delay

signature: delay(delay: number | Date, scheduler: Scheduler): Observable

Delay emitted values by given time.

Why use delay?

This operator is your go-to when simulating real-world scenarios such as network latency or introducing a pause before a value is emitted. The delay operator allows you to hold back values for a specified duration before they're released to subscribers.

Keep in mind that delay won’t prevent the original observable from emitting values. It merely postpones the delivery to its subscribers. This is a gotcha as it could look like your data is lagging or not in sync with the source, especially when multiple observables are at play.

Examples

Example 1: Delay to recognize long press

( StackBlitz )

import { fromEvent, of } from 'rxjs';
import { mergeMap, delay, takeUntil } from 'rxjs/operators';

const mousedown$ = fromEvent(document, 'mousedown');
const mouseup$ = fromEvent(document, 'mouseup');

mousedown$
  .pipe(mergeMap(event => of(event).pipe(delay(700), takeUntil(mouseup$))))
  .subscribe(event => console.log('Long Press!', event));

Example 2: Delay for increasing durations

( StackBlitz )

// RxJS v6+
import { of, merge } from 'rxjs';
import { mapTo, delay } from 'rxjs/operators';

//emit one item
const example = of(null);
//delay output of each by an extra second
const message = merge(
  example.pipe(mapTo('Hello')),
  example.pipe(mapTo('World!'), delay(1000)),
  example.pipe(mapTo('Goodbye'), delay(2000)),
  example.pipe(mapTo('World!'), delay(3000))
);
//output: 'Hello'...'World!'...'Goodbye'...'World!'
const subscribe = message.subscribe(val => console.log(val));

Related Recipes

• Battleship Game

• Progress Bar

• Save Indicator

• Swipe To Refresh

Additional Resources

• delay 📰 - Official docs

• Transformation operator: delay and delayWhen 🎥 💵 - André Staltz

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