convert.Date.ISO.timestamp.js

const toLower = (str) => str.toLowerCase()
const isEqual = (a, b) => a === b
const AND = (args) => args.reduce((r, t) => r && t , true)

const getTSFromISODate = (date) => new Date(date).getTime()
const getISODateFromTS = (date) => new Date(date)

const isTSFromISODate = (from, to) => 
  AND([
    isEqual(toLower(from), 'iso'), 
    isEqual(toLower(to), 'timestamp')
  ])

const convertDate = (date, {from = 'iso', to = 'timestamp'}) =>
  (isTSFromISODate(from, to))
    ? getTSFromISODate(date)
    : getISODateFromTS(date)


console.log(
  'iso to ts',
  new Date().toISOString(),
  convertDate(
    new Date().toISOString(),
    {from: 'iso', to: 'timestamp'}
  )
)

console.log(
  'ts to iso',
  Date.now(),
  convertDate(
    Date.now(), 
    {from: 'timestamp', to: 'iso'}
  )
)