Cannot read property 'subscribe' of undefined in nested calls in Angular 2 - rest

Question

I want to subscribe in company-list.component on getCompanies() from the company.service. However I get the following error:
Cannot read property 'subscribe' of undefined
This is the code:
company.service.ts
getCompaniesOfUser() {
let r;
this.userService.getUser().subscribe(
res=> r = res,
err => console.log(err),
()=>(this.getCompaniesWithSpecificLink())
)
}
getCompaniesWithSpecificLink() {
if (isAdmin == false) {
url = '/user/companies';
} else {
url = '/companies';
}
return this.getCompanies(url);
}
getCompanies(url):Observable<Company[]> {
return this.http.get(this.host + url)
.map((res:Response) => res.json());
}
company-list.component.ts
companies:Company[];
public getTheCompanies() {
this._companyService.getCompaniesOfUser()
.subscribe(companies => this.companies = companies); <-- the error occurred here
}

Answer 1

Subscribe method must be used on an observable but your getCompaniesOfUser() method is not returning anything thus is a void method.
If you want to call back-to-back rest services. Like one's input depending on the others' output. You should use the flatMap operator.
Check this answer for example: https://stackoverflow.com/a/36712707/5706293
Something like this should work:
getCompaniesOfUser() {
return this.userService.getUser().flatMap( (resp) => {
return this.getCompaniesWithSpecificLink();
});
}