Quite often you come to a point where you start with one type of Observable and you want it to turn into something else.
import { of } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import { flatMap, map } from 'rxjs/operators';
let stream$ = of(1,2,3)
.pipe(
flatMap((val) => {
return of(val).pipe(
ajax({ url : url + })).pipe(
map((e) => e.response)
);
)
);
}
stream.subscribe((val) => console.log(val))
// { id : 1, name : 'Darth Vader' },
// { id : 2, name : 'Emperor Palpatine' },
// { id : 3, name : 'Luke Skywalker' }
So here we have a case of starting with values 1,2,3 and wanting those to lead up to an ajax call each
--1------2-----3------>
--json-- json--json -->
The reason for us NOT doing it like this with a .map()
operator
import { of } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import { map } from 'rxjs/operators';
let stream$ = of(1,2,3).pipe(
map((val) => {
return of(val).pipe(
ajax({ url : url + }) ).pipe(
map((e) => e.response )
)
})
);
is that it would not give the result we want instead the result would be:
// Observable, Observable, Observable
because we have created a list of observables, so three different streams. The flatMap()
operator however is able to flatten these three streams into one stream called a metastream
. There is however another interesting operator that we should be using when dealing with ajax generally and it's called switchMap()
. Read more about it here Cascading calls