Functional way.

functional_hell.js

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

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

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()