export const formatTime = (date: Date) => {
  const year = date.getFullYear()
  const month = date.getMonth() + 1
  const day = date.getDate()
  const hour = date.getHours()
  const minute = date.getMinutes()
  const second = date.getSeconds()

  return (
    [year, month, day].map(formatNumber).join('/') +
    ' ' +
    [hour, minute, second].map(formatNumber).join(':')
  )
}

const formatNumber = (n: number) => {
  const s = n.toString()
  return s[1] ? s : '0' + s
}

export function groupBy<T>(items: Iterable<T>, callbackFn: (element: T, index: number) => any): Record<string, T[]> {
  const obj = Object.create(null);
  let k = 0;
  for (const value of items) {
    const key = callbackFn(value, k++);
    if (key in obj) { obj[key].push(value); } else { obj[key] = [value]; }
  }
  return obj;
}