Next.js App Router relies heavily on React Server Components (RSC). When you render a timestamp or use window.localStorage during SSR, the server output will differ from the client output. React immediately throws a hydration error.
Additionally, poor dependency management inside useEffect is the leading cause of browser-freezing infinite loops.
Quick Fix Summary
- Hydration error: Use a custom useIsClient hook to delay rendering until the component mounts.
- Infinite loops: Ensure dependency arrays inside useEffect are correct and memoize object references.
- Prop drilling: Refactor to Context API or Redux.
Root Causes
Next.js Hydration Mismatch
The HTML generated on the server (like a Date timestamp) does not match what the client browser initially renders.
<div>{new Date().getTime()}</div>useEffect Infinite Loops
Objects and arrays passed as dependencies are compared by reference, not value, triggering constant re-renders.
Step-by-Step Fix Guide
Create a useIsClient Hook
Force the component to only render on the client side.
import { useState, useEffect } from 'react';
export function useIsClient() {
const [isClient, setIsClient] = useState(false);
useEffect(() => setIsClient(true), []);
return isClient;
}Implement useIsClient in your Component
Use the hook to conditionally render browser-only data.
export default function ThemeToggle() {
const isClient = useIsClient();
if (!isClient) return null;
return <button>Toggle Theme</button>;
}Stuck in an infinite loop?
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Get React HelpRelated Errors
Cannot read properties of undefined (reading 'map')
Ensure the array exists before mapping: array?.map(...) or array && array.map(...)
Prevention Strategy
- Always use optional chaining (?.) when fetching data.
- Never pass inline objects or arrays to a useEffect dependency array.
- Use suppressHydrationWarning only for simple text nodes.