index.ts

import { of } from 'rxjs';
import { map, mergeMap } from 'rxjs/operators';

of(1).pipe(
  map(async () => await Promise.resolve('hello?')),
  // mergeMap incuded here because of how it's written in the original code.
  // It has no effect on the async value that's passed in.
  mergeMap(customerId => {
    // Promise { <pending> }
    console.log(customerId);
    return [];
  }),
).subscribe();

of(1).pipe(
  map(() => Promise.resolve('hello?')),
  mergeMap(customerId => {
    // Promise { 'hello?' }
    console.log(customerId);
    return [];
  }),
).subscribe();

// 'hello!'
of(1).pipe(mergeMap(() => Promise.resolve('hello!'))).subscribe(console.log);