ECC/skills/nuxt4-patterns/SKILL.md

name, description, metadata

name	description	metadata
nuxt4-patterns	Nuxt 4 app patterns for hydration safety, performance, route rules, lazy loading, and SSR-safe data fetching with useFetch and useAsyncData. Use when building or reviewing a Nuxt 4 app, or debugging hydration mismatches and SSR-safe data fetching.	origin ECC

Nuxt 4 Patterns

Use when building or debugging Nuxt 4 apps with SSR, hybrid rendering, route rules, or page-level data fetching.

When to Activate

• Hydration mismatches between server HTML and client state
• Route-level rendering decisions such as prerender, SWR, ISR, or client-only sections
• Performance work around lazy loading, lazy hydration, or payload size
• Page or component data fetching with useFetch, useAsyncData, or $fetch
• Nuxt routing issues tied to route params, middleware, or SSR/client differences

Hydration Safety

• Keep the first render deterministic. Do not put Date.now(), Math.random(), browser-only APIs, or storage reads directly into SSR-rendered template state.
• Move browser-only logic behind onMounted(), import.meta.client, ClientOnly, or a .client.vue component when the server cannot produce the same markup.
• Use Nuxt's useRoute() composable, not the one from vue-router.
• Do not use route.fullPath to drive SSR-rendered markup. URL fragments are client-only, which can create hydration mismatches.
• Treat ssr: false as an escape hatch for truly browser-only areas, not a default fix for mismatches.

Data Fetching

• Prefer await useFetch() for SSR-safe API reads in pages and components. It forwards server-fetched data into the Nuxt payload and avoids a second fetch on hydration.
• Use useAsyncData() when the fetcher is not a simple $fetch() call, when you need a custom key, or when you are composing multiple async sources.
• Give useAsyncData() a stable key for cache reuse and predictable refresh behavior.
• Keep useAsyncData() handlers side-effect free. They can run during SSR and hydration.
• Use $fetch() for user-triggered writes or client-only actions, not top-level page data that should be hydrated from SSR.
• Use lazy: true, useLazyFetch(), or useLazyAsyncData() for non-critical data that should not block navigation. Handle status === 'pending' in the UI.
• Use server: false only for data that is not needed for SEO or the first paint.
• Trim payload size with pick and prefer shallower payloads when deep reactivity is unnecessary.

const route = useRoute()

const { data: article, status, error, refresh } = await useAsyncData(
  () => `article:${route.params.slug}`,
  () => $fetch(`/api/articles/${route.params.slug}`),
)

const { data: comments } = await useFetch(`/api/articles/${route.params.slug}/comments`, {
  lazy: true,
  server: false,
})

Route Rules

Prefer routeRules in nuxt.config.ts for rendering and caching strategy:

export default defineNuxtConfig({
  routeRules: {
    '/': { prerender: true },
    '/products/**': { swr: 3600 },
    '/blog/**': { isr: true },
    '/admin/**': { ssr: false },
    '/api/**': { cache: { maxAge: 60 * 60 } },
  },
})

• prerender: static HTML at build time
• swr: serve cached content and revalidate in the background
• isr: incremental static regeneration on supported platforms
• ssr: false: client-rendered route
• cache or redirect: Nitro-level response behavior

Pick route rules per route group, not globally. Marketing pages, catalogs, dashboards, and APIs usually need different strategies.

Lazy Loading and Performance

• Nuxt already code-splits pages by route. Keep route boundaries meaningful before micro-optimizing component splits.
• Use the Lazy prefix to dynamically import non-critical components.
• Conditionally render lazy components with v-if so the chunk is not loaded until the UI actually needs it.
• Use lazy hydration for below-the-fold or non-critical interactive UI.

<template>
  <LazyRecommendations v-if="showRecommendations" />
  <LazyProductGallery hydrate-on-visible />
</template>

• For custom strategies, use defineLazyHydrationComponent() with a visibility or idle strategy.
• Nuxt lazy hydration works on single-file components. Passing new props to a lazily hydrated component will trigger hydration immediately.
• Use NuxtLink for internal navigation so Nuxt can prefetch route components and generated payloads.

Review Checklist

• First SSR render and hydrated client render produce the same markup
• Page data uses useFetch or useAsyncData, not top-level $fetch
• Non-critical data is lazy and has explicit loading UI
• Route rules match the page's SEO and freshness requirements
• Heavy interactive islands are lazy-loaded or lazily hydrated