compact.ts

type Nullable<T> = T | undefined | null;

function isDefined<T>(item: T): item is NonNullable<T> {
  return item !== undefined && item !== null;
}

function compact<T, R>(
  list: Nullable<T[]>,
  mapFn: (item: NonNullable<T>) => Nullable<R>
): R[] {
  if (!list) {
    return [];
  }
  return list.reduce((acc: R[], item) => {
    if (isDefined(item)) {
      const mappedItem = mapFn(item);
      if (isDefined(mappedItem)) {
        acc.push(mappedItem);
      }
    }
    return acc;
  }, []);
}