ticking_clock.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 civilianTime = 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, 
        civilianTime
    )(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()