grafana/plugin-bundle-size

Grafana plugin bundle size optimisation

module.js is the render-blocking entry point for every Grafana app plugin. The smaller it is, the less impact the plugin has on Grafana's overall startup time. A well-split plugin should have a module.js under ~200 KB that contains nothing but lazy-loaded wrappers — all feature code loads on demand.

Target: ~15–25 JS chunks total. Fewer means too little splitting; far more (50+) means over-engineering.

Risk levels

Not all splitting opportunities carry the same risk. Apply them in this order:

| Level | What | Risk | Impact | | ---------------- | -------------------------------------------------------------------- | ---------------------------------- | ---------------------------------------- | | Safe | module.tsx lazy wrappers (Priority 1) | Very low — no behaviour change | Highest — module.js drops 90%+ | | Safe | Route-level lazy() (Priority 2) | Low — each route is self-contained | High — one chunk per route | | Safe | Extension lazy() (Priority 3) | Low — extensions are isolated | Medium — independent chunk per extension | | Moderate | Component registries / tab panels (Priority 4) | Medium — verify Suspense placement | Medium — splits heavy pages further | | Do not touch | Vendor libraries (@grafana/scenes, @reduxjs/toolkit) | N/A | N/A — webpack splits these automatically | | Do not touch | Shared utility components (Markdown, Spinner) used across many files | High churn, many callsites | Low — already in shared vendor chunks |

When in doubt, stop after Priority 2. Routes alone typically reduce module.js by 95%+.

---

Step 1: Add bundle size CI reporting (recommended)

Add the grafana/plugin-actions/bundle-size action to get automatic bundle size comparison comments on every PR. This posts a table showing entry point size changes, file count diffs, and total bundle impact — making regressions visible before merge.

Root-level plugins (plugin at repo root):

# .github/workflows/bundle-size.yml
name: Bundle Size
on:
  pull_request:
  push:
    branches: [main]
  workflow_dispatch:

jobs:
  bundle-size:
    runs-on: ubuntu-x64-large
    permissions:
      contents: write
      id-token: write
      pull-requests: write
      actions: read
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version-file: .nvmrc
      - name: Install and build
        run: yarn install
      - name: Bundle Size
        uses: grafana/plugin-actions/bundle-size@a66a1c96cdbb176f9cccf10cf23593e250db7cce # bundle-size/v1.1.0

Subdirectory plugins (e.g. plugin/ in a monorepo):

The action's install step runs at the repo root and cannot find yarn.lock in a subdirectory. Work around this by installing deps yourself and symlinking to root:

# .github/workflows/bundle-size.yml
name: Bundle Size
on:
  pull_request:
    paths: ['plugin/**']
  push:
    branches: [main]
    paths: ['plugin/**']
  workflow_dispatch:

jobs:
  bundle-size:
    runs-on: ubuntu-x64-large
    permissions:
      contents: write
      id-token: write
      pull-requests: write
      actions: read
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version-file: ./plugin/.nvmrc

      # Auth for private npm registries (if needed)
      # - uses: google-github-actions/auth@v3 ...
      # - run: npx google-artifactregistry-auth ...

      - name: Install dependencies
        working-directory: ./plugin
        run: yarn install

      - name: Symlink plugin to root for bundle-size action
        run: |
          ln -s plugin/yarn.lock yarn.lock
          ln -s plugin/package.json package.json
          ln -s plugin/.yarnrc.yml .yarnrc.yml
          ln -s plugin/node_modules node_modules

      - name: Bundle Size
        uses: grafana/plugin-actions/bundle-size@a66a1c96cdbb176f9cccf10cf23593e250db7cce # bundle-size/v1.1.0
        with:
          working-directory: ./plugin

How it works:

• On push to main: builds and uploads a main-branch-stats artifact as baseline
• On PRs: builds the PR, downloads the baseline, compares, and posts a comment
• First run on main generates the baseline — use workflow_dispatch to trigger manually after adding the workflow
• The threshold input (default 5) controls whether the comment is posted (only when entry point diff exceeds N%)

Reference: grafana-k8s-plugin workflow (root-level), cloud-onboarding #10388 (subdirectory)

---

Step 2: Detect plugin context

# Confirm this is an app plugin (type: "app" — datasource/panel plugins have different needs)
jq -r '"\(.id) — \(.type)"' src/plugin.json

# Locate the entry point
ls src/module.ts src/module.tsx 2>/dev/null

# Measure the current PRODUCTION bundle size BEFORE making any changes
# Dev builds are unminified and much larger — always measure production
yarn build 2>/dev/null || npm run build
echo "=== module.js ==="
ls -lah dist/module.js
echo "=== all JS chunks ==="
ls -lah dist/*.js | sort -k5 -rh | head -20
echo "=== chunk count ==="
ls dist/*.js | wc -l

Record the baseline. A pre-split plugin commonly has a module.js of 1–3 MB with no other JS chunks.

---

Step 3: Check and update create-plugin

The @grafana/create-plugin tool controls .config/webpack/, .config/jest/, and other build scaffolding. Updating it often unlocks faster SWC compilation and better chunk output.

# Check current version
cat .config/.cprc.json 2>/dev/null || grep '"@grafana/create-plugin"' package.json

# Get the latest version
npm view @grafana/create-plugin version

# Update if outdated
npx @grafana/create-plugin@latest update

After updating, review the diff (especially .config/webpack/webpack.config.ts) and run a test build before proceeding. If the update introduces breaking changes, fix them first.

Custom webpack configs: If the plugin has a top-level webpack.config.ts that webpack-merges the scaffolded base config, review the merge carefully after updating — custom rules or plugins may conflict with new scaffolding.

---

Step 4: Analyse the codebase — find what to split

Read these files in order of impact. Do not start implementing until you have read all of them.

Entry point:

cat src/module.ts 2>/dev/null || cat src/module.tsx

Look for: direct (non-lazy) imports of App, ConfigPage, and any exposeComponent / addComponent targets.

Root App component:

# Common locations
cat src/App.tsx src/components/App.tsx src/feature/app/components/App.tsx 2>/dev/null | head -80

Look for: direct imports of page/route components that should be lazy-loaded.

Extension registrations:

grep -r "exposeComponent\|addComponent\|addLink" src/ --include="*.ts" --include="*.tsx" -n

Every component registered here is loaded by other Grafana apps — each should be an independent chunk.

Component registries:

# Arrays of objects that contain React components
grep -rn "component:" src/ --include="*.ts" --include="*.tsx" | grep -v "node_modules" | head -20

Exported side-effect singletons:

# Values initialised and exported at module level (e.g. Faro, analytics clients)
grep -n "^export const\|^export let" src/module.ts src/module.tsx 2>/dev/null
# Find all files that import from module.ts — these create circular deps after splitting
grep -rn "from '.*module'" src/ --include="*.ts" --include="*.tsx" | grep -v node_modules

Look for: export const faro = initializeFaro() or similar. These must be extracted to a dedicated file before lazy-loading (see Step 4 Priority 1 note on singletons).

Heavy synchronous imports:

# Libraries that are large and only needed in specific pages
grep -rn "from 'monaco-editor\|@codemirror\|d3\b\|recharts\|chart\.js" \
  src/ --include="*.ts" --include="*.tsx" | grep -v node_modules

Now prioritise. A good rule: if a file is imported by module.ts directly (even transitively), it ends up in module.js. Everything reachable from the lazy boundary is its own chunk.

---

Step 5: Implement splits — in priority order

Named vs default exports: React.lazy() requires the target module to have a default export. Most Grafana plugin components use named exports — these need a .then() re-map:

// Named export (e.g. export function MyComponent)
const LazyMyComp = lazy(() => import('./MyComponent').then((m) => ({ default: m.MyComponent })));

// Default export — works directly
const LazyMyComp = lazy(() => import('./MyComponent'));

When creating new extension or page files, prefer export default so the lazy() call stays clean. For existing files with named exports, use .then().

Priority 1: module.tsx (highest impact, always do this first)

If the entry point is module.ts, rename it:

git mv src/module.ts src/module.tsx

Make module.tsx import nothing from feature code except through lazy():

// src/module.tsx
import React, { lazy, Suspense } from 'react';
import { AppPlugin, AppRootProps } from '@grafana/data';
import { LoadingPlaceholder } from '@grafana/ui';

// Use `import type` for prop interfaces — this is erased at compile time
// and does NOT pull the component module into the eager bundle
import type { MyExtensionProps } from './extensions/MyExtension';
import type { JsonData } from './features/app/state/slice';

// ── Faro (lazy init — keeps @grafana/faro-react out of module.js) ────────────
let faroInitialized = false;
async function initFaro() {
  if (faroInitialized) {
    return;
  }
  faroInitialized = true;
  const { initializeFaro } = await import('faro');
  initializeFaro();
}

// ── Root page ─────────────────────────────────────────────────────────────────
const LazyApp = lazy(async () => {
  await initFaro();
  return import('./features/app/App').then((m) => ({ default: m.App }));
});

function App(props: AppRootProps<JsonData>) {
  return (
    <Suspense fallback={<LoadingPlaceholder text="" />}>
      <LazyApp {...props} />
    </Suspense>
  );
}

// ── Extension components ──────────────────────────────────────────────────────
const LazyMyExtension = lazy(() => import('./extensions/MyExtension').then((m) => ({ default: m.MyExtension })));

function MyExtension(props: MyExtensionProps) {
  return (
    <Suspense fallback={<LoadingPlaceholder text="" />}>
      <LazyMyExtension {...props} />
    </Suspense>
  );
}

// ── Plugin registration ───────────────────────────────────────────────────────
// Pass the JsonData generic so setRootPage() types match the App wrapper
export const plugin = new AppPlugin<JsonData>().setRootPage(App);

plugin.exposeComponent({
  id: 'my-plugin/my-extension/v1',
  title: 'My Extension',
  component: MyExtension,
});

Key details:

• import type for props: Always use import type when importing interfaces or types for the lazy wrapper's props. A regular import creates a real module dependency that webpack follows, pulling the component code into the eager bundle and defeating the split.
• AppPlugin<JsonData> generic: If the original App component uses AppRootProps<JsonData> (a custom type for plugin.json settings), pass that generic to AppPlugin<JsonData>(). Without it, setRootPage() expects AppRootProps<KeyValue<any>> which won't match.
• Remove the ComponentClass type cast: If the original module.ts used App as unknown as ComponentClass<AppRootProps>, remove the cast entirely. The lazy wrapper function App(props) is a valid React function component and setRootPage() accepts it directly.

Expected impact: module.js drops from MB range to ~50–200 KB.

---

Side note: singletons (e.g. Faro) — lazy init, not eager

If module.ts has a synchronous Faro init like export const faro = initializeFaro(), do not keep it as a top-level import in module.tsx. That pulls the entire @grafana/faro-react library into module.js.

Instead, dynamically import and initialise Faro inside the lazy() callback, before the App import resolves. This moves the Faro library into the App chunk:

// src/module.tsx — Faro initialises lazily, before App renders
let faroInitialized = false;
async function initFaro() {
  if (faroInitialized) {
    return;
  }
  faroInitialized = true;
  const { initializeFaro } = await import('faro');
  initializeFaro();
}

const LazyApp = lazy(async () => {
  await initFaro();
  return import('./features/app/App').then((m) => ({ default: m.App }));
});

This pattern (from grafana-collector-app) ensures:

1. @grafana/faro-react and its deps stay out of module.js — they load with the App chunk
2. Faro initialises before any component renders (it runs inside lazy() before the App import resolves)
3. The faroInitialized guard prevents double-init if the lazy factory runs again

If other source files import the Faro instance from module.ts (e.g. import { faro } from '../module'), first check:

grep -rn "from '.*module'" src/ --include="*.ts" --include="*.tsx" | grep -v node_modules

If files import from module.ts, extract the singleton to a dedicated file before renaming:

1. Move to src/faro.ts (or if it's already in a separate file like src/faro/index.ts, skip this)
2. Update internal imports from '*/module' → '*/faro'
3. In module.tsx, use the lazy initFaro() pattern above instead of importing and re-exporting

---

Priority 2: Route-based splitting in App.tsx

Replace every direct import of a page component with lazy():

// src/components/App.tsx
import React, { lazy, Suspense } from 'react';
import { Route, Routes } from 'react-router-dom';
import { LoadingPlaceholder } from '@grafana/ui';

// One lazy() per route — each becomes its own JS chunk
const HomePage = lazy(() => import('../pages/Home'));
const SettingsPage = lazy(() => import('../pages/Settings'));
const DetailPage = lazy(() => import('../pages/Detail'));
// ... add one per route

function App(props: AppRootProps) {
  return (
    // A single Suspense at the Routes level is enough — no need for one per route
    <Suspense fallback={<LoadingPlaceholder text="" />}>
      <Routes>
        <Route path="home" element={<HomePage />} />
        <Route path="settings" element={<SettingsPage />} />
        <Route path="detail/:id" element={<DetailPage />} />
        <Route path="" element={<HomePage />} />
      </Routes>
    </Suspense>
  );
}

export default App;

Bypass barrel files: When a component is re-exported through an index.ts barrel, target the actual component file in the import(), not the barrel. If the barrel re-exports multiple things, importing it pulls them all into the same chunk:

// Risky — barrel may re-export other heavy modules into this chunk
const Catalog = lazy(() => import('features/catalog'));

// Better — only pulls in the Catalog component's tree
const Catalog = lazy(() => import('features/catalog/Catalog').then((m) => ({ default: m.Catalog })));

Priority 3: Extension components

Each extension file should export default its component so webpack can split it cleanly. If it needs context (e.g. AppProviders), include that in the export:

// src/extensions/MyExtension.tsx
import React from 'react';
import { AppProviders } from '../components/AppProviders';

function MyExtensionContent(props: MyExtensionProps) {
  return <div>...</div>;
}

// Default export wraps with providers so the lazy consumer in module.tsx stays simple
export default function MyExtension(props: MyExtensionProps) {
  return (
    <AppProviders>
      <MyExtensionContent {...props} />
    </AppProviders>
  );
}

fallback={null} for extensions: Extension components often load quickly; a <LoadingPlaceholder> flash is more disruptive than no indicator. Use fallback={null} unless the component is genuinely slow:

function MyExtension(props: MyExtensionProps) {
  return (
    <Suspense fallback={null}>
      <LazyMyExtension {...props} />
    </Suspense>
  );
}

Surgical split — lazy-load the inner component, not the wrapper: If the extension wrapper must stay eager in module.tsx (e.g. it has complex props setup), lazy-load the heavy component it renders instead of restructuring the entry point:

// src/components/Extensions/InstallAlloyHelmExtension.tsx
// The wrapper is still imported eagerly in module.tsx — only the heavy inner component is lazy
import React, { lazy, Suspense } from 'react';
import { LoadingPlaceholder } from '@grafana/ui';

const InstallAlloyHelm = lazy(() => import('components/scenes/Config/ClusterConfig/InstallAlloyHelm'));

export function InstallAlloyHelmExtension() {
  return (
    <Suspense fallback={<LoadingPlaceholder text="" />}>
      <InstallAlloyHelm />
    </Suspense>
  );
}

This is the right approach when you can't restructure module.tsx but one extension pulls in a disproportionately large component tree.

Priority 4: Component registries and tab panels (if present)

If you have an array of objects containing React components (e.g. tab panels on a details page), apply lazy loading per entry. This is moderate risk — verify a <Suspense> boundary exists where the component is rendered.

Tab panel example (from cloud-onboarding Source page):

// Before — all tab components load upfront even though only one is shown at a time
import { ConfigurationDetails } from './ConfigurationDetails';
import { ConnectorOverview } from './ConnectorOverview';
import { Metrics } from './Metrics';

const tabs = [
  { id: 'overview', component: ConnectorOverview },
  { id: 'config', component: ConfigurationDetails }, // 67+ file tree!
  { id: 'metrics', component: Metrics },
];

// After — each tab component loads only when selected
const ConfigurationDetails = lazy(() =>
  import('./ConfigurationDetails/ConfigurationDetails').then((m) => ({ default: m.ConfigurationDetails }))
);
const ConnectorOverview = lazy(() =>
  import('./ConnectorOverview/ConnectorOverview').then((m) => ({ default: m.ConnectorOverview }))
);
const Metrics = lazy(() => import('./Metrics').then((m) => ({ default: m.Metrics })));

const tabs = [
  { id: 'overview', component: ConnectorOverview },
  { id: 'config', component: ConfigurationDetails },
  { id: 'metrics', component: Metrics },
];

Critical: add a Suspense boundary where the tab content renders:

// In the parent component that renders the active tab
<TabContent>
  <Suspense fallback={<LoadingPlaceholder text="" />}>{ActiveTab && <ActiveTab />}</Suspense>
</TabContent>

React.lazy() returns a valid component reference that can be stored in arrays and rendered later — no special handling needed in the registry itself. The Suspense boundary just needs to exist somewhere above the render point.

General component registry example:

const LazyConfigEditor = lazy(() => import('./editors/ConfigEditor'));
const LazyQueryEditor = lazy(() => import('./editors/QueryEditor'));

const panels = [
  {
    id: 'config',
    component: (props: ConfigEditorProps) => (
      <Suspense fallback={<LoadingPlaceholder text="" />}>
        <LazyConfigEditor {...props} />
      </Suspense>
    ),
  },
  {
    id: 'query',
    component: (props: QueryEditorProps) => (
      <Suspense fallback={<LoadingPlaceholder text="" />}>
        <LazyQueryEditor {...props} />
      </Suspense>
    ),
  },
];

Datasource plugins: setConfigEditor, setQueryEditor, and support editors

Datasource plugins (type: "datasource") apply the same pattern to setConfigEditor(), setQueryEditor(), and the editor/QueryEditor fields on VariableSupport and AnnotationSupport. Rename module.ts → module.tsx and lazy-load all four:

// src/module.tsx (datasource plugin)
import React, { Suspense } from 'react';
import { DataSourcePlugin } from '@grafana/data';
import { DataSource, DSOptions } from './datasource';
import { Query } from './types';
import type { KGQueryEditorProps } from './components/QueryEditor';

// Named exports → re-map to default with .then()
const LazyConfigEditor = React.lazy(() =>
  import('./components/ConfigEditor').then((m) => ({ default: m.ConfigEditor }))
);
const LazyQueryEditor = React.lazy(() => import('./components/QueryEditor').then((m) => ({ default: m.QueryEditor })));

function ConfigEditor(props: DataSourcePluginOptionsEditorProps<DSOptions>) {
  return (
    <Suspense fallback={null}>
      <LazyConfigEditor {...props} />
    </Suspense>
  );
}
function QueryEditor(props: KGQueryEditorProps) {
  return (
    <Suspense fallback={null}>
      <LazyQueryEditor {...props} />
    </Suspense>
  );
}

export const plugin = new DataSourcePlugin<DataSource, Query, DSOptions>(DataSource)
  .setConfigEditor(ConfigEditor)
  .setQueryEditor(QueryEditor);

For VariableSupport and AnnotationSupport, rename the .ts file to .tsx and assign the lazy-wrapped component:

// src/datasource/VariableSupport.tsx (renamed from .ts)
import React, { Suspense } from 'react';
import type { VariableQueryEditorProps } from './components/VariableQueryEditor';

const LazyVariableQueryEditor = React.lazy(() =>
  import('./components/VariableQueryEditor').then((m) => ({ default: m.VariableQueryEditor }))
);
function VariableQueryEditorWithSuspense(props: VariableQueryEditorProps) {
  return (
    <Suspense fallback={null}>
      <LazyVariableQueryEditor {...props} />
    </Suspense>
  );
}

export class MyVariableSupport extends CustomVariableSupport<DataSource, MyVariableQuery> {
  editor = VariableQueryEditorWithSuspense;
  // ...
}

Same pattern for AnnotationSupport.QueryEditor. Use import type for props interfaces to avoid pulling the component into the module's eager load.

---

Step 6: Group related chunks if over-splitting

If the build produces more than ~25 JS files, use webpack magic comments to group related pages into a single chunk:

// These two pages land in the same "fleet.js" chunk
const FleetList = lazy(() => import(/* webpackChunkName: "fleet" */ '../pages/FleetList'));
const FleetDetail = lazy(() => import(/* webpackChunkName: "fleet" */ '../pages/FleetDetail'));

Use one webpackChunkName per logical feature area. Don't group unrelated pages — the point is to load code only when needed.

---

Step 7: Measure and verify

yarn build 2>/dev/null || npm run build

echo "=== module.js ==="
ls -lah dist/module.js

echo "=== all JS chunks (largest first) ==="
ls -lah dist/*.js | sort -k5 -rh | head -30

echo "=== chunk count ==="
ls dist/*.js | wc -l

Healthy outcome:

| Metric | Target | | -------------------- | ---------------------- | | module.js size | < 200 KB | | Total JS chunk count | 15–25 | | Largest single chunk | < 1 MB | | Chunk per route | ✓ (verify in DevTools) |

If a chunk is unexpectedly large (> 1 MB), check what it imports:

# Analyse bundle composition (if webpack-bundle-analyzer is available)
npx webpack-bundle-analyzer dist/stats.json 2>/dev/null || \
  yarn build --env production --profile 2>/dev/null

---

Step 8: Test the running plugin

Lazy loading can expose runtime errors that were previously hidden by eager loading.

1. Open the plugin in a Grafana instance (refer to the deploy-plugin-pr skill to deploy a test build)
2. Navigate to every route in the app — each triggers a new chunk download
3. Check browser DevTools → Network → JS tab: confirm lazy chunks load on navigation, not all upfront
4. Check browser Console for errors
5. Test any exposeComponent extensions from other Grafana apps that use them

---

Troubleshooting

| Symptom | Cause | Fix | | ------------------------------------------------------------ | ----------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------- | | module.js barely shrank | Entry point still transitively imports feature code | Read module.tsx carefully — any direct import pulls its entire tree in | | Route shows blank page | Component is rendered outside its Suspense boundary | Add <Suspense> wrapping in the parent, or move the boundary up | | Extension crashes | Missing AppProviders context | Wrap the default export in the extension file with <AppProviders> | | Too many chunks (50+) | Every subcomponent split | Use webpackChunkName to group related pages | | module.js barely shrank after rename | Entry point re-exports a singleton (faro, analytics) that pulls in its whole init tree | Extract singleton to src/faro.ts; module.tsx re-exports it with export { faro } from './faro' | | Circular dependency warning after split | Feature files import from module.ts (e.g. faro) and module.tsx lazy-imports them back | Extract the exported value to a dedicated file (see singleton note in Step 4) | | Build fails after rename | swc-loader or ts-loader needs tsx support | Ensure tsconfig.json has "jsx": "react-jsx" and "tsx" in the parser config | | lazy() throws "does not provide an export named 'default'" | Component uses a named export, not a default export | Use .then(m => ({ default: m.ComponentName })) (see named export note in Step 4) | | Datasource editor blank after split | Suspense missing on VariableSupport.editor or AnnotationSupport.QueryEditor | Wrap the assigned component with a Suspense boundary (see datasource plugin section) | | React.lazy not available | Very old React or CommonJS module output | Requires React ≥ 16.6 and esModuleInterop: true in tsconfig | | Chunks not loading in prod | output.publicPath mismatch | Verify publicPath in webpack config matches public/plugins/<PLUGIN_ID>/ | | ESLint import/no-unused-modules error after rename | ignoreExports glob only matches .ts, not .tsx | Add './src/*.tsx' to ignoreExports in eslint config | | Chunks cache forever after deploy | chunkFilename missing content hash | Add [contenthash] to output.chunkFilename in webpack config | | setRootPage() type error after adding JsonData generic | AppPlugin not parameterised | Use new AppPlugin<JsonData>() so setRootPage() expects AppRootProps<JsonData> | | Dev build sizes are huge (multi-MB) | Measuring dev instead of production | Always clean (rm -rf dist node_modules/.cache) and build with --env production for measurements |

rspack compatibility: All React.lazy() / dynamic import patterns work identically with rspack. webpackChunkName magic comments are also supported. If the plugin uses .config/rspack/, no changes are needed to the build config.

---

References

• grafana-collector-app — app plugin reference: 2.7 MB → 100 KB after splitting
• cloud-onboarding #10380 — app plugin: 18 MB → 234 KB, route + tab + extension splits
• grafana-k8s-plugin #1730 — extension params file pattern + surgical lazy split: ~700 KB → ~100 KB
• grafana-k8s-plugin #2409 — surgical lazy inside extension wrapper
• asserts-app-plugin #2705 — datasource plugin: setConfigEditor / setQueryEditor / VariableSupport / AnnotationSupport
• Web.dev — code splitting with lazy and Suspense
• SurviveJS — webpack code splitting chapter
• webpack magic comments — webpackChunkName for grouping chunks