Page loaded at: {new Date().toLocaleTimeString()}
; // Server renders "10:00:00 AM", client renders "10:00:01 AM" — mismatch } // WRONG — conditional on window export function ThemeIcon() { const isDark = typeof window !== "undefined" && window.matchMedia("(prefers-color-scheme: dark)").matches; return {isDark ? "🌙" : "☀️"}; // Server: always renders "☀️"; client: may render "🌙" — mismatch } ``` La console du navigateur affiche : ``` Error: Hydration failed because the initial UI does not match what was rendered on the server. ``` ## Pourquoi cela se produit L'agent écrit des composants qui semblent corrects isolément. Il ne simule pas deux passes de rendu séparées (SSR + hydratation client) et ne sait pas que toute valeur différente entre les deux brisera l'hydratation. ## Comment le repérer - `Date.now()`, `new Date()`, `Math.random()` appelés directement dans JSX ou lors du rendu sans être dans un initialiseur `useEffect` ou `useState`. - Les gardes `typeof window !== "undefined"` dans le retour du rendu — le serveur prend toujours la branche `false`. - Lecture de `localStorage`, `sessionStorage`, ou `navigator` pendant le rendu. - Balises HTML ou valeurs d'attributs différentes entre la sortie serveur et client (visible dans l'avertissement d'hydratation de React DevTools). ## Comment le corriger Reportez les valeurs propres au navigateur après le montage, ou générez des valeurs stables côté serveur et transmettez-les en tant que props. ```tsx // CORRECT — defer client-only value to after mount "use client"; import { useState, useEffect } from "react"; export function Timestamp() { const [time, setTime] = useStateLoading time…
; // matches server output returnPage loaded at: {time}
; } ``` ```tsx // CORRECT — theme icon: use CSS / data attribute, avoid JS render branch // Set data-theme on in a blocking inline script (not React) // Then use CSS: [data-theme="dark"] .icon { content: "🌙" } // Or use Next.js next-themes which handles SSR safely: import { useTheme } from "next-themes"; import { useEffect, useState } from "react"; export function ThemeIcon() { const { resolvedTheme } = useTheme(); const [mounted, setMounted] = useState(false); useEffect(() => setMounted(true), []); if (!mounted) return ; // stable placeholder return {resolvedTheme === "dark" ? "🌙" : "☀️"}; } ``` ```txt [ ] No Date.now() / Math.random() / new Date() called directly during render [ ] No typeof window / localStorage / navigator accessed during render [ ] Browser-only state initializes to null/undefined, set in useEffect [ ] Render a stable placeholder (same as server output) until after mount [ ] Use suppressHydrationWarning only on elements where the mismatch is intentional (e.g. timestamp) [ ] Run "next build && next start" and check browser console for hydration errors ``` ## Prompt de correction ```txt title="Fix Prompt" This component causes a React hydration mismatch because it reads browser-only values (Date, window, localStorage, Math.random) during render. Fix it: move all browser-only reads into a useEffect that sets state after mount, render a stable placeholder on first render that matches what the server produces, and never conditionally return different JSX trees based on typeof window. Explain why each change prevents the mismatch. ``` ## Test ```bash # Build and start, then check for hydration errors with curl diff next build 2>&1 | grep -i "hydrat\|mismatch" && echo "FAIL: hydration error in build" || echo "Build OK" # Runtime: open browser console after next start and look for hydration warnings next start & sleep 3 curl -s http://localhost:3000 | grep -i "data-nextjs-scroll-focus-boundary" > /dev/null && echo "Server rendered OK" ```