前言

上一篇 介绍了 dayjs 的源码中的 dayjs 函数。接下来，本篇将解读一下 index.js 中的 Dayjs 类。

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.$d.getMonth() // 月（注意从 0 开始）    this.$D = this.$d.getDate() // 日    this.$W = this.$d.getDay() // 周几（0 表示周日）    this.$H = this.$d.getHours() // 时    this.$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 // 语言，默认为 'en'  }  // 下面的都是实例方法  $utils() {    return Utils  }  isValid() {    return !(this.$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 'ddd':          return getShort(locale.weekdaysShort, this.$W, weekdays, 3)        case 'dddd':          return weekdays[this.$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()  }}

上面是类 Dayjs 中的所有代码。

首先，我们看 Dayjs 的构造函数，该构造函数调用了 Dayjs 实例方法 parse ，传入 cfg 参数（cfg 在 有讲到）。

在 parse 实例方法中，会使用 parseDate 函数，传入 cfg.date 属性，得到 JavaScript 中原生的 Date 对象，且将 Date 对象保存在 Dayjs 实例属性（实例中的属性）中。然后再调用实例方法 init 来初始化 Dayjs 实例的其他属性。

在 init 实例方法中，使用实例属性 $d（为 JavaScript 原生 Date 对象）来获取 年、月、日、周几、时、分、秒、毫秒，且保存在实例中。并且调用 parseLocale 函数来获取语言，赋值给 Dayjs 实例的 $L 属性。

再下面的代码，便是实例方法了，即在 所讲的 api 都是实例方法，源码都可以在这里找到。

在 Dayjs 类中 add、subtract 等方法，都会返回一个新的 Dayjs 实例，是通过使用 Dayjs 类中的 clone() 方法实现的。所以，通过这个 clone() 方法，实现了 dayjs 的 immutable 特性。

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