How to Update React Hydration in React

React hydration is the process where client-side JavaScript attaches event listeners to server-rendered HTML, turning a static page into a fully interactive application. This article provides a direct guide on how to update your React application to use the modern React 18 hydration API, resolve common hydration mismatch errors, and safely update state after the initial hydration process is complete.

Upgrading to the React 18 Hydration API

In React 18, the legacy ReactDOM.hydrate method was deprecated. To update your hydration process to the latest standard, you must use the hydrateRoot API from the react-dom/client package. This enables modern features like Selective Hydration and concurrent rendering.

To update your entry point, replace your old hydration code with the following implementation:

import { hydrateRoot } from 'react-dom/client';
import App from './App';

const container = document.getElementById('root');

// Update to React 18 hydrateRoot
hydrateRoot(container, <App />);

Handling Client-Server State Updates Correctly

A common issue during hydration is a “hydration mismatch” error, which occurs when the server-rendered HTML does not match the initial HTML rendered on the client. This often happens when displaying dates, times, or client-specific data like localStorage.

To update your UI with client-specific data without breaking hydration, you should use a two-pass rendering strategy with useEffect.

import { useState, useEffect } from 'react';

function MyComponent() {
  const [isClient, setIsClient] = useState(false);

  useEffect(() => {
    // This runs only on the client after hydration is complete
    setIsClient(true);
  }, []);

  return (
    <div>
      {isClient ? 'Rendered on Client' : 'Rendered on Server'}
    </div>
  );
}

By delaying the client-specific state update until the useEffect hook runs, you ensure that the initial client render matches the server HTML exactly, preventing hydration errors.

Suppressing Unavoidable Mismatches

If a minor mismatch is unavoidable (such as a timestamp difference of a few milliseconds) and does not affect the layout, you can suppress the warning by adding the suppressHydrationWarning attribute to the specific element.

<span suppressHydrationWarning>
  Current time: {new Date().toLocaleTimeString()}
</span>

Note that this attribute only works one level deep, and it should be used sparingly as a fallback rather than a primary solution for structural mismatches.