fix: replace typeof window with typeof document for Node.js 25+ SSR compatibility

[EduardF1]

Problem

Node.js 25+ ships an experimental Web Storage API that aliases window to globalThis. This breaks next-themes on Node.js 25+ because the SSR guard evaluates at module load time:

const isServer = typeof window === 'undefined'
// Node.js 25: typeof window → "object" (not "undefined") ❌

This causes:

• isServer = false on the server
• getTheme() calls localStorage.getItem() on the server
• ThemeScript nonce rendered empty during SSR
• TypeError: localStorage.getItem is not a function on startup

Reported in #389.

Fix

Replace typeof window === 'undefined' with typeof document === 'undefined'.

document is a browser-only API — Node.js does not expose it, even in v25+. This is a reliable cross-version SSR guard:

const isServer = typeof document === 'undefined'
// Node.js 25: typeof document → "undefined" ✅
// Browser: typeof document → "object" ✅

Changes

• next-themes/src/index.tsx — 2 occurrences updated

Testing

# Existing build still passes
pnpm build

# On Node.js 25: isServer correctly evaluates to true
node -e "console.log(typeof document === 'undefined')"  # true ✅
node -e "console.log(typeof window === 'undefined')"    # false ❌ (Node 25+)

Tested on Node.js v24.15.0, pnpm v10.32.1.

[EduardF1]

Summary

Swaps typeof window for typeof document in the SSR guard, restoring Node.js 25+ compatibility where window is exposed on globalThis but document remains undefined in SSR contexts — one-line fix, no API surface change.

Status

• CI: ✅ Both Playwright and unit tests passing
• No unresolved review threads
• Ready for review

@pacocoursey happy to add more tests or adjust anything if needed. Thanks!

[EduardF1]

Friendly bump on this fix for Node.js 25+ compatibility. CI is green and it's ready whenever you have time. Thank you!

[EduardF1]

Hi! Just checking in on this — happy to address any feedback. Thanks!

[EduardF1]

Pushed a deterministic regression test (__tests__/ssr-node25.test.tsx, node vitest environment) that recreates the Node.js 25 window→globalThis alias with a throwing localStorage.getItem. It asserts ThemeProvider renders to static markup without throwing and never reads localStorage on the server. The test fails on the previous typeof window guard and passes with the typeof document guard, locking in the fix against regressions. Full suite: 31 passed.