Hydration

tl;dr hydration is the process that adds interactivity to the static HTML that was pre-rendered by the server.

When you load a page that uses Server-Side Rendering (SSR), the server generates all the HTML and sends it to your browser. SSR gives you a smooth experience: when you load the page, it looks complete and nice right away.

But wait a second—it’s just static content, pretty much nothing more than a snapshot of the page at that moment. None of the React components are functional yet.

Next thing you know, the client-side JavaScript kicks in. React re-creates the component tree using the Virtual DOM and attaches event listeners, making the components functional and interactive. This process is known as hydration.

If I am to sound corny, I’d say React breathes life into the components.

Warning: Hydration Mismatch

Here comes the tricky part: if the server-rendered HTML doesn’t match perfectly with the Virtual DOM React creates on the client, aka the notorious “hydration mismatch”, things can get ugly.

React might have trouble attaching event listeners to where they belong. You might get buttons that don’t respond to clicks or dropdowns that refuse to toggle.

If the actual DOM is way too different from the Virtual DOM, React may try to reconcile the differences. It might even re-render the entire component tree. This may increase load times or cause a Flash of Unstyled Content (FOUC), where styles are briefly missing or applied incorrectly.

If the initial server-rendered HTML layout differs significantly from the one rendered after hydration, elements might act unexpectedly. You might have content jumping around on the page. This can occur if you have client-only logic modifying the layout after hydration, like checking window.innerWidth.

Sometimes you get lucky, the issues are rather subtle. For instance, in a scenario where text rendered on the server is different due to timestamps, React will just warn you and update the content. Although a flicker of text changing could still be perceivable, it’s relatively minor.

And sometimes a hydration mismatch acts in the form of state and prop inconsistencies. These can lead to incorrect data being displayed or break the component’s functionality, especially when the UI’s rendering and behaviors depends on the state.

Take hydration mismatches seriously. Be an adult and stop fooling yourself with "well, if it's not broken." These harmless tiny act-ups can snowball into a debugging hell before you notice, especially as your application grows or when performance becomes a concern.

Culprits behind Hydration Mismatches

If you think about it, anything that might act differently between the server and client could lead to a mismatch. Here are some of the convicted felons and ways to tame them:

Random and Time-Sensitive Values

Values that are unpredictable and change every time they are called.

Random numbers: Math.random( ), UUID

Time-based values: Date.now( )

They are inherently variable and if they’re used during SSR, the HTML will differ from what React generates on the client.

How to avoid: Use stable, predictable values for rendering during SSR, update these values on the client using useEffect or similar hooks once the component has mounted.

TYPESCRIPT

const [randomValue, setRandomValue] = useState(0);
useEffect(() => {
  setRandomValue(Math.random());
}, []);

Using a hash combined with an index for unique list keys is an option alternative to UUIDs, should the need arise.

Client-Side Specific Logic

Logic that depends on browser-specific conditions or APIs, for example, window.innerWidth, document.title and localStorage during SSR. Styles that generated dynamically based on client-side logic.

I had the first-hand experience with window.innerWidth myself. It wasn’t until deployment that I realized the server doesn’t have a window at all.

How to avoid: Use useEffect or similar hooks to run client-side logic only after the page has hydrated. Namely, make sure browser-dependent code only execute on the client side.

Here’s how I fixed the resize logic:

TYPESCRIPT

useEffect(() => {
  if (typeof window === "undefined") return;

  const handleResize = () => {
    const isLargeViewport = window.innerWidth >= 1100;
    // Logic based on viewport size
  };

  handleResize();
  window.addEventListener("resize", handleResize);
  return () => window.removeEventListener("resize", handleResize);
}, []);

Asynchronous Data and State Management

Data that is fetched asynchronously or managed through state can lead to mismatches if the data changes between SSR and hydration.

This could lead to visible content changes and of course, hydration warnings.

How to avoid: Use a consistent data-fetching strategy.

getServerSideProps in Next.js is a way to go. Implementing caching mechanisms is another option.

TYPESCRIPT

// Fetch data consistently using getServerSideProps
export async function getServerSideProps() {
  const data = await fetchDataFromAPI();
  return { props: { initialData: data } };
}

Conditional Rendering

Rendering based on conditions that might differ between server and client.

For instance, if the server can’t determine a user’s authentication status, the user profile menu might be missing on the initial render and show up after hydration.

How to avoid: Use default values, placeholders, or deferring client-only conditions with useEffect.

TYPESCRIPT

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

useEffect(() => setIsClient(true), []);

return isClient && isAuthenticated ? <UserProfileMenu /> : <Placeholder />;

Very often, there are more than one culprits. You might have a component depending on both asynchronous data fetching and client-side specific logic that throws you into the maelstrom of inconsistencies.

Playing Detective

Debugging hydration mismatches can be incredibly frustrating and ambiguous.

Vague error messages

When React catches a hydration mismatch, it doesn’t give you enough context. “Text content does not match server-rendered HTML”, “Extra attributes from the server”… Where does it come from? The cause could be buried deeply in the component tree, making it challenging to pinpoint.

Ambiguous outcomes

While React does a good job telling you something goes wrong, can you really spot it?

Sometimes a button that previously worked stops working due to a hydration mismatch, but you might not notice it because you’re working on a drawer. All you can see is a warning showing up in your console, and you start questioning your entire codebase.

Sometimes, the layout shifts, you just can’t connect it to a hydration mismatch because the potential causes are endless.

Case: Nested Tags

This is a particular mixed culprit scenario I found frustrating yet interesting. Conditional rendering and client-side specific logic are involved.

One thing I should’ve paid more attention when I was reading the Next.js docs is,

The layout file is used to define a layout in your Next.js application.

I took this feature for granted without much thought, thinking, “Great, now Next wraps my pages in my layout nicely by default.” Then I went ahead and lay in this bed I made:

TYPESCRIPT

import { useEffect, useState } from "react";
import { parseHTMLContent } from "@/lib/html-parser"; // Custom HTML parser

export default function BlogPost({ post }) {
    const [parsedContent, setParsedContent] = useState("");

    useEffect(() => {
        if (post.content) {
            const parsed = parseHTMLContent(post.content);
            setParsedContent(parsed);
        }
    }, [post.content]);

    return (
        <div className="blog-post">
            <h1>{post.title}</h1>

            {/* Disaster strikes: Parsed HTML content changes on the client */}
            <div dangerouslySetInnerHTML={{ __html: parsedContent || post.content }} />
        </div>
    );
}

The actual component was more complicated, but this is about the gist of that.

I had some rich HTML content in post.content that needed to be parsed into a more structured format. The goal seemed simple and efficient at the time:

Offloading Parsing to the Client: I wrote a custom parseHTMLContent function to transform the HTML, but I made it run only on the client using useEffect. My thinking was that this would keep the initial server render fast and lightweight.

Fallback Strategy: To ensure content was always displayed, I added a fallback using dangerouslySetInnerHTML to render the original, unparsed post.content if the parsed version wasn’t ready yet. This felt like a safe and pragmatic approach.

It all made sense in isolation—until the ignorance of Next.js’s internal mechanisms threw me off the cliff.

Server-Side Rendering (SSR) and hydration processes depend on an exact match between the server-rendered HTML and the client-rendered Virtual DOM.

Here’s how it fell apart:

The server-rendered HTML is more than a component’s output.

Next wraps your pages in your layout, by adding HTML tags around your components to keep the structure consistent.

The React tree you pass to hydrateRoot needs to produce the same output as it did on the server.

Keep in mind that Next.js is a React framework, and under the hood, it uses React’s hydrateRoot API for hydration. This API enforces the exact-match requirement between SSR HTML and CSR Virtual DOM.

dangerouslySetInnerHTML could make hydration mismatches worse.

dangerouslySetInnerHTML is a React prop that allows you to insert raw HTML into the DOM. It’s called “dangerous” for a reason: it bypasses React’s DOM diffing, making hydration issues even trickier to debug.

Putting It All Together: A Perfect Recipe for Disaster

I underestimated how Next.js wraps HTML around my content, complicating the initial structure.

Then, my client-side parsing logic drastically altered the HTML after hydration, violating hydrateRoot’s requirement for a perfect match between the server and client. There’s another risk here: the server might produce a well-structured HTML, but a <div> might end up in a <p> after client-side parsing.

What’s worse, using dangerouslySetInnerHTML crippled React’s ability to reconcile the DOM, making it impossible for React to handle these changes smoothly.

Strategies

Debugging hydration mismatches can be a nightmare but it follows the same systematic approach: start by checking the console for warnings, which may be vague but still provide useful clues.

Use browser DevTools to compare the server-rendered HTML and the client-side Virtual DOM, identifying any differences.

Try isolating the issue by temporarily disabling or commenting out components, then reintroduce them one by one to pinpoint the problem.

Look into client-only logic, like accessing window or document, and ensure it’s wrapped in useEffect so it doesn’t interfere with server-side rendering.

Use React DevTools to inspect the component tree, verify props and state, and log data flow to see how they are passed and rendered.

The best way to cope is to write clean code. I keep reminding myself: maybe next time I’ll be smarter—keep data consistent, check if components are mounted or if it’s running on the client side, and make sure what runs on the client stays at the client.

And remember to stay hydrated!