groupBy.ts

function groupBy<T extends Record<string, unknown>, K extends keyof T>(array: T[], key: K): Record<T[K] & PropertyKey, T[]> {
  return array.reduce((result, current) => {
    const currentKey = current[key] as T[K] & PropertyKey
    if (Array.isArray(result[currentKey])) {
      result[currentKey].push(current)
    } else {
      result[currentKey] = [current]
    }
    return result
  }, {} as Record<T[K] & PropertyKey, T[]>)
}