SSR & Hydration

This guide covers server-side rendering (SSR) with Better i18n and TanStack Start, ensuring consistent rendering between server and client.

How SSR Works

1. Request arrives at server
2. Middleware detects locale (URL, cookie, header)
3. Loader fetches messages from CDN
4. React renders HTML with translations
5. HTML sent to client
6. Client hydrates with same messages
7. No flash of untranslated content!

Loading Messages on Server

Use getMessages in your route loaders:

app/routes/__root.tsx

import { getMessages } from "@better-i18n/use-intl/server"

export const Route = createRootRouteWithContext<{ locale: string }>()({
  loader: async ({ context }) => { 
    const messages = await getMessages({ 
      project: "org/project", 
      locale: context.locale || "en", 
    }) 

    return { messages, locale: context.locale } 
  },

  component: RootComponent,
})

Providing Messages to Client

Pass messages to the provider to avoid client-side fetching:

function RootComponent() {
  const { messages, locale } = Route.useLoaderData()

  return (
    <BetterI18nProvider
      project="org/project"
      locale={locale} 
      messages={messages}  // Pre-loaded from server
      timeZone="UTC"
    > // [!code ++]
      <Outlet />
    </BetterI18nProvider> 
  )
}

When messages is provided, the provider skips client-side CDN fetching entirely.

Timezone Configuration

use-intl v4+ requires a timezone during SSR to prevent hydration mismatches:

<BetterI18nProvider
  project="org/project"
  locale={locale}
  messages={messages}
  timeZone="UTC"  // Required for SSR
>
  {children}
</BetterI18nProvider>

Why Timezone Matters

Date formatting can differ between server and client:

// Server (UTC): "January 18, 2026"
// Client (PST): "January 17, 2026"  ← Hydration mismatch!

By setting a consistent timeZone, both render identically.

User Timezone

To use the user's timezone, detect it on the client and update:

function RootComponent() {
  const { messages, locale } = Route.useLoaderData()
  const [timeZone, setTimeZone] = useState("UTC")

  useEffect(() => {
    setTimeZone(Intl.DateTimeFormat().resolvedOptions().timeZone)
  }, [])

  return (
    <BetterI18nProvider
      project="org/project"
      locale={locale}
      messages={messages}
      timeZone={timeZone}
    >
      <Outlet />
    </BetterI18nProvider>
  )
}

Route-Level Messages

For route-specific namespaces, you can load additional messages:

app/routes/dashboard.tsx

export const Route = createFileRoute("/dashboard")({
  loader: async ({ context }) => {
    // Dashboard-specific translations
    const dashboardMessages = await getMessages({
      project: "org/project",
      locale: context.locale,
      namespace: "dashboard",  // Load only dashboard namespace
    })

    return { dashboardMessages }
  },
})

Server-Side Translation

Use translations in loaders for metadata:

app/routes/about.tsx

import { getMessages, createServerTranslator } from "@better-i18n/use-intl/server"

export const Route = createFileRoute("/about")({
  loader: async ({ context }) => {
    const messages = await getMessages({
      project: "org/project",
      locale: context.locale,
    })

    const t = createServerTranslator({
      locale: context.locale,
      messages,
      namespace: "about",
    })

    return {
      messages,
      meta: {
        title: t("meta.title"),
        description: t("meta.description"),
      },
    }
  },

  head: ({ loaderData }) => ({
    meta: [
      { title: loaderData.meta.title },
      { name: "description", content: loaderData.meta.description },
    ],
  }),

  component: AboutPage,
})

Caching Considerations

Messages are cached at multiple levels:

Level	Duration	Invalidation
CDN	1 hour	On publish
Server	Request	Per request
Client	Session	On navigation

For production, consider adding server-side caching:

const messageCache = new Map<string, Messages>()

async function getCachedMessages(locale: string) {
  if (!messageCache.has(locale)) {
    messageCache.set(locale, await getMessages({
      project: "org/project",
      locale,
    }))
  }
  return messageCache.get(locale)!
}

Debug Mode

Enable debug logging to troubleshoot SSR issues:

const messages = await getMessages({
  project: "org/project",
  locale: context.locale,
  debug: true,  // Logs CDN requests
})

Common Issues

Hydration Mismatch

Symptom: Console warning about hydration mismatch.

Solution: Ensure timeZone is set consistently:

<BetterI18nProvider timeZone="UTC" ... />

Flash of English

Symptom: Page briefly shows English before correct locale.

Solution: Pre-load messages in the loader:

loader: async ({ context }) => {
  const messages = await getMessages({ ... })
  return { messages }  // Pass to provider
}

Wrong Locale on First Load

Symptom: First page load shows wrong locale.

Solution: Ensure middleware sets locale in context:

// middleware/i18n.ts
export const i18nMiddleware = createBetterI18nMiddleware({
  project: "org/project",
  defaultLocale: "en",
})

Middleware - Locale detection
Routing - Path-based locales
getMessages - Server utility

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