clock_functional.js

const oneSecond = () => 1000;
const getCurrentTime = () => new Date();
const clear = () => console.clear();
const log = (message) => console.log(message)

const compose = (...fns) => (arg) => fns.reduce((composed, f) => f(composed), arg)

const serializeClockTime = (date) => ({
  hours: date.getHours(),
  minutes: date.getMinutes(),
  seconds: date.getSeconds()
})

const civilianHours = (clockTime) => ({
  ...clockTime,
  hours: (clockTime.hours > 12) ?
    clockTime.hours - 12 :
    clockTime.hours
})

const appendAMPM = (clockTime) => ({
  ...clockTime,
  ampm: (clockTime.hours >= 12) ? "PM" : "AM"
})

const display = target => time => target(time)

const formatClock = format => time => format.replace("hh", time.hours).
  replace("mm", time.minutes).
  replace("ss", time.seconds).
  replace("tt", time.ampm)

const prependZero = key => clockTime => ({
  ...clockTime,
  [key]: (clockTime[key] < 10) ? "0" + clockTime[key] : clockTime[key]
})

const convertToCivilianTime = clockTime => compose(appendAMPM, civilianHours)(clockTime)

const doubleDigits = civilianTime => compose(
  prependZero("hours"),
  prependZero("minutes"),
  prependZero("seconds")
)(civilianTime)

const startTicking = () => setInterval(
  compose(
    clear,
    getCurrentTime,
    serializeClockTime,
    convertToCivilianTime,
    doubleDigits,
    formatClock("hh:mm:ss tt"),
    display(log)
  ), oneSecond()
)

startTicking()