do / tap

signature: do(nextOrObserver: function, error: function, complete: function): Observable

Transparently perform actions or side-effects, such as logging.

If you are using as a pipeable operator, do is known as tap!

Examples

Example 1: Logging with do

( jsBin |
jsFiddle )

import { of } from 'rxjs/observable/of';
import { tap, map } from 'rxjs/operators';
const source = of(1, 2, 3, 4, 5);
//transparently log values from source with 'do'
const example = source.pipe(
  tap(val => console.log(`BEFORE MAP: ${val}`)),
  map(val => val + 10),
  tap(val => console.log(`AFTER MAP: ${val}`))
);
//'do' does not transform values
//output: 11...12...13...14...15
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

• do
  - Official docs
• Logging a stream with do
  - John Linquist
• Utility operator: do
  - André Staltz

Source Code:
https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/do.ts