calendar/packages/lib/fromEntriesWithDuplicateKeys.ts

export function fromEntriesWithDuplicateKeys(entries: IterableIterator<[string, string]> | null) {
  const result: Record<string, string | string[]> = {};

  if (entries === null) {
    return result;
  }

  // Consider setting atleast ES2015 as target
  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
  // @ts-ignore
  for (const [key, value] of entries) {
    if (result.hasOwnProperty(key)) {
      let currentValue = result[key];
      if (!Array.isArray(currentValue)) {
        currentValue = [currentValue];
      }
      currentValue.push(value);
      result[key] = currentValue;
    } else {
      result[key] = value;
    }
  }
  return result;
}