SubscribeTap is a RxJS' operator that will execute defined function for you when the source gets subscribed.

SubscribeTap is available on npm. To install type:

npm install -S rxjs-subscribetap

Firstly you need to import operator you have just installed:

import { subscribeTap } from 'rxjs-subscribetap';

Then use it on any Observable!

of(1, 2, 3)
 .pipe(
 subscribeTap(() => console.log('callback!')),
 )
 .subscribe((value: number) => console.log(value));

If you intend to use multiple subscribeTaps you need to know, that operator "wraps" your original Observable. This wrapper creates new Observable that will execute specified callback and feed items from original Observable to the new one.

So if you use two subscribeTaps one after another you will end up with "wrapper 1" over "wrapper 2". If you subscribe to such a result callback from "wrapper 2" will be executed first.

of(1, 2, 3)
 .pipe(
 subscribeTap(() => console.log('callback 1')), // executed second
 subscribeTap(() => console.log('callback 2')), // executed first
 )
 .subscribe((value: number) => console.log(value));

To avoid any unexpected behavior it is advised to use only single subscribeTap and as a first operator in the pipeline:

of(1, 2, 3)
 .pipe(
 subscribeTap(() => {
 console.log('callback 1'); // executed first
 console.log('callback 2'); // executed second
 }), 
 )
 .subscribe((value: number) => console.log(value));

Dual licensed under "The Unlicense" and "MIT" .

"The Unlicense" is recommended for most cases, because it let's you use operator with no restrictions under no conditions. If "The Unlicense" is for some reason not suitable for you, take it under MIT license.

In any case, feel free to use this piece of software for any purposes (including commercial use).