[Fixed] Troubleshooting React issue: warning 'Expected server HTML to contain a matching <header> in <div>' with server-side rendering and – Express

The Problem:

While setting up server-side rendering (SSR) with Express and React, the server sends an HTML response that React renders on the client side. However, the HTML response does not contain a matching <header> element as expected by React, resulting in a warning. The component includes a <header> element, and the HTML generated by the server also contains the <header> element. The issue persists even after removing nested components or searching for malformed HTML, leaving the cause of the mismatch uncertain.

The Solutions:

Solution 1: Correcting HTML Template

The issue stemmed from a mismatch between the structure of the HTML template and the rendered React component. React requires the injected component to be inline, without any intervening newlines.

The following correction was made to the HTML template:

<div class="root"><%- reactComponent %></div>

Instead of:

<div class="root">
  <!-- Rendered React component will be injected here -->
  <%- reactComponent %>
</div>

This ensured that the HTML template aligned with React’s requirement for inline component injection, resolving the "Expected server HTML to contain a matching <header> in <div>" warning.

Solution 2: Use a data fetching library that supports server side rendering

This error can occur when certain parts of your component use client-only data or perform operations that aren’t executed on the server. To resolve this issue, consider using a data fetching library that supports server side rendering, such as react-query or SWR. These libraries allow you to fetch data on the server side and make it available to your components when they are rendered. This ensures that the data is available when the component is first rendered, eliminating the mismatch between the server-rendered HTML and the client.

Solution 3: Handle HTML Whitespace Correctly

Ensure that there’s no extra whitespace (including newlines) around the React-generated HTML within the root node. This is a common cause of hydration errors, as described in the React docs. To resolve this, inline the HTML by joining the different parts of the response using the `join()` method, as shown in the provided code snippet.

Video Explanation:

The following video, titled "I spent an hour debugging this react error - YouTube", provides additional insights and in-depth exploration related to the topics discussed in this post.

issue took my about an hour to debug bug, but if I spent some time re-reading my code, I could have fixed it in 5 minutes. Programming sucks ...