defer

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

Create an observable with given subscription function.

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


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

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