 18 and can now be used for data fetching (e.g., with libraries like React Query or Relay). Additionally, React 18 introduces SuspenseList to coordinate multiple Suspense boundaries and handle them in a more controlled way.

4. Server-Side Rendering (SSR) and Hydration (React 18)

• React 17: Server-Side Rendering was supported, but hydration (the process of making a server-rendered page interactive) was a more straightforward and less flexible approach.
• React 18: React 18 improves Server-Side Rendering (SSR) and hydration. It introduces Streaming SSR and Concurrent Rendering for SSR, which allows you to stream HTML to the client progressively while React is still working to hydrate and become interactive, improving performance for large applications.

5. useId Hook (React 18)

• React 17: No built-in hook for generating unique IDs.

• React 18: React 18 introduces the useId hook to generate stable, unique IDs that are safe for use in both the client and server during SSR. This is helpful in preventing mismatches between server-rendered and client-rendered IDs, especially in forms and elements requiring IDs.

  import { useId } from 'react';
  const id = useId();

6. Concurrent Features and startTransition (React 18)

• React 17: Did not have concurrent features like startTransition.

• React 18: React 18 introduces the startTransition API to mark updates as non-urgent, helping React prioritize more important updates (e.g., user interactions). This allows for smoother rendering of lower-priority updates, such as state updates triggered by network responses or other non-urgent updates.

  import { startTransition } from 'react';
  
  startTransition(() => {
  // Updates that are lower priority
  });

7. Gradual Adoption of New Features (React 18)

• React 17: React 17 focused mainly on making it easier to upgrade React apps without breaking things. It didn’t introduce major new features.
• React 18: React 18 is more focused on introducing new features like concurrent rendering, Suspense for data fetching, and improved SSR. However, it also allows for gradual adoption of these features. This means you can opt into concurrent rendering and other new features on a per-component basis, making it easier to adopt them over time.

8. Improved TypeScript Support

• React 17: React 17 had improved TypeScript support but was still limited by the lack of native support for some newer TypeScript features.
• React 18: React 18 provides better TypeScript support, including better typing for hooks like useId and other new APIs.

9. Legacy Features and Backward Compatibility

• React 17: React 17 focused on easing the upgrade process and improving backward compatibility.
• React 18: React 18 continues to maintain backward compatibility, but it also introduces breaking changes in how you need to initialize your app (using createRoot instead of render).

10. React 18 New APIs and Features

• ReactDOM.createRoot: As mentioned, you need to use this method for initializing the React app, enabling concurrent rendering.
• useTransition: A new hook for handling transitions.
• startTransition: Used for prioritizing urgent updates over non-urgent ones.
• useId: A new hook for generating unique IDs.

Summary of Key Differences:

Conclusion:

React 18 introduces significant performance improvements with concurrent rendering, automatic batching, and features for smoother user experiences, such as Suspense for data fetching. It’s an opt-in upgrade, meaning you don’t need to switch everything over at once but can adopt features gradually. React 17 was more about making React easier to upgrade, whereas React 18 focuses on improving how React works behind the scenes.