defer

signature: defer(observableFactory: function(): SubscribableOrPromise): Observable

Create an observable with given subscription function.

💡 defer is used as part of the iif operator!

Ultimate RxJS

Examples

Example 1: Defer to get current date/time at the time of subscription

( StackBlitz )

// RxJS v6+
import { defer, of, timer, merge } from 'rxjs';
import { switchMap } from 'rxjs/operators';
const s1 = of(new Date()); //will capture current date time
const s2 = defer(() => of(new Date())); //will capture date time at the moment of subscription
console.log(new Date());
timer(2000)
.pipe(switchMap(_ => merge(s1, s2)))
.subscribe(console.log);
/*
OUTPUT =>
2019-02-10T12:38:30.000Z (currrent date/time from first console log)
2019-02-10T12:38:30.000Z (date/time in s1 console log, captured date/time at the moment of observable creation)
2019-02-10T12:38:32.000Z (date/time in s2 console log, captured date/time at the moment of subscription)
*/
/*//NOTE: 'traditional' js equivalent of timer code above is:
setTimeout(() => {
s1.subscribe(console.log);
s2.subscribe(console.log);
}, 2000);
*/

Additional Resources

  • defer 📰 - Official docs

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