JS_Challenge_2.js

const day = d => moment(d, 'dddd').format('ddd')

const decimal = time => moment().startOf('d').add(time, 'h')

const interval = ({start, end}) => {
  const f = 'h:mm A'
  return `${decimal(start).format(f)} - ${decimal(end).format(f)}`
}

const toHTML = item => `<li>${item}</li>`

const append = (html, index) => document.querySelector(`#box${index + 1} ul`).innerHTML = html

const filter = item => !!item

function format({time, names}) {
  if (names.length === 1) {
    return `${day(names[0])}: ${time}`
  } else {
    return `${day(names[0])} - ${day(names[names.length - 1])}: ${time}`
  }
}

function toIntervals({days, order}) {
  let key = 0
  let prev

  return order.reduce((group, day) => {
    if (!days[day]) {
      key++
      return group
    }

    const time = interval(days[day])

    if (prev !== time) {
      key++
      prev = time
    }

    if (!group[key]) {
      group[key] = {names: [], time}
    }

    group[key].names.push(day)

    return group
  }, [])
}

const convert = src => toIntervals(src)
  .filter(filter)
  .map(format)
  .map(toHTML)
  .join(' ');

[source, source2, source3]
  .map(convert)
  .forEach(append)

sources.js

const source = {
  order: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'],
  days: {
    Monday: {
      start: 10,
      end: 19.5
    },
    Tuesday: {
      start: 10.75,
      end: 23.5
    },
    Wednesday: {
      start: 10.75,
      end: 23.5
    },
    Thursday: {
      start: 10.75,
      end: 23.5
    },
    Saturday: {
      start: 9,
      end: 18
    },
    Sunday: {
      start: 9,
      end: 18
    }
  }
}
const source2 = {
  order: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'],
  days: {
    Monday: {
      start: 9,
      end: 18.35
    },
    Tuesday: {
      start: 10,
      end: 19.2
    },
    Wednesday: {
      start: 9,
      end: 18.35
    },
    Thursday: {
      start: 10,
      end: 19.2
    },
    Saturday: {
      start: 10,
      end: 19.2
    },
    Sunday: {
      start: 10,
      end: 19.2
    }
  }
}
const source3 = {
  order: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'],
  days: {
    Monday: {
      start: 7.5,
      end: 18.5
    },
    Tuesday: {
      start: 7.5,
      end: 18.5
    },
    Saturday: {
      start: 7.5,
      end: 18.5
    },
    Sunday: {
      start: 9,
      end: 18
    }
  }
}