Add output.resourceHints.dedupe to skip prefetch for already preloaded/prefetched chunks - #21386
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…hunks Some browsers (Chrome) issue a second network request when a <link rel="prefetch"> is inserted for a resource already hinted via markup. The new opt-in output.resourceHints.dedupe flag makes the JSONP and ESM chunk-loading runtimes scan existing <link> elements and skip the prefetch when a matching preload/prefetch already exists. Default false, so builds that do not need it pay no runtime cost. Refs #17496
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Summary
Some browsers (Chrome) fire a second network request when a
<link rel="prefetch">is inserted for a resource that was already hinted via markup (e.g. an SSR/HTML-injected<link rel="preload">). This adds an opt-inoutput.resourceHints.dedupe(defaultfalse) — a new boolean on the existingoutput.resourceHintsgroup — that makes the JSONP and ESM chunk-loading runtimes scan existing<link>elements and skip the prefetch when a matching preload/prefetch already exists. webpack already dedupes its own preload→prefetch viainstalledChunks, so this only covers externally-authored hints; keeping it opt-in means builds that don't need it pay no runtime cost. Refs #17496.What kind of change does this PR introduce?
feat — a new opt-in
output.resourceHintssub-option.Did you add tests for your changes?
Yes —
test/configCases/web/prefetch-preload-dedupe(classic/JSONP output) andtest/configCases/web/prefetch-preload-dedupe-module(ESM output), each asserting the already-preloaded chunk is skipped while others still prefetch; plus updatedDefaults, and CLI-flags snapshots.Does this PR introduce a breaking change?
No — the option defaults to
false, so existing builds are unchanged.If relevant, what needs to be documented once your changes are merged or what have you already documented?
The new
output.resourceHints.dedupeboolean should be documented alongside the otheroutput.resourceHintsoptions on webpack.js.org.Use of AI
AI (Claude) was used to reproduce the double-request in Chromium, implement the option across schema/defaults/runtime, write the tests, and regenerate the types/validators. All changes were reviewed and verified locally (targeted Jest suites +
tsc).Note
Low Risk
Opt-in config defaulting to false; behavior change is limited to browser prefetch injection when dedupe is explicitly enabled.
Overview
Adds
output.resourceHints.dedupe(defaultfalse) on the existingoutput.resourceHintsobject. When enabled, the JSONP and ESM chunk-loading runtimes scan existing<link>elements before appending a runtime<link rel="prefetch">and bail out if the samehrefis already covered byrel="prefetch",preload, ormodulepreload—addressing duplicate fetches in Chrome when SSR/HTML already injected preload hints.Wiring includes defaults (
lib/config/defaults.js), generated TypeScript declarations, a minor changeset, and an AGENTS.md note that runtime-emitting code must stay consistent across chunk-loading targets.Reviewed by Cursor Bugbot for commit debdbea. Bugbot is set up for automated code reviews on this repo. Configure here.