asp.net web api - Angular2 filter observable with switchMap -

i'm doing term search using angular2 , webapi using method described in numerous angular2 tutorials.

in service:

search(term: string): observable<userlist[]> {     return this.http.get(this.usersearchurl + term)         .map((response: response) => response.json())         .catch(this.handleerror); } 

in component:

ngoninit(): void {     this.users = this.searchtermstream         .debouncetime(300)         .distinctuntilchanged()         .switchmap((term: string) =>             term ? this.userservice.search(term) : observable.of<userlist[]>([]))         .catch(error => {             console.log(error);             return observable.of<userlist[]>([]);         }); } 

what on service side keep track of last search. if initial search "b" , search "bo" should not have make call webapi since have results need need filter down further. assume doing observables out of question/difficult since subscribe stream don't know observables yet. thought maybe doing promise might easier, i'm not sure how handle component side of subject() , switchmap() seem observables

this like use on service side:

searchpromise(term: string): promise<userlist[]> {     if (this.lastusedterm.length > 0 && term.indexof(this.lastusedterm) == 0) {         this.lastusedsearch = this.lastusedsearch.then(x => x.filter(z => z.firstname.startswith(term)));     }     else {         this.lastusedsearch = this.http.get(this.usersearchurl + term)             .topromise()             .then((response: response) => response.json())             .catch(this.handleerror);     }      this.lastusedterm = term;     return this.lastusedsearch.then(x => x.filter(z => z.firstname.startswith("bob"))); }