What is the "Hydration Failed" Error in Next.js and How to Avoid It

In Next.js, the error message "Hydration failed because the initial UI does not match what was rendered on the server" is a frequent source of frustration, especially for developers working with components that depend on client-side behaviors or effects. This article will explain what this issue means, why it occurs, and provide strategies to avoid it in the future.

What is Hydration?

Hydration is the process by which Next.js takes the server-rendered HTML sent to the browser and enhances it with interactivity using React. During server-side rendering (SSR), Next.js generates static HTML for your components. When this HTML is loaded in the browser, React attaches event listeners and restores any dynamic functionality.

If the HTML generated during SSR does not match the HTML React generates in the browser, a hydration mismatch occurs, leading to the error:

"Hydration failed because the initial UI does not match what was rendered on the server."

Why Does the Error Occur?

The error occurs when there are differences between the HTML generated on the server and the DOM generated on the client. These mismatches often result from:

1. Client-Side-Only Features: Components relying on browser-specific APIs (like window, document, or navigator) or effects that only run in the client (useEffect, useState) can't be rendered accurately on the server.

2. Timing Issues: Components that calculate their appearance dynamically (e.g., using a viewport check like useInView) may behave differently on the server and client.

3. Asynchronous Behaviors: If data fetched or computed during client rendering is different from what was used during server rendering, it can cause discrepancies.

4. Mutable State: Differences between initial state values on the server and client (e.g., useState) lead to mismatches.

5. Dynamic Content or Dependencies: Components like ReactPlayer, animations, or any library that performs client-side rendering can cause this error if they attempt to execute on the server.

Common Scenarios Leading to Hydration Mismatches

1. Using Browser APIs on the Server: Accessing window, document, or similar browser-specific APIs during SSR.

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2. State-Based Content Differences: A button's text or an element's visibility depends on a useState value initialized differently on the server and client.

3. Rendering Client-Side Libraries: Libraries like ReactPlayer, useInView, or those that depend on browser events can lead to mismatches when rendered on the server.

4. Images and Placeholders: The next/image component may load placeholders differently on the client and server, causing visual discrepancies.

Strategies to Avoid Hydration Mismatches

Here are some actionable techniques to prevent this issue:

1. Use Dynamic Imports for Client-Side-Only Components

When working with components or libraries that rely on browser APIs or client-side rendering, you should load them dynamically using dynamic with ssr: false.

Example:

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By disabling SSR for such components, they will only render on the client side, avoiding hydration mismatches.

2. Conditionally Access Browser-Specific APIs

Check whether you're in the browser before using APIs like window or document.

Example:

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3. Ensure Consistent State Between Server and Client

State mismatches are a common cause of hydration errors. Ensure that any useState or useEffect logic results in the same initial output on the server and client.

Example:

Instead of this:

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Use:

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4. Handle Dynamic Content After Hydration

For content that depends on client-side computations (like viewport checks or animations), defer rendering until the component has mounted.

Example:

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5. Use the priority Attribute for Images

For images loaded with the next/image component, adding the priority attribute ensures that they are preloaded and reduces the likelihood of hydration mismatches.

Example:

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6. Debugging Hydration Errors

Use the following tips to identify and debug hydration mismatches:

• Inspect the Server HTML: Use the browser’s "View Page Source" to examine the server-rendered HTML.
• Inspect Client DOM: Use the browser's developer tools to compare the DOM after React has hydrated.
• Wrap Components in Error Boundaries: Catch and debug errors during hydration.

Example:

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The "Hydration failed because the initial UI does not match what was rendered on the server" error arises when server-rendered HTML differs from client-rendered HTML in Next.js. By understanding the causes—browser-specific APIs, dynamic content, or mismatched state—and applying strategies like dynamic imports, conditional rendering, and consistent state management, you can effectively prevent hydration mismatches.