React & TypeScript
Next.js Hydration Error Matcher & Fixer
Paste Next.js or React hydration error logs to instantly identify root causes (Error 418, 423, nesting violations, next-themes flash) and generate clean, copy-paste code fixes.
100% In-Browser & Private
HIGHDynamic & Non-Deterministic Values
Full Hydration Guide Dynamic Value (Date, Time, or Random Number) in Initial Render
A value that changes between server render time and client execution time (e.g. new Date(), Date.now(), or Math.random()) was directly printed to JSX.
Root Cause Breakdown
Server HTML is built at one millisecond (or at build time via SSG), and client execution happens seconds or minutes later in a different timezone or random seed. The rendered text differs, forcing React to discard server HTML and trigger a full client re-render.
Recommended Code Fix
Problematic Pattern (Causes Mismatch)
// Unsafe: Non-deterministic render
export function LiveClock() {
// Server generates time at server timestamp; client generates current client time
return <div>Last updated: {new Date().toLocaleTimeString()}</div>;
}Hydration-Safe Pattern (Fixed)
// Fixed Option A: suppressHydrationWarning on the specific element
export function LiveClock({ timestamp }: { timestamp: string }) {
return (
<div suppressHydrationWarning>
Last updated: {new Date(timestamp).toLocaleTimeString()}
</div>
);
}
// Fixed Option B: Render only after client mount
export function FormattedDate({ dateString }: { dateString: string }) {
const [mounted, setMounted] = useState(false);
useEffect(() => setMounted(true), []);
if (!mounted) return <span>Loading date...</span>;
return <span>{new Date(dateString).toLocaleDateString()}</span>;
}Key Prevention Principles
- Use suppressHydrationWarning on individual date/timestamp text nodes if timezone variance is acceptable.
- Pass ISO-8601 timestamps as props from the server rather than generating new Date() inside the component.
- Use the mounted state pattern for user-local relative timestamps (e.g. "5 minutes ago").
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How to use
- 1Paste your Next.js hydration error log or terminal trace into the Diagnose Error Log tab, or click one of the quick presets.
- 2Read the instant diagnosis breakdown, including root cause analysis and severity level.
- 3Review the side-by-side Before and After code examples, and click "Copy Code Fix" to grab the verified solution.
- 4Switch to the "Scan JSX Snippet" tab to paste your React component code and scan for risky patterns like direct new Date(), Math.random(), or invalid HTML nesting.
Frequently asked questions
What is a Next.js hydration error?
A hydration error occurs when the initial HTML string rendered on the server differs from the virtual DOM React builds on the client. When React mounts, it expects the server DOM and client DOM to match exactly. Any discrepancy (such as timestamp variations, browser globals like window, or invalid HTML tag nesting) causes React to discard the server HTML and trigger a full client re-render.
How do I fix "Text content does not match server-rendered HTML"?
This error usually happens when dates, random numbers, or client-only data are evaluated during the initial render. Fix it by: (1) using a mounted state guard with useEffect to defer client rendering, (2) adding the suppressHydrationWarning attribute to the specific element if slight timestamp drift is acceptable, or (3) using Next.js dynamic() with ssr: false for client-only components.
What is Minified React Error #418 and #423?
In production builds, React strips human-readable warning strings to minimize bundle sizes and replaces them with error codes. Error #418 means "Hydration failed because the initial UI does not match what was rendered on the server", and Error #423 means "There was an error while hydrating. Because the error happened outside of a Suspense boundary, the entire root will switch to client rendering." Run your app locally in development mode (npm run dev) to see the full diff.
Why does next-themes trigger a className mismatch on <html>?
The server renders HTML with the default or fallback theme because it has no access to client localStorage. When the browser executes the theme script, it immediately sets class="dark" on <html>, which conflicts with the server HTML. Add suppressHydrationWarning to <html lang="en" suppressHydrationWarning> in app/layout.tsx to resolve this issue.
Does this debugger send code or stack traces to a server?
No. The entire diagnostic engine runs 100% locally in your browser using client-side JavaScript regex pattern matching and static analysis. No code, logs, or user data are uploaded or logged.
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