Accept a null source in MessageEventInit - #7470
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jasnell merged 1 commit intoSep 23, 2026
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The HTML spec declares `MessageEventSource? source = null` in MessageEventInit, but the initializer typed source as jsg::Optional<jsg::Ref<MessagePort>>, which rejects null with a TypeError. Message events fired by a WebSocket or an EventSource have a null source, so copying one with `new MessageEvent(event.type, event)` threw. partysocket copies events that way. Compatibility: no flag. An omitted source and a MessagePort source behave as before; the only change is that null no longer throws. Before 79e36b4 the initializer read nothing but data, so code passing a null source worked until then, and this restores that.
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danielmlr
marked this pull request as ready for review
September 22, 2026 18:42
jasnell
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Sep 23, 2026
jasnell
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Sep 23, 2026
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Lets
new MessageEvent()accept a null source again, as the HTML spec allows (MessageEventSource? source = null).Since #7053 the initializer types source as optional but not nullable, so null throws
TypeError: Incorrect type for the 'source' field on 'Initializer'. Every message event a WebSocket or EventSource fires has a null source, so copying one withnew MessageEvent(event.type, event)throws too. partysocket copies events that way, and emdash patches it to leave the source out.A port or an omitted source is unchanged. Before #7053 the initializer ignored the source altogether, so accepting null again needs no compatibility flag.
messageEventNullSourceinevents-test.jscovers an explicit null, a copy of a user-constructed event and a copy of aWebSocketPairmessage event. Without the fix it fails with the TypeError above in the@,@all-compat-flagsand@all-autogatesvariants; with it,bazel test //src/...passes on macOS arm64 (1951 tests, 8 skipped).