摘要：前言上一篇源碼解析三構造函數介紹了的源碼中的函數。接下來，本篇將解讀一下中的類。首先，我們看的構造函數，該構造函數調用了實例方法，傳入參數在上一篇有講到。下一篇源碼解析五插件詳解

上一篇 dayjs 源碼解析（三）（dayjs 構造函數）介紹了 dayjs 的源碼中的 dayjs 函數。接下來，本篇將解讀一下 index.js 中的 Dayjs 類。

class Dayjs {
  constructor(cfg) {
    this.parse(cfg) // for plugin
  }

  parse(cfg) {
    this.$d = parseDate(cfg.date)
    this.init(cfg)
  }

  init(cfg) {
    this.$y = this.$d.getFullYear() // 年
    this.$M = this.$d.getMonth() // 月（注意從 0 開始）
    this.$D = this.$d.getDate() // 日
    this.$W = this.$d.getDay() // 周幾（0 表示周日）
    this.$H = this.$d.getHours() // 時
    this.$m = this.$d.getMinutes() // 分
    this.$s = this.$d.getSeconds() // 秒
    this.$ms = this.$d.getMilliseconds() // 毫秒
    this.$L = this.$L || parseLocale(cfg.locale, null, true) || L // 語言，默認為 "en"
  }

  // 下面的都是實例方法
  $utils() {
    return Utils
  }

  isValid() {
    return !(this.$d.toString() === "Invalid Date")
  }

  isLeapYear() {
    return ((this.$y % 4 === 0) && (this.$y % 100 !== 0)) || (this.$y % 400 === 0)
  }

  $compare(that) {
    return this.valueOf() - dayjs(that).valueOf()
  }

  isSame(that) {
    return this.$compare(that) === 0
  }

  isBefore(that) {
    return this.$compare(that) < 0
  }

  isAfter(that) {
    return this.$compare(that) > 0
  }

  year() {
    return this.$y
  }

  month() {
    return this.$M
  }

  day() {
    return this.$W
  }

  date() {
    return this.$D
  }

  hour() {
    return this.$H
  }

  minute() {
    return this.$m
  }

  second() {
    return this.$s
  }

  millisecond() {
    return this.$ms
  }

  unix() {
    return Math.floor(this.valueOf() / 1000)
  }

  valueOf() {
    // timezone(hour) * 60 * 60 * 1000 => ms
    return this.$d.getTime()
  }

  startOf(units, startOf) { // startOf -> endOf
    const isStartOf = !Utils.isUndefined(startOf) ? startOf : true
    const unit = Utils.prettyUnit(units)
    const instanceFactory = (d, m) => {
      const ins = wrapper(new Date(this.$y, m, d), this)
      return isStartOf ? ins : ins.endOf(C.D)
    }
    const instanceFactorySet = (method, slice) => {
      const argumentStart = [0, 0, 0, 0]
      const argumentEnd = [23, 59, 59, 999]
      return wrapper(this.toDate()[method].apply( // eslint-disable-line prefer-spread
        this.toDate(),
        isStartOf ? argumentStart.slice(slice) : argumentEnd.slice(slice)
      ), this)
    }
    switch (unit) {
      case C.Y:
        return isStartOf ? instanceFactory(1, 0) :
          instanceFactory(31, 11)
      case C.M:
        return isStartOf ? instanceFactory(1, this.$M) :
          instanceFactory(0, this.$M + 1)
      case C.W:
        return isStartOf ? instanceFactory(this.$D - this.$W, this.$M) :
          instanceFactory(this.$D + (6 - this.$W), this.$M)
      case C.D:
      case C.DATE:
        return instanceFactorySet("setHours", 0)
      case C.H:
        return instanceFactorySet("setMinutes", 1)
      case C.MIN:
        return instanceFactorySet("setSeconds", 2)
      case C.S:
        return instanceFactorySet("setMilliseconds", 3)
      default:
        return this.clone()
    }
  }

  endOf(arg) {
    return this.startOf(arg, false)
  }

  $set(units, int) { // private set
    const unit = Utils.prettyUnit(units)
    switch (unit) {
      case C.DATE:
        this.$d.setDate(int)
        break
      case C.M:
        this.$d.setMonth(int)
        break
      case C.Y:
        this.$d.setFullYear(int)
        break
      case C.H:
        this.$d.setHours(int)
        break
      case C.MIN:
        this.$d.setMinutes(int)
        break
      case C.S:
        this.$d.setSeconds(int)
        break
      case C.MS:
        this.$d.setMilliseconds(int)
        break
      default:
        break
    }
    this.init()
    return this
  }


  set(string, int) {
    return this.clone().$set(string, int)
  }

  add(number, units) {
    number = Number(number) // eslint-disable-line no-param-reassign
    const unit = Utils.prettyUnit(units)
    const instanceFactory = (u, n) => {
      const date = this.set(C.DATE, 1).set(u, n + number)
      return date.set(C.DATE, Math.min(this.$D, date.daysInMonth()))
    }
    if (unit === C.M) {
      return instanceFactory(C.M, this.$M)
    }
    if (unit === C.Y) {
      return instanceFactory(C.Y, this.$y)
    }
    let step
    switch (unit) {
      case C.MIN:
        step = C.MILLISECONDS_A_MINUTE
        break
      case C.H:
        step = C.MILLISECONDS_A_HOUR
        break
      case C.D:
        step = C.MILLISECONDS_A_DAY
        break
      case C.W:
        step = C.MILLISECONDS_A_WEEK
        break
      case C.S:
        step = C.MILLISECONDS_A_SECOND
        break
      default: // ms
        step = 1
    }
    const nextTimeStamp = this.valueOf() + (number * step)
    return wrapper(nextTimeStamp, this)
  }

  subtract(number, string) {
    return this.add(number * -1, string)
  }

  format(formatStr) {
    const str = formatStr || C.FORMAT_DEFAULT
    const zoneStr = Utils.padZoneStr(this.$d.getTimezoneOffset())
    const locale = this.$locale()
    const {
      weekdays, months
    } = locale
    const getShort = (arr, index, full, length) => (
      (arr && arr[index]) || full[index].substr(0, length)
    )
    return str.replace(C.REGEX_FORMAT, (match) => {
      if (match.indexOf("[") > -1) return match.replace(/[|]/g, "")
      switch (match) {
        case "YY":
          return String(this.$y).slice(-2)
        case "YYYY":
          return String(this.$y)
        case "M":
          return String(this.$M + 1)
        case "MM":
          return Utils.padStart(this.$M + 1, 2, "0")
        case "MMM":
          return getShort(locale.monthsShort, this.$M, months, 3)
        case "MMMM":
          return months[this.$M]
        case "D":
          return String(this.$D)
        case "DD":
          return Utils.padStart(this.$D, 2, "0")
        case "d":
          return String(this.$W)
        case "dd":
          return getShort(locale.weekdaysMin, this.$W, weekdays, 2)
        case "ffffd":
          return getShort(locale.weekdaysShort, this.$W, weekdays, 3)
        case "ffffdd":
          return weekdays[this.$W]
        case "H":
          return String(this.$H)
        case "HH":
          return Utils.padStart(this.$H, 2, "0")
        case "h":
        case "hh":
          if (this.$H === 0) return 12
          return Utils.padStart(this.$H < 13 ? this.$H : this.$H - 12, match === "hh" ? 2 : 1, "0")
        case "a":
          return this.$H < 12 ? "am" : "pm"
        case "A":
          return this.$H < 12 ? "AM" : "PM"
        case "m":
          return String(this.$m)
        case "mm":
          return Utils.padStart(this.$m, 2, "0")
        case "s":
          return String(this.$s)
        case "ss":
          return Utils.padStart(this.$s, 2, "0")
        case "SSS":
          return Utils.padStart(this.$ms, 3, "0")
        case "Z":
          return zoneStr
        default: // "ZZ"
          return zoneStr.replace(":", "")
      }
    })
  }

  diff(input, units, float) {
    const unit = Utils.prettyUnit(units)
    const that = dayjs(input)
    const diff = this - that
    let result = Utils.monthDiff(this, that)
    switch (unit) {
      case C.Y:
        result /= 12
        break
      case C.M:
        break
      case C.Q:
        result /= 3
        break
      case C.W:
        result = diff / C.MILLISECONDS_A_WEEK
        break
      case C.D:
        result = diff / C.MILLISECONDS_A_DAY
        break
      case C.H:
        result = diff / C.MILLISECONDS_A_HOUR
        break
      case C.MIN:
        result = diff / C.MILLISECONDS_A_MINUTE
        break
      case C.S:
        result = diff / C.MILLISECONDS_A_SECOND
        break
      default: // milliseconds
        result = diff
    }
    return float ? result : Utils.absFloor(result)
  }

  daysInMonth() {
    return this.endOf(C.M).$D
  }

  $locale() { // get locale object
    return Ls[this.$L]
  }

  locale(preset, object) {
    const that = this.clone()
    that.$L = parseLocale(preset, object, true)
    return that
  }

  clone() {
    return wrapper(this.toDate(), this)
  }

  toDate() {
    return new Date(this.$d)
  }

  toArray() {
    return [
      this.$y,
      this.$M,
      this.$D,
      this.$H,
      this.$m,
      this.$s,
      this.$ms
    ]
  }

  toJSON() {
    return this.toISOString()
  }

  toISOString() {
    // ie 8 return
    // new Dayjs(this.valueOf() + this.$d.getTimezoneOffset() * 60000)
    // .format("YYYY-MM-DDTHH:mm:ss.SSS[Z]")
    return this.toDate().toISOString()
  }

  toObject() {
    return {
      years: this.$y,
      months: this.$M,
      date: this.$D,
      hours: this.$H,
      minutes: this.$m,
      seconds: this.$s,
      milliseconds: this.$ms
    }
  }

  toString() {
    return this.$d.toUTCString()
  }
}

上面是類 Dayjs 中的所有代碼。

首先，我們看 Dayjs 的構造函數，該構造函數調用了 Dayjs 實例方法 parse ，傳入 cfg 參數（cfg 在 上一篇 有講到）。

在 parse 實例方法中，會使用 parseDate 函數，傳入 cfg.date 屬性，得到 JavaScript 中原生的 Date 對象，且將 Date 對象保存在 Dayjs 實例屬性（實例中的屬性）中。然后再調用實例方法 init 來初始化 Dayjs 實例的其他屬性。

在 init 實例方法中，使用實例屬性 $d（為 JavaScript 原生 Date 對象）來獲取 年、月、日、周幾、時、分、秒、毫秒，且保存在實例中。并且調用 parseLocale 函數來獲取語言，賦值給 Dayjs 實例的 $L 屬性。

再下面的代碼，便是實例方法了，即在 第一篇 所講的 api 都是實例方法，源碼都可以在這里找到。

在 Dayjs 類中 add、subtract 等方法，都會返回一個新的 Dayjs 實例，是通過使用 Dayjs 類中的 clone() 方法實現的。所以，通過這個 clone() 方法，實現了 dayjs 的 immutable 特性。

下一篇：dayjs 源碼解析（五）（dayjs 插件詳解）