fetchpipe.ts

interface IFetchResponse {
  text: () => Promise<string>;
  json: () => Promise<string>;
}
declare var fetch: (url: string) => Promise<IFetchResponse>;

@Pipe({
  name: 'fetch'
})
class FetchJsonPipe {
  private fetchedValue:any;
  private fetchPromise:Promise<any>;
  transform(value:string, args:string[]):any {

    if (!this.fetchPromise) {
      this.fetchPromise = fetch(value)
        .then(result => result.json())
        .then(json => {
          this.fetchedValue = json;
        });
    }

    return this.fetchedValue;
  }
}