How to chain Http calls in Angular2?

Multiple HTTP calls in the Angular 2 can be chained using the mergeMap operator.

//example
import { mergeMap } from 'rxjs/operators';
this.http.get('./customer.json').pipe(
  mergeMap(customer => this.http.get(customer.contractUrl))
).subscribe(res => this.contract = res);

This merge operator is used by the Angular version above 4.3.