fl.ts

import { of } from 'rxjs/observable/of';
import { map } from 'rxjs/operators';
import { Observable } from 'rxjs/Observable';

const identity = <T, M extends keyof T>(...props: M[]) => map<T, { [M in keyof T]: T[M] }>(val => props.reduce((props, propName) => {
  props[propName] = val[propName];
  return props;
}, {} as { [M in keyof T]: T[M] }));

interface Somethin {
  value: number;
  str: string;
}

const smth: Somethin = { value: 0, str: 'str' };

of(smth).pipe(
  map(val => val),
  identity('str'),
  map(({ str }) => str.toLowerCase())
).subscribe();