PR Description

This PR refreshes prefetch/preload support across Worker chunks and new URL() assets, aligns the behavior with dynamic import(), and introduces new runtime helpers.

fixes

• fixes feat: support webpackPrefetch/webpackPreload/webpackFetchPriority for new Worker syntax #19377
• fixes support webpackPrefetch and webpackPreload for new Worker() chunk #19373
• fixes feat: support webpackPrefetch for new URL(...) #18524

What kind of change does this PR introduce?

Adds magic comment support to configure resource hints:

• Worker chunks (numeric ordering + fetch priority)
• new URL() assets (boolean flags + fetch priority + explicit preload attribute overrides)
• Introduces runtime helpers to inject <link> elements at runtime:
• prefetchAsset and preloadAsset

Supported magic comments

• Worker chunks (new Worker / SharedWorker / service worker register):
  • /* webpackPrefetch: true | number */
  • /* webpackPreload: true | number */
  • /* webpackFetchPriority: "high" | "low" | "auto" */
• new URL() assets:
  • /* webpackPrefetch: true */
  • /* webpackPreload: true */
  • /* webpackFetchPriority: "high" | "low" | "auto" */
  • /* webpackPreloadAs: "<request-destination>" */
  • /* webpackPreloadType: "<mime/type>" */
  • /* webpackPreloadMedia: "<media-query>" */

Notes:

• Worker accepts number ordering (same as import()), URL assets do not.
• Explicit webpackPreloadAs/webpackPreloadType/webpackPreloadMedia override the auto-detected defaults for URL assets.
• The as attribute accepts Fetch Standard request destinations. Allowed values include: audio, audioworklet, document, embed, fetch, font, image, manifest, object, paintworklet, report, script, sharedworker, serviceworker, style, track, video, worker, xslt.
• Warnings are emitted for invalid values.

Example Usage

// Prefetch an asset for future use
const imageUrl = new URL(
  /* webpackPrefetch: true */
  './assets/hero.jpg',
  import.meta.url
);

// Preload with high priority
const criticalFont = new URL(
  /* webpackPreload: true */
  /* webpackFetchPriority: "high" */
  './fonts/main.woff2',
  import.meta.url
);

// Works with Worker syntax
const worker = new Worker(
  new URL(
    /* webpackPrefetch: true */
    './heavy-computation.worker.js',
    import.meta.url
  )
);

// Override as/type/media explicitly (defaults are auto-detected)
const fontUrl = new URL(
  /* webpackPreload: true */
  /* webpackPreloadAs: "font" */
  /* webpackPreloadType: "font/woff2" */
  /* webpackPreloadMedia: "(max-width: 600px)" */
  './fonts/heading.woff2',
  import.meta.url
);

// Accepts Fetch Request Destinations (e.g., sharedworker)
const helperUrl = new URL(
  /* webpackPreload: true */
  /* webpackPreloadAs: "sharedworker" */
  './scripts/worker-helper.js',
  import.meta.url
);

Did you add tests for your changes?

Yes

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

• Worker: true | number for webpackPrefetch/webpackPreload, fetchpriority supported.
• URL:
  • webpackPrefetch: true / webpackPreload: true
  • webpackFetchPriority: "high" | "low" | "auto"
  • Auto-detected as (based on file type), with explicit overrides:
    • webpackPreloadAs: "<request-destination>" (Fetch Request Destinations; includes audio, audioworklet, document, embed, fetch, font, image, manifest, object, paintworklet, report, script, sharedworker, serviceworker, style, track, video, worker, xslt)
    • webpackPreloadType: "<mime/type>"
    • webpackPreloadMedia: "<media-query>"
  • Invalid values emit warnings.

rel

• ESM Module Output #17121