Refactoring: Build a Date Picker in 15mins Using Javascript/React from Scratch

/** Your style of below or React.use-what-ever */
import React, { useState, useEffect, useRef, createRef, useReducer } from 'react'
import {
  getDateStringFromTimestamp,
  getMonthDetails,
  monthMap
} from './shared/dates'
import PropTypes from 'prop-types'

/**
 * We wont show stylesheet in this refactor example.
 * @see https://medium.com/swlh/build-a-date-picker-in-15mins-using-javascript-react-from-scratch-f6932c77db09
 */
import './DatePicker.scss'

/** We'll use useReducer to manage our state **/
const date = new Date()
const oneDay = 60 * 60 * 24 * 1000
const todayTimestamp = date.getTime() - (date.getTime() % oneDay) + (date.getTimezoneOffset() * 1000 * 60)

const initialState = {
  todayTimestamp: todayTimestamp, // or todayTimestamp, for short
  year: date.getFullYear(),
  month: date.getMonth(),
  selectedDay: todayTimestamp,
  monthDetails: getMonthDetails(date.getFullYear(), date.getMonth())
}

export function DatePicker(props) {
  const el = useRef(null)
  const inputRef = createRef()
  const [state, dispatch] = useReducer(reducer, initialState)
  /** Maybe you could add this to initialState 🤷🏽‍♂️ */
  const [showDatePicker, setShowDatePicker] = useState(false)

  const addBackDrop = e => {
    if(showDatePicker && (el && !el.current.contains(e.target))) {
      setShowDatePicker(false);
    }
  }

  const setDateToInput = (timestamp) => {
    const dateString = getDateStringFromTimestamp(timestamp)
    inputRef.current.value = dateString
  }

  useEffect(() => {
    /** 
     * Only needed when using SSR ie Next.js 
     * Uncomment if you're using SSR:
     * if (!process.browser) { return }
     */

    window.addEventListener('click', addBackDrop)
    setDateToInput(state.selectedDay)

    // returned function will be called on component unmount 
    return () => {
      window.removeEventListener('click', addBackDrop)
    }
  }, [showDatePicker])

  const isCurrentDay = day => day.timestamp === todayTimestamp
  const isSelectedDay = day =>  day.timestamp === state.selectedDay
  const getMonthStr = month => monthMap[Math.max(Math.min(11, month), 0)] || 'Month'

  const onDateClick = day => {
    dispatch({type: 'selectedDay', value: day.timestamp})
    setDateToInput(day.timestamp)

    /** Pass data to parent */
    props.onChange(day.timestamp)
  }

  const setYear = offset => {
    const year = state.year + offset
    dispatch({type: 'year', value: year})
    dispatch({type: 'monthDetails', value: getMonthDetails(year, state.month)})
  }

  const setMonth = offset => {
    let year = state.year
    let month = state.month + offset
    if(month === -1) {
        month = 11
        year--
    } else if(month === 12) {
        month = 0
        year++
    }

    dispatch({type: 'year', value: year})
    dispatch({type: 'month', value: month})
    dispatch({type: 'monthDetails', value: getMonthDetails(year, month)})
  }

  const setDate =dateData=> {
    const selectedDay = new Date(dateData.year, dateData.month - 1, dateData.date).getTime()
    dispatch({type: 'selectedDay', value: selectedDay})

    /** Pass data to parent */
    props.onChange(selectedDay)
  }

  const getDateFromDateString = dateValue => {
    const dateData = dateValue.split('-').map(d => parseInt(d, 10))

    if (dateData.length < 3) {
      return null
    }

    const year = dateData[0]
    const month = dateData[1]
    const date = dateData[2]
    return {year, month, date}
 }

  const updateDateFromInput =()=> {
    const dateValue = inputRef.current.value
    const dateData = getDateFromDateString(dateValue)

    if (dateData !== null) { 
        setDate(dateData)
        dispatch({type: 'year', value: dateData.year})
        dispatch({type: 'month', value: dateData.month - 1})
        dispatch({type: 'monthDetails', value: getMonthDetails(dateData.year, dateData.month -1)})
    }
  }

  const daysMarkup = (
    state.monthDetails.map((day, index) => (
      <div className={'c-day-container ' + (day.month !== 0 ? ' disabled' : '') + 
          (isCurrentDay(day) ? ' highlight' : '') + (isSelectedDay(day) ? ' highlight-green' : '')} key={index}>
          <div className='cdc-day'>
              <span onClick={() => onDateClick(day)}>
                  {day.date}
              </span>
          </div>
      </div>
    ))
  )

  const calendarMarkup = (
    <div className='c-container'>
        <div className='cc-head'>
            {['SUN', 'MON', 'TUE', 'WED', 'THU', 'FRI', 'SAT'].map((d,i)=><div key={i} className='cch-name'>{d}</div>)}
        </div>
        <div className='cc-body'>
            {daysMarkup}
        </div>
    </div>
  )

  return (
    <div ref={el} className='MyDatePicker'>
      <div className='mdp-input' onClick={()=> setShowDatePicker(true)}>
          <input 
            type='date' 
            ref={inputRef}
            onChange={updateDateFromInput}
          />
      </div>
      {showDatePicker ? (
          <div className='mdp-container'>
            <div className='mdpc-head'>
              <div className='mdpch-button'>
                <div className='mdpchb-inner' onClick={()=> setYear(-1)}>
                  <span className='mdpchbi-left-arrows'></span>
                </div>
              </div>
              <div className='mdpch-button'>
                <div className='mdpchb-inner' onClick={()=> setMonth(-1)}>
                  <span className='mdpchbi-left-arrow'></span>
                </div>
              </div>
              <div className='mdpch-container'>
                <div className='mdpchc-year'>{state.year}</div>
                <div className='mdpchc-month'>{getMonthStr(state.month)}</div>
              </div>
              <div className='mdpch-button'>
                <div className='mdpchb-inner' onClick={()=> setMonth(1)}>
                  <span className='mdpchbi-right-arrow'></span>
                </div>
              </div>
              <div className='mdpch-button' onClick={()=> setYear(1)}>
                <div className='mdpchb-inner'>
                  <span className='mdpchbi-right-arrows'></span>
                </div>
              </div>
            </div>
            <div className='mdpc-body'>
                {calendarMarkup}
            </div>
          </div>
      ) : ''}
    </div>
  )
}

/** Fancy using switch statement? Go ahead */
function reducer(state, action) {

    if (state.hasOwnProperty(action.type)) {
      return {
        ...state,
        [`${action.type}`]: action.value
      }
    }

    console.log(`Unknown key in state: ${action.type}`)
}

DatePicker.propTypes = {
  onChange: PropTypes.func.isRequired
}