Error: Expected server HTML to contain a matching

in

in Next.js: Causes and How to Fix

Mismatch Between Client and Server Rendering

• All page components in Next.js can be hydrated on the client-side after being pre-rendered on the server. A mismatch between the server-rendered HTML and what's expected by the React component can trigger this error. This typically arises when the server HTML structure does not match the DOM expected or generated by client-side React components. For instance, if dynamic data (like user-specific content) alters the structure differently on the client from the pre-rendered server output, a matching `
  ` might not be found.

Conditional Rendering

• Errors commonly stem from components that are conditionally rendered based on client-side state. When relying on client state not available or different during server rendering, React will fail to match the HTML structures. If an element is only rendered based on a state that changes after the initial render, the server might render a different structure than the initial client-side build, causing a mismatch. For example:

// Server-side rendered state might be false, skipping the div
const Component = () => {
  const [toggle, setToggle] = useState(false);

  return (
    <div>
      {toggle && <div>Dynamic Content</div>}
    </div>
  );
}

Async Data Fetching

• Another frequent cause is data fetched asynchronously on the client side that influences the component's HTML structure but hasn't been reflected during server-side rendering (SSR). This happens when the server render doesn't include fetched data, leading the client to render a different structure when the data arrives asynchronously. For example, they may lead to different numbers of elements being rendered:

// Data fetched asynchronously may alter the DOM structure
const DynamicList = () => {
  const [items, setItems] = useState([]);

  useEffect(() => {
    fetch('/api/data')
      .then(response => response.json())
      .then(data => setItems(data));
  }, []);

  return (
    <ul>
      {items.map(item => <li key={item.id}>{item.name}</li>)}
    </ul>
  );
}

Environment-Specific Code

• Using environment-specific attributes that differ between client and server renderings, such as checking for `window` or `document` without proper guards, can cause discrepanices. Since properties like `window` and `document` are not available in SSR, conditional logic or effects relying on them can lead to different HTML outputs between server and client.

Dependence on Global State

• Errors can also emerge from rendering logic that depends on global state, which is inherently different on client and server sides. Global application state persisting across renders on the client might not be initialized in exactly the same way during the first render on the server, leading to differences.