pluck

values = [1, 1, 2, 2, 2, 1, 1, 2, 3, 3, 4, 4, 3, 1, 1, 2];
// not shown; this would be like { key: 1 }, { key: 1 }, { key: 2 } ...
input = timer(0, 1000).pipe(
  map(key => ({ key: this.values[key] })),
  take(this.values.length),
);
// These function identically
display = this.input.pipe(map(val => val.key));
output = this.input.pipe(pluck('key'));