Observable.onSubscribe equivalent in RxJs

Another example that builds on Mark's answer is to use the factory functions:

Rx.Observable.prototype.doOnSubscribe = function(onSubscribe) {
  const source = this; 
  return Rx.Observable.defer(() => {
      onSubscribe();
      return source;
  });
}

Really like Mark's answer, but if you're using rxjs version 5 or later, I'd prefer using pure functions instead of patching the prototype.

Update:

Implemented without empty concat hack using defer as suggested by Ray

import {defer} from 'rxjs/observable/defer';
import {Observable} from 'rxjs/Observable';

/** Example
import {from} from 'rxjs/observable/from';

from([1, 2, 3])
    .pipe(doOnSubscribe(() => console.log('subscribed to stream')))
    .subscribe(x => console.log(x), null, () => console.log('completed'));
*/

export function doOnSubscribe<T>(onSubscribe: () => void): (source: Observable<T>) =>  Observable<T> {
    return function inner(source: Observable<T>): Observable<T> {
        return defer(() => {
          onSubscribe();
          return source;
        });
    };
}

https://gist.github.com/evxn/750702f7c8e8d5a32c7b53167fe14d8d

Original Answer:

import {empty} from 'rxjs/observable/empty';
import {concat, tap} from 'rxjs/operators';
import {Observable} from 'rxjs/Observable';

/** Example
import {from} from 'rxjs/observable/from';
from([1, 2, 3])
    .pipe(doOnSubscribe(() => console.log('subscribed to stream')))
    .subscribe(x => console.log(x), null, () => console.log('completed'));
*/

export function doOnSubscribe<T>(callback: () => void): (source: Observable<T>) =>  Observable<T> {
    return function inner(source: Observable<T>): Observable<T> {
        return empty().pipe(
            tap(null, null, callback),
            concat<T>(source)
        );
    };
}

This function is easily implemented by composition of the existing functions of RxJs5.

Rx.Observable.prototype.doOnSubscribe = function (onSubscribe) {
  return Rx.Observable.empty()
    .do(null,null, onSubscribe)
    .concat(this);
};

Rx.Observable.from([1,2,3])
  .doOnSubscribe(() => console.log('subscribed to stream'))
  .subscribe(console.log, null, () => console.log('completed'))

<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.3/Rx.js"></script>

When downstream subscribes to the doOnSubscribe we first emit an empty observable and use the .do() onComplete callback as doOnSubscribe. Then we .concat() the upstream to this and return this modified stream.