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✏️ Editing: bindCallbackInternals.ts

import { SchedulerLike } from '../types';
import { isScheduler } from '../util/isScheduler';
import { Observable } from '../Observable';
import { subscribeOn } from '../operators/subscribeOn';
import { mapOneOrManyArgs } from '../util/mapOneOrManyArgs';
import { observeOn } from '../operators/observeOn';
import { AsyncSubject } from '../AsyncSubject';

export function bindCallbackInternals(
  isNodeStyle: boolean,
  callbackFunc: any,
  resultSelector?: any,
  scheduler?: SchedulerLike
): (...args: any[]) => Observable<unknown> {
  if (resultSelector) {
    if (isScheduler(resultSelector)) {
      scheduler = resultSelector;
    } else {
      // The user provided a result selector.
      return function (this: any, ...args: any[]) {
        return (bindCallbackInternals(isNodeStyle, callbackFunc, scheduler) as any)
          .apply(this, args)
          .pipe(mapOneOrManyArgs(resultSelector as any));
      };
    }
  }

// If a scheduler was passed, use our `subscribeOn` and `observeOn` operators
  // to compose that behavior for the user.
  if (scheduler) {
    return function (this: any, ...args: any[]) {
      return (bindCallbackInternals(isNodeStyle, callbackFunc) as any)
        .apply(this, args)
        .pipe(subscribeOn(scheduler!), observeOn(scheduler!));
    };
  }

// We're using AsyncSubject, because it emits when it completes,
  // and it will play the value to all late-arriving subscribers.
  const subject = new AsyncSubject<any>();

return function (this: any, ...args: any[]): Observable<any> {
    // If this is true, then we haven't called our function yet.
    let uninitialized = true;
    return new Observable((subscriber) => {
      // Add our subscriber to the subject.
      const subs = subject.subscribe(subscriber);

if (uninitialized) {
        uninitialized = false;
        // We're going to execute the bound function
        // This bit is to signal that we are hitting the callback asychronously.
        // Because we don't have any anti-"Zalgo" gaurantees with whatever
        // function we are handed, we use this bit to figure out whether or not
        // we are getting hit in a callback synchronously during our call.
        let isAsync = false;

// This is used to signal that the callback completed synchronously.
        let isComplete = false;

// Call our function that has a callback. If at any time during this
        // call, an error is thrown, it will be caught by the Observable
        // subscription process and sent to the consumer.
        callbackFunc.apply(
          // Pass the appropriate `this` context.
          this,
          [
            // Pass the arguments.
            ...args,
            // And our callback handler.
            (...results: any[]) => {
              if (isNodeStyle) {
                // If this is a node callback, shift the first value off of the
                // results and check it, as it is the error argument. By shifting,
                // we leave only the argument(s) we want to pass to the consumer.
                const err = results.shift();
                if (err != null) {
                  subject.error(err);
                  // If we've errored, we can stop processing this function
                  // as there's nothing else to do. Just return to escape.
                  return;
                }
              }
              // If we have one argument, notify the consumer
              // of it as a single value, otherwise, if there's more than one, pass
              // them as an array. Note that if there are no arguments, `undefined`
              // will be emitted.
              subject.next(1 < results.length ? results : results[0]);
              // Flip this flag, so we know we can complete it in the synchronous
              // case below.
              isComplete = true;
              // If we're not asynchronous, we need to defer the `complete` call
              // until after the call to the function is over. This is because an
              // error could be thrown in the function after it calls our callback,
              // and if that is the case, if we complete here, we are unable to notify
              // the consumer than an error occured.
              if (isAsync) {
                subject.complete();
              }
            },
          ]
        );
        // If we flipped `isComplete` during the call, we resolved synchronously,
        // notify complete, because we skipped it in the callback to wait
        // to make sure there were no errors during the call.
        if (isComplete) {
          subject.complete();
        }

// We're no longer synchronous. If the callback is called at this point
        // we can notify complete on the spot.
        isAsync = true;
      }

// Return the subscription fron adding our subscriber to the subject.
      return subs;
    });
  };
}

MR SKB