tap.js

const noop = () => { };
const throwError = (e) => { throw e; };

Observable.prototype.interceptOnNext = (onNext, thisArg) => {
  const source = this;
  return Observable.create(o => {
    let i = 0;
    return source.subscribe(
      x => {
        try {
          return onNext.call(thisArg, x, i++, source);
        } catch (e) {
          o.onError(e);
        }
        o.onNext(x);
      }
    );
  });
};

Observable.prototype.interceptOnError = (onError, thisArg) => {
  const source = this;
  return Observable.create(o => {
    return source.subscribe(
      noop,
      err => {
        try {
          return onError.call(thisArg, err, source);
        } catch (e) {
          o.onError(e);
        }
        o.onError(err);
      }
    );
  });
};


Observable.prototype.interceptOnCompleted = (onCompleted, thisArg) => {
  const source = this;
  return Observable.create(o => {
    return source.subscribe(
      noop,
      throwError,
      () => {
        try {
          return onCompleted.call(thisArg, source);
        } catch (e) {
          o.onError(e);
        }
        o.onCompleted();
      }
    );
  });
};