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What is hydration in Next.js and what causes a hydration error?

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01

Understand the problem

Attaching React to server HTML and why mismatches break it.

nextjshydrationssrreact
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Hydration is when React attaches event handlers to the server-rendered HTML on the client. A hydration error happens when the client render differs from the server HTML — e.g. using Date.now(), random values, or browser-only APIs during render — so the trees don't match.

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04

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Avoiding a hydration mismatch
'use client';
import { useState, useEffect } from 'react';

export default function Clock() {
  const [time, setTime] = useState<string | null>(null);
  // ✅ compute browser-only value AFTER mount, not during render
  useEffect(() => setTime(new Date().toLocaleTimeString()), []);
  return <span suppressHydrationWarning>{time ?? 'loading…'}</span>;
}
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