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name: audit-bundle-size description: > Analyse and shrink a web app's JavaScript bundle. Use when "reduce bundle size", "code splitting", "slow initial load", or "why is the bundle so big". Runtime → audit-performance. license: MIT effort: high

audit-bundle-size — Find and Eliminate Bundle Bloat

Degree of freedom: MIXED — Offender triage and swap choice [HIGH freedom]; Phase 1 production build + analyser [LOW freedom — run exactly].

Audit-and-fix exception. Measure the payload, then shrink it. Runtime slowness → audit-performance.

Fix only what the measurement named, in the files it named; a repo-wide dependency sweep the report did not list is a different session. Edit surgically — the offending import, dynamic() boundary, or chunk config — rather than rewriting whole files. Pre-existing bugs met on the way go in the report as follow-ups. Add or change tests only where the repo already keeps them for that surface; scratch checks and generated analyser reports (client.html, bundle-report.html) stay out of the commit. An ANALYZE=true-guarded analyser config stays only if the repo already keeps one.

Every kilobyte of JavaScript the browser must download, parse, and compile before showing anything costs real users real time. A large initial bundle is the #1 avoidable cause of slow LCP and poor Core Web Vitals. Find exactly what is bloating it and fix each item.

How to reason — Observe → Interpret → Classify → Severity

1. Observe — quote First Load JS / chunk gzip and the treemap contributor (file + package)
2. Interpret — is this unused code on the critical path, a duplicate, or a missing split?
3. Classify — giant-vendor / full-library import / missing-lazy / unused-dep / dev-in-prod / duplicate
4. Severity — by gzip on the initial route: >250 KB First Load JS = review; do not delete a feature to save KB

Worked example

Observe: Next.js First Load JS 420 KB gzip. Treemap: default lodash import in lib/utils.ts; moment with all locales in the shared layout. Interpret: two libraries pull unused code onto every route. Classify: full-library import + oversized date lib. Severity: High — over the 250 KB review threshold. Finding: named lodash/debounce + replace moment with dayjs; remesure First Load JS before claiming a save.

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Phase 0: Detect bundler and existing setup [HIGH freedom]

package.json scripts.build   → build command and framework
vite.config.*                → Vite + rollupOptions
next.config.*                → Next.js (webpack / Turbopack)
webpack.config.*             → standalone Webpack
astro.config.*               → Astro (islands, Vite underneath)

Identify:

• Bundler: Vite / Webpack / Rollup / esbuild / Turbopack / Next.js built-in
• Analyser available: rollup-plugin-visualizer, webpack-bundle-analyzer, @next/bundle-analyzer, source-map-explorer
• Framework: Next.js (App / Pages), SvelteKit, Astro, Remix, Nuxt, Vite SPA

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Phase 1: Run a production build with analysis [LOW freedom — run exactly]

Vite / Rollup

If rollup-plugin-visualizer is not installed:

npm install --save-dev rollup-plugin-visualizer

Add temporarily to vite.config.ts:

import { visualizer } from 'rollup-plugin-visualizer';
// in plugins array:
visualizer({ open: false, filename: 'dist/bundle-report.html', gzipSize: true })

Then build:

npm run build 2>&1 | tail -40

Read dist/bundle-report.html for the treemap, or parse dist/stats.json if the plugin is configured to output JSON.

Next.js

ANALYZE=true npm run build 2>&1 | tail -60

This requires @next/bundle-analyzer in next.config.*:

const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true',
});
module.exports = withBundleAnalyzer({ /* your config */ });

Read the generated client.html report. Key number: First Load JS per route. Next.js prints this in the build output — capture it for before/after comparison.

If no analyser is installed

Use source-map-explorer on the build output:

npx source-map-explorer 'dist/**/*.js' --html dist/bundle-report.html

Or for a quick size summary:

find dist -name '*.js' | xargs ls -lh | sort -k5 -hr | head -20
find dist -name '*.css' | xargs ls -lh | sort -k5 -hr | head -5

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Phase 2: Parse the results — find the problems [HIGH freedom]

For each chunk or entry point, record:

Chunk	Raw size	Gzip size	Largest contributors
main / page.js	...	...	[dep@version, ...]
vendor	...	...	[dep@version, ...]

Red flags to look for

Problem	Signal	Impact
One giant vendor chunk	Single vendor.js > 200 KB gzip	High — blocks first paint
Duplicate dependency	Same library listed twice (e.g. lodash + lodash-es)	Medium
Full library import	import _ from 'lodash' (imports everything)	High
Missing lazy routes	All routes in one bundle	High
Unused package	Large dep that appears in bundle but only 1–2 exports used	High
Dev-only dep in prod bundle	faker, debug, chalk in client code	Medium
Moment.js	67 KB gzip with all locales	Medium — replace with date-fns or dayjs
Material UI / Ant Design full import	Full icon library loaded	High
react-icons full package	50+ MB raw, huge when not tree-shaken	High

Also check:

• React Compiler on (delete manual memo it makes redundant — smaller bundle, fewer bugs)
• Barrel imports routed through experimental.optimizePackageImports / direct paths (icon libs, lodash, date libs)
• Route prefetch is deliberate — Next.js 16 prefetch is conservative by default; <Link prefetch> only on likely-next routes, and don't double-prefetch what Speculation Rules already cover

Below-fold widgets → dynamic() with a Suspense skeleton; navigation prefetch/prerender → enhance-web-instant-nav.

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Phase 3: Research current alternatives [HIGH freedom]

For the largest offenders, check current alternatives:

firecrawl:firecrawl_search
{
  "query": "replace <package-name> smaller alternative bundle size [current year]",
  "limit": 3,
  "sources": [{ "type": "web" }]
}

Common swaps (research to confirm current state):

• moment → date-fns or dayjs (much smaller, tree-shakeable)
• lodash → native JS or lodash-es with named imports
• Full icon set → per-icon imports or SVG sprites
• axios → native fetch (if browser targets allow)
• Large chart libs → check if a lighter alternative exists for the charts used

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Self-critique before applying [LOW freedom — do not skip]

1. Measured — First Load JS / gzip from a production build, not "looks big"
2. Behavior preserved — dynamic import has a loading state; do not delete a feature to save KB
3. Right owner — runtime slowness without payload evidence → audit-performance
4. Analyzer honest — if the analyser was skipped, say what you used
5. Remeasure after each fix class — no save claimed from inspection alone

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Phase 4: Fix — ordered by size saved [HIGH freedom]

Fix 1: Named imports (tree shaking)

// Before — pulls in the whole library
import _ from 'lodash';
import { BellIcon } from '@heroicons/react/24/solid'; // only needs Bell

// After — only the export you need
import debounce from 'lodash/debounce';
import { BellIcon } from '@heroicons/react/24/solid'; // already correct for heroicons v2

For icon libraries that do not tree-shake well, use per-icon deep imports or an SVG sprite sheet.

Fix 2: Dynamic imports for routes (code splitting)

Next.js App Router — code-splits by route automatically. If you have heavy components inside a page, split them:

import dynamic from 'next/dynamic';
const HeavyChart = dynamic(() => import('../components/HeavyChart'), {
  loading: () => <ChartSkeleton />,
  ssr: false, // if it uses browser-only APIs
});

Vite / React Router:

const LazyPage = React.lazy(() => import('./pages/LazyPage'));
// Wrap in <Suspense fallback={<PageSkeleton />}>

SvelteKit — routes are code-split by default. For heavy components:

{#await import('./HeavyComponent.svelte') then { default: Component }}
  <Component />
{/await}

Fix 3: Replace or remove over-sized dependencies

Follow the research from Phase 3. For any replacement:

1. Read the new library's docs via Context7 or Firecrawl before touching code.
2. Make the swap in one file, run npm run build, compare sizes before merging everywhere.

Fix 4: Bundle splitting configuration

Vite — split large deps into their own cacheable chunks:

build: {
  rollupOptions: {
    output: {
      manualChunks: {
        'react-vendor': ['react', 'react-dom'],
        'router': ['react-router-dom'],
        'charts': ['recharts'], // or whatever charting lib
      },
    },
  },
},

Next.js — configure splitChunks in next.config.* only if the default chunking is creating sub-optimal splits. Prefer dynamic() over manual config.

Fix 5: Remove dev-only packages from the client bundle

Any package that should only run server-side or in tests must not be imported from client-side code. Move to devDependencies and verify it disappears from the bundle after rebuild.

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Phase 5: Measure the improvement [LOW freedom — run exactly]

After fixes, rebuild and re-run Phase 1:

Before: First Load JS = X KB gzip
After:  First Load JS = Y KB gzip
Saved:  Z KB (N%)

For each route (Next.js build output shows this), confirm the numbers improved. Run a quick Playwright check to confirm the app still works:

PW="npx --yes @playwright/cli@latest"
$PW -s=bundle-check open --headed "<app-url>"    # then `goto` each key route
$PW -s=bundle-check snapshot                      # no blank screen or error
$PW -s=bundle-check console                       # no new errors

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Quick-reference: size targets

Asset	Target	Review if
Initial JS (gzip)	< 100 KB	> 250 KB
Route chunk (gzip)	< 50 KB	> 150 KB
CSS total (gzip)	< 20 KB	> 50 KB
Largest single image	< 200 KB	> 500 KB
Total page weight	< 500 KB	> 1.5 MB