lib.dom.d.ts: Transformer is missing the cancel member that WHATWG Streams defines #65008

Summary

lib.dom.d.ts declares interface Transformer without the cancel member, but
the WHATWG Streams standard includes it and Node implements it. Any
new TransformStream({ ..., cancel() {} }) therefore fails to compile with
TS2353, even though the handler is valid and gets called when the readable side
is cancelled.

Reproduced on TypeScript 6.0.3 and 7.0.2.

Standards reference

WHATWG Streams, Transformer dictionary
(https://streams.spec.whatwg.org/#dictdef-transformer) defines six members:

start, transform, flush, cancel, readableType, writableType

lib.dom.d.ts declares five of the six. cancel is missing:

interface Transformer<I = any, O = any> {
    flush?: TransformerFlushCallback<O>;
    readableType?: undefined;
    start?: TransformerStartCallback<O>;
    transform?: TransformerTransformCallback<I, O>;
    writableType?: undefined;
}

Minimal reproduction

const ts = new TransformStream<Uint8Array, Uint8Array>({
  transform(chunk, controller) { controller.enqueue(chunk); },
  cancel() { /* client disconnected */ },
});
export { ts };

$ tsc --noEmit --target es2022 --lib dom,es2022 probe.ts
probe.ts(3,3): error TS2353: Object literal may only specify known properties,
and 'cancel' does not exist in type
'Transformer<Uint8Array<ArrayBufferLike>, Uint8Array<ArrayBufferLike>>'.

Identical output on 6.0.3 and 7.0.2.

The runtime does call it

Node 24.18.0, stream/web:

const ts = new TransformStream({
  transform(c, ctrl) { ctrl.enqueue(c); },
  cancel(reason) { console.log("transformer.cancel INVOKED", reason); },
});
await ts.readable.cancel("client-disconnect");
// -> transformer.cancel INVOKED client-disconnect

So cancel is not a dead optional member. It is the only hook that fires when
the readable side is cancelled, which for an SSE proxy is exactly the
client-disconnect case. Handlers here typically clear heartbeat and progress
intervals; removing them to satisfy the checker leaks a timer per abandoned
stream.

Why a declaration-merge workaround does not work

Filing this after trying the obvious local patch. Inside a module:

declare global {
  interface Transformer<I = unknown, O = unknown> {
    cancel?: (reason?: unknown) => void | PromiseLike<void>;
  }
}

This does not merge. Interface declarations must agree on their type
parameters, and lib.dom.d.ts uses <I = any, O = any> while the natural
patch uses unknown. With skipLibCheck: true the resulting TS2428 is
suppressed, so the merge silently fails and cancel stays absent. Matching
any makes it merge, but then the default type arguments on every
TransformStream in the codebase degrade to any, which trades one problem
for another.

That dead end is the reason this is filed as a lib gap rather than handled
locally.

Suggested fix

Add the member to Transformer in dom.generated.d.ts, with a callback type
next to the other three:

interface Transformer<I = any, O = any> {
    flush?: TransformerFlushCallback<O>;
    readableType?: undefined;
    start?: TransformerStartCallback<O>;
    transform?: TransformerTransformCallback<I, O>;
    cancel?: TransformerCancelCallback;
    writableType?: undefined;
}

type TransformerCancelCallback = (reason?: any) => void | PromiseLike<void>;

lib.webworker.d.ts has the same interface and needs the same change.

Notes

• Checked 6.0.3 and 7.0.2; both reproduce.
• Searched the issue tracker for Transformer cancel and lib.dom TransformStream
  before filing; the results were unrelated, so this does not look like a
  duplicate.
• TypeScript version 7 ships lib files inside the platform binary, so the
  reproduction uses the CLI rather than reading lib.dom.d.ts from disk.