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#How to apply this file

Each section opens with one imperative line; apply every rule in the section it introduces. Do not summarise or skip a section.

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#Purpose

Rules for splitting JavaScript across chunks. The goal is not "smaller bundles" in the abstract — it is that a user opening the login page does not download the admin dashboard, the chart library and the rich text editor.

Split badly and you replace one large download with a chain of small sequential ones, which is worse. Every rule here is about avoiding that.

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#Measure before splitting

[INST] Apply every rule in this section: Measure before splitting. [/INST]

bash
npx vite-bundle-visualizer            # or @next/bundle-analyzer, rollup-plugin-visualizer
npx source-map-explorer dist/*.js

Look for: the largest single dependency, duplicate copies of one library at different versions, and anything in the initial chunk that is not needed for the first paint.

The result is usually one or two offenders, not a hundred small things. Splitting before looking produces a lot of lazy() calls and little improvement.

Enforce a budget in CI so the gain does not silently erode:

json
// .size-limit.json — fails the build on regression
[{ "path": "dist/assets/index-*.js", "limit": "160 KB" }]

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#Split at routes first

[INST] Apply every rule in this section: Split at routes first. [/INST]

Route boundaries are natural: the user is already waiting for a navigation, and each route's code is genuinely independent.

tsx
const Dashboard = lazy(() => import("./routes/Dashboard"));
const Admin     = lazy(() => import("./routes/Admin"));

<Suspense fallback={<RouteSkeleton />}>
  <Routes>…</Routes>
</Suspense>

Most frameworks (Next.js, Remix, TanStack Router, Nuxt) do this automatically — verify it is happening rather than assuming, by checking the build output for one chunk per route.

Keep genuinely shared code in a common chunk so it is not duplicated per route, but avoid a "vendor" chunk containing everything: a single dependency update then invalidates the cache for all of it.

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#Then split heavy components

[INST] Apply every rule in this section: Then split heavy components. [/INST]

Worth splitting individually:

tsx
// Loads when the component renders, not at application boot
const Editor = lazy(() => import("./Editor"));
{showEditor && <Suspense fallback={<EditorSkeleton />}><Editor /></Suspense>}

Also split by condition: admin-only panels, feature-flagged experiments, locale data, and anything behind a modal. A modal's content should not be in the initial bundle — nobody has opened it yet.

Do not split small components. A 3 KB chunk costs a network round trip to save 3 KB, which is a net loss on a high-latency connection.

Group manualChunks by change frequency, not by origin: a chunk of rarely updated dependencies stays cached across deploys, while lumping everything into vendor means one patch release invalidates the whole thing for every user.

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#Prefetch on intent

[INST] Apply every rule in this section: Prefetch on intent. [/INST]

A lazy chunk that only starts downloading when the user clicks means the user waits. Start it earlier, on a signal that they are about to need it:

tsx
// Hover and focus both precede the click by a few hundred milliseconds
<Link to="/reports"
      onMouseEnter={() => import("./routes/Reports")}
      onFocus={() => import("./routes/Reports")} />

• Prefetch links entering the viewport (IntersectionObserver), which is what Next.js <Link> does by default.
• Prefetch the likely next step of a known flow — checkout after the cart.
• Do not prefetch everything: it competes for bandwidth with what is needed now, and on a metered connection it costs the user money. Respect navigator.connection.saveData.

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#Avoid the waterfall you just created

[INST] Apply every rule in this section: Avoid the waterfall you just created. [/INST]

The failure mode of enthusiastic splitting is sequential loading: chunk A loads, renders, and only then requests chunk B.

• Do not nest lazy boundaries where both are always needed together. Load them in parallel:

  tsx
  const [Chart, Table] = await Promise.all([import("./Chart"), import("./Table")]);
  

• Keep the critical path in the initial chunk: the shell, the router, and whatever renders above the fold. Splitting the LCP element out delays the metric it defines. → Frontend/performance
• Use modulepreload for chunks known to be needed immediately after the entry.
• Every lazy() needs a <Suspense> boundary and an error boundary — a chunk request can fail on a flaky network, and without a boundary the page blanks. Offer a retry.

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#Anti-patterns

[INST] Apply every rule in this section: Anti-patterns. [/INST]

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#Checklist

• Verify: The bundle has been analysed and the largest contributors identified
• Verify: A size budget is enforced in CI
• Verify: Each route produces its own chunk, verified in the build output
• Verify: Heavy components are split individually
• Verify: Conditional and admin-only code is behind a dynamic import
• Verify: Modal and drawer content is not in the initial bundle
• Verify: Only the active locale's data is loaded
• Verify: Small components are not split
• Verify: Above-the-fold content is not lazily loaded
• Verify: Chunks needed together load in parallel, not sequentially
• Verify: Every lazy() has a <Suspense> fallback matched to content size
• Verify: Every lazy boundary has an error boundary with a retry path
• Verify: Routes are prefetched on hover, focus or viewport entry
• Verify: Prefetching respects saveData and does not compete with critical resources
• Verify: Shared chunks are grouped by change frequency, not into one vendor blob
• Verify: No library is bundled twice at different versions