date.js

export const newDate = (date, updates) => {
  date = new Date(date)
  Object.keys(updates).forEach((method) => {
    const methodName = 'set' + method.replace(/^\w/, (c) => c.toUpperCase())
    if (!date[methodName]) throw new Error(`Key '${method}' is not supported!`)
    date[methodName](updates[method])
  })
  return date
}

export const startOf = (unit, date = new Date()) => {
  const strategy = {
    day: ['setHours', 'setMinutes', 'setSeconds', 'setMilliseconds'],
    month: ['day', 'setDate'],
    year: ['setMonth', 'month'],
  }

  strategy[unit].forEach((method) => {
    date[method]
      ? date[method](Number(method === 'setDate'))
      : startOf(method, date)
  })

  return date
}

// unit supports: "days", "years", "seconds", "minutes", "hours" etc.
export const dateAdd = (date, unit, val) => {
  date = new Date(date)
  const map = { days: 'date', years: 'fullYear' }
  const method = (map[unit] || unit).replace(/^\w/, (c) => c.toUpperCase())
  date[`set${method}`](date[`get${method}`]() + val)
  return date
}

export const isValidDate = (d) => d instanceof Date && !isNaN(d)

const padZero = (val, n) => ('0' + val).slice(n * -1)

export const formatDate = (d, mask) => {
  const flags = {
    d: d.getDate(),
    m: d.getMonth() + 1,
    y: d.getFullYear(),
  }

  return mask.replace(/[d,m,y]+/g, (x) => {
    if (x !== x[0].repeat(x.length))
      throw new Error(`Format "${mask}" is invalid!`)
    return x.length > 1 ? padZero(flags[x[0]], x.length) : flags[x[0]]
  })
}

// works: mm/dd/yyyy, dd.mm.yy, yyyy-mm-dd, m/d/yy, d/m/yy