1. Time Helper

参考 moment.js

export function formatDate(date) {
  let day = date.getDate();
  if (day < 10) {
    day = '0' + day;
  }
  let month = date.getMonth() + 1;
  if (month < 10) {
    month = '0' + month;
  }
  let year = date.getFullYear();
  return (
    year +
    '-' +
    month +
    '-' +
    day +
    ' ' +
    [date.getHours(), date.getMinutes(), date.getSeconds()].join(':')
  );
}

1.1. Declaratively Programming Demo(functional programming)

/**
 * compose
 * @param {functions} fns a list of functions
 * reduce first para: function
   second para: initial value
   cb first para is initial value(arg in this case)
   cb second para is a item(one item of fns in this case)
   目的是执行第一个函数，得到结果再交给第二个函数执行
   compose(f1, f2) 是构建传入 function，compose(f1, f2)(initialValue) 是返回最终结果
 */
const compose = (...fns) => (arg) => fns.reduce((accu, f) => f(accu), arg);

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

const abstractClockTime = (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],
});

// very important!!!
// reduce first para: function
// second para: initial value
// cb first para is initial value(arg in this case)
// cb second para is a item(one item of fns in this case)
// 目的是执行第一个函数，得到结果再交给第二个函数执行
// compose(f1, f2) 是构建传入 function，compose(f1, f2)(initialValue) 是返回最终结果
const compose = (...fns) => (arg) => fns.reduce((accu, f) => f(accu), arg);

// clockTime 作为 arg，初始值。先 appendAMPM(clockTime)，返回的对象作为 civilianHours 的参数，执行 civilianHours
const convertToCivilianTime = (clockTime) => compose(appendAMPM, civilianHours)(clockTime);

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

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

startTicking();

1.2. 多久之前发布评论

const second = 1000,
  minute = 60 * second,
  hour = 60 * minute,
  day = 24 * hour,
  timeframe = { second, minute, hour, day },
  breakpoints = {
    second: 60,
    minute: 60,
    hour: 24,
    day: 30,
  };

const toDate = (timeStampString) => new Date(timeStampString);

const getDiff = (timestamp, now) => toDate(now) - toDate(timestamp);

const isUnderTime = (diff, timeframe, time) => diff / timeframe < time;

const diffOverTimeframe = (diff, timeframe) => Math.floor(diff / timeframe);

const printResult = (result, timeframeName) =>
  `${result} ${timeframeName + (result > 1 ? 's' : '')}`;

const checkDate = (diff, timeframeName, underTime, timeframe) =>
  isUnderTime(diff, timeframe[timeframeName], underTime)
    ? printResult(diffOverTimeframe(diff, timeframe[timeframeName]), timeframeName)
    : null;

const printFullDate = (dateTime) =>
  `${dateTime.getMonth() + 1}/${dateTime.getDate()}/${dateTime.getFullYear()}`;

const lessThanAMinute = (timeString) =>
  timeString.match(/seconds/) ? 'less than a minute' : timeString + ' ago';

const _checkNext = (result, callback) => (result ? lessThanAMinute(result) : callback());

const checkNext = ([tfName, ...rest], timeframe, timestamp, now) =>
  _checkNext(checkDate(getDiff(timestamp, now), tfName, breakpoints[tfName], timeframe), () =>
    howLongAgo(rest, timeframe, timestamp, now),
  );

const howLongAgo = (remainingTimeframe, timeframe, timestamp, now) =>
  !remainingTimeframe.length
    ? printFullDate(toDate(timestamp))
    : checkNext(remainingTimeframe, timeframe, timestamp, now);

export const ago = (timestamp, now = new Date().toString()) =>
  howLongAgo(Object.keys(timeframe), timeframe, timestamp, now);