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next.jsreact-19dark-modezustandperformance

Next.js Dark Mode uden blink eller React 19-advarsler

Sådan erstattes next-themes med Zustand + useServerInsertedHTML til flimmerfri mørk tilstand i Next.js 15+.

Udgivet 11. april 20268 min læsning

TL;DR

next-themes renderer et <script>-tag inde i en React Client Component for at forhindre temaflimmer (FOUC). React 19 advarer om dette. Biblioteket er ikke opdateret siden marts 2025. Løsning: Zustand-store + useServerInsertedHTML. Nul afhængigheder, nul FOUC, nul advarsler.

Problemet

Hvis du bruger next-themes med Next.js 15+ og React 19, får du denne konsollfejl ved hver sideindlæsning:

Encountered a script tag while rendering React component.
Scripts inside React components are never executed when rendering on the client.
Consider using template tag instead.

src/providers/theme-provider.tsx (7:10) @ ThemeProvider

Ingen hydreringsfejl. React 19 advarer om, at <script>-tags i klientkomponenter aldrig vil blive udført. Scriptet virker i SSR, men React markerer det.

Hvorfor det sker

next-themes sætter temaklasse inden hydrering ved at injicere et inline-script via React.createElement:

next-themes internals (minified)
// next-themes renders a <script> inside a Client Component
return React.createElement(Provider, { value },
  React.createElement("script", {
    suppressHydrationWarning: true,
    dangerouslySetInnerHTML: { __html: `(...theme init code...)` }
  }),
  children
)

React 19 ændrede adfærden — script-tags i komponenter markeres nu. suppressHydrationWarning hjælper ikke her.

Hvad vi prøvede (og hvorfor det mislykkedes)

Vi prøvede systematisk hver tilgang:

AttemptResult
suppressHydrationWarning on <html>Suppresses hydration mismatch, but NOT the script tag warning
Delay mount with useState + useEffectWarning disappears, but causes FOUC (theme flash)
Raw <script> in layout <head>React 19 catches it — same warning even in Server Components
next/script with beforeInteractiveStill rendered inside React tree — same warning
Remove <head>, put Script in bodySame warning — it's inside <html> which is React-managed
next-themes@1.0.0-beta.0Beta, no release date, unclear if fixed

Løsningen: Zustand + useServerInsertedHTML

useServerInsertedHTML injicerer HTML i SSR-strømmen uden for React-træet. Scriptet er i HTML, men React "ser" det ikke på klienten. Kombineret med Zustand til reaktiv tematilstand — komplet erstatning, nul afhængigheder.

Sådan fungerer det

SSR (server):
  useServerInsertedHTML → injects <script> into HTML stream
  ↓
  Browser receives HTML with theme script already in it
  ↓
  Script runs BEFORE React hydrates → correct class on <html>
  ↓
  No flash. No mismatch. No warning.

Client (after hydration):
  useEffect → _init() → Zustand store syncs with localStorage
  ↓
  useTheme() → reactive theme state for components

Trin 1: Zustand-store

Storen håndterer tematilstand, DOM-klasser, systemdetektering og synkronisering på tværs af faner. _init() returnerer en oprydningsfunktion til useEffect:

store/use-theme-store.ts
import { create } from "zustand";

const STORAGE_KEY = "theme";
const MEDIA_QUERY = "(prefers-color-scheme: dark)";

function getSystemTheme(): "light" | "dark" {
  return window.matchMedia(MEDIA_QUERY).matches ? "dark" : "light";
}

function applyTheme(resolved: string, disableTransition: boolean) {
  const d = document.documentElement;

  if (disableTransition) {
    const style = document.createElement("style");
    style.appendChild(
      document.createTextNode(
        "*,*::before,*::after{transition:none!important}"
      )
    );
    document.head.appendChild(style);
    window.getComputedStyle(document.body);
    setTimeout(() => document.head.removeChild(style), 1);
  }

  d.classList.remove("light", "dark");
  d.classList.add(resolved);
  d.style.colorScheme = resolved;
}

interface ThemeState {
  theme: string;
  systemTheme: "light" | "dark";
  resolvedTheme: string;
  setTheme: (theme: string) => void;
  _init: (disableTransition: boolean) => () => void;
}

export const useThemeStore = create<ThemeState>((set, get) => ({
  theme: "system",
  systemTheme: "light",
  resolvedTheme: "light",

  setTheme: (newTheme) => {
    const { systemTheme, disableTransitionOnChange } = get();
    const resolved = newTheme === "system" ? systemTheme : newTheme;
    localStorage.setItem(STORAGE_KEY, newTheme);
    applyTheme(resolved, disableTransitionOnChange);
    set({ theme: newTheme, resolvedTheme: resolved });
  },

  _init: (disableTransition) => {
    const stored = localStorage.getItem(STORAGE_KEY) || "system";
    const system = getSystemTheme();
    const resolved = stored === "system" ? system : stored;

    set({ theme: stored, systemTheme: system, resolvedTheme: resolved });
    applyTheme(resolved, disableTransition);

    // System theme changes
    const mq = window.matchMedia(MEDIA_QUERY);
    const onChange = (e: MediaQueryListEvent) => {
      const newSystem = e.matches ? "dark" : "light";
      const { theme } = get();
      set({ systemTheme: newSystem,
            resolvedTheme: theme === "system" ? newSystem : theme });
      if (theme === "system") applyTheme(newSystem, disableTransition);
    };
    mq.addEventListener("change", onChange);

    // Cross-tab sync
    const onStorage = (e: StorageEvent) => {
      if (e.key !== STORAGE_KEY || !e.newValue) return;
      const system = getSystemTheme();
      const resolved = e.newValue === "system" ? system : e.newValue;
      set({ theme: e.newValue, resolvedTheme: resolved });
      applyTheme(resolved, disableTransition);
    };
    window.addEventListener("storage", onStorage);

    return () => {
      mq.removeEventListener("change", onChange);
      window.removeEventListener("storage", onStorage);
    };
  },
}));

export function useTheme() {
  return useThemeStore((s) => ({
    theme: s.theme,
    setTheme: s.setTheme,
    resolvedTheme: s.resolvedTheme,
    systemTheme: s.systemTheme,
  }));
}

Trin 2: ThemeProvider

Provideren injicerer FOUC-forebyggende script via useServerInsertedHTML og initialiserer Zustand-storen ved montering:

providers/theme-provider.tsx
"use client"

import { useEffect } from "react"
import { useServerInsertedHTML } from "next/navigation"
import { useThemeStore } from "@/store/use-theme-store"

const THEME_INIT_SCRIPT = `(function(){
  try {
    var t = localStorage.getItem("theme") || "system";
    var r = t;
    if (t === "system") {
      r = window.matchMedia("(prefers-color-scheme: dark)").matches
        ? "dark" : "light";
    }
    document.documentElement.classList.remove("light", "dark");
    document.documentElement.classList.add(r);
    document.documentElement.style.colorScheme = r;
  } catch(e) {}
})()`

export function ThemeProvider({
  children,
  disableTransitionOnChange = false,
}) {
  // Injects script into SSR HTML outside the React tree
  // — prevents FOUC without triggering React 19 warning
  useServerInsertedHTML(() => (
    <script dangerouslySetInnerHTML={{ __html: THEME_INIT_SCRIPT }} />
  ))

  useEffect(() => {
    return useThemeStore.getState()._init(disableTransitionOnChange)
  }, [disableTransitionOnChange])

  return <>{children}</>
}

Trin 3: Layout

app/layout.tsx
import { ThemeProvider } from "@/providers/theme-provider"

export default function RootLayout({ children }) {
  return (
    <html lang="en" suppressHydrationWarning>
      <body>
        <ThemeProvider disableTransitionOnChange>
          {children}
        </ThemeProvider>
      </body>
    </html>
  )
}

Trin 4: Brug det

components/theme-switch.tsx
import { useTheme } from "@/store/use-theme-store"

export function ThemeSwitch() {
  const { theme, setTheme } = useTheme();

  return (
    <button onClick={() => setTheme(
      theme === "dark" ? "light" : "dark"
    )}>
      {theme === "dark" ? "☀️" : "🌙"}
    </button>
  );
}

Migration fra next-themes

API'et er bevidst identisk. Migration er én enkelt importændring per fil:

migration
- import { useTheme } from "next-themes"
+ import { useTheme } from "@/store/use-theme-store"

  // API is identical — no other changes needed
  const { theme, setTheme, resolvedTheme, systemTheme } = useTheme()

Sammenligning

next-themesZustand + useServerInsertedHTML
React 19 warningYes — <script> in client componentNo
FOUC preventionYesYes
Cross-tab syncYesYes
System theme detectionYesYes
disableTransitionOnChangeYesYes
Bundle size~3.5 KB~1.5 KB (uses existing Zustand)
Dependencies+1 (next-themes)0 (Zustand already in project)
Maintenance riskAbandoned since March 2025Your own code

Hvorfor ikke andre alternativer?

@wrksz/themes

Drop-in-erstatning der bruger useServerInsertedHTML. Virker, men endnu en afhængighed med én vedligeholder. Hvis next-themes lærte os noget — afhængigheder bliver opgivet. Med ~100 linjer kode ejer du hele løsningen.

next-themes@1.0.0-beta.0

Findes på npm, ingen udgivelsesdato, ingen ændringslog, uklart om React 19-advarslen er rettet. Ikke værd at satse produktionskode på.

Kun CSS (prefers-color-scheme)

Virker til systemdetektering, kan ikke håndtere localStorage-persistens, manuel skift eller "system"-muligheden. Kræver JavaScript.

Konklusioner

  1. next-themes er reelt opgivet — seneste udgivelse marts 2025, React 19-advarsel urettet
  2. useServerInsertedHTML er den korrekte Next.js-primitiv til scriptinjektion uden React-advarsler
  3. Zustand giver reaktiv tematilstand med mindre kode end en Context-provider
  4. Hele løsningen ~100 linjer, nul nye afhængigheder, du ejer hver linje

Kilder

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