Definition
Observable defines Subscriber
[Exposed=*]
interface Subscriber {
undefined next(any value);
undefined error(any error);
undefined complete();
undefined addTeardown(VoidFunction teardown);
// True after the Subscriber is created, up until either
// complete()/error() are invoked, or the subscriber unsubscribes. Inside
// complete()/error(), this attribute is true.
readonly attribute boolean active;
readonly attribute AbortSignal signal;
};