A TransformStream that allows its Transformer to be notified when the stream has shutdown, either due to an error or from end-of-input.

The standard TransformStream does not support notifying its Transformer if the stream encounters an error.

To be notified of stream shutdown, either provide a close() method on the Transformer object, or register a listener on the Controller's signal property (an AbortSignal) in the Transformer's start() method:

new ShutdownAwareTransformStream({
  transformer: {
    start(controller) {
      controller.signal.addEventListener(
        "abort",
        () => console.log("stream aborted"),
      );
    },
    close() {
      console.log("stream has shutdown");
    },
    transform(chunk, controller) {
      // ...
    },
  },
});