tap / do

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

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

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

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Examples

Example 1: Logging with tap

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { of } from 'rxjs';
import { tap, map } from 'rxjs/operators';

const source = of(1, 2, 3, 4, 5);
// transparently log values from source with 'tap'
const example = source.pipe(
  tap(val => console.log(`BEFORE MAP: ${val}`)),
  map(val => val + 10),
  tap(val => console.log(`AFTER MAP: ${val}`))
);

//'tap' does not transform values
//output: 11...12...13...14...15
const subscribe = example.subscribe(val => console.log(val));

Example 2: Using tap with object

( StackBlitz)

// RxJS v6+
import { of } from 'rxjs';
import { tap, map } from 'rxjs/operators';

const source = of(1, 2, 3, 4, 5);

// tap also accepts an object map to log next, error, and complete
const example = source
  .pipe(
    map(val => val + 10),
    tap({
      next: val => {
        // on next 11, etc.
        console.log('on next', val);
      },
      error: error => {
        console.log('on error', error.message);
      },
      complete: () => console.log('on complete')
    })
  )
  // output: 11, 12, 13, 14, 15
  .subscribe(val => console.log(val));

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Additional Resources

• tap 📰 - Official docs

• tap - In Depth Dev Reference

• Logging a stream with do 🎥 💵 - John Linquist

• Utility operator: do 🎥 💵 - André Staltz

• Build your own tap operator 🎥 - Kwinten Pisman

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