fix: Add weekday (locale aware day of week) plugin

fix #559

src/plugin/weekday/index.js

@@ -0,0 +1,13 @@
+export default (o, c) => {
+  const proto = c.prototype
+  proto.weekday = function (input) {
+    const weekStart = this.$locale().weekStart || 0
+    const { $W } = this
+    const weekday = ($W < weekStart ? $W + 7 : $W) - weekStart
+    if (this.$utils().u(input)) {
+      return weekday
+    }
+    return this.subtract(weekday, 'day').add(input, 'day')
+  }
+}
+

test/plugin/weekday.test.js

@@ -0,0 +1,43 @@
+import MockDate from 'mockdate'
+import moment from 'moment'
+import dayjs from '../../src'
+import weekday from '../../src/plugin/weekday'
+import '../../src/locale/zh-cn'
+import '../../src/locale/ar'
+
+dayjs.extend(weekday)
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+  moment.locale('en')
+  dayjs.locale('en')
+})
+
+it('Sunday is the first day of the week', () => {
+  expect(dayjs().weekday()).toBe(moment().weekday())
+  expect(dayjs().weekday(0).date()).toBe(moment().weekday(0).date())
+  expect(dayjs().weekday(-7).format()).toBe(moment().weekday(-7).format())
+  expect(dayjs().weekday(7).format()).toBe(moment().weekday(7).format())
+})
+
+it('Monday is the first day of the week', () => {
+  moment.locale('zh-cn')
+  dayjs.locale('zh-cn')
+  expect(dayjs().weekday()).toBe(moment().weekday())
+  expect(dayjs().weekday(0).date()).toBe(moment().weekday(0).date())
+  expect(dayjs().weekday(-7).format()).toBe(moment().weekday(-7).format())
+  expect(dayjs().weekday(7).format()).toBe(moment().weekday(7).format())
+})
+
+it('Saturday is the first day of the week', () => {
+  moment.locale('ar')
+  dayjs.locale('ar')
+  expect(dayjs().weekday()).toBe(moment().weekday())
+  expect(dayjs().weekday(0).date()).toBe(moment().weekday(0).date())
+  expect(dayjs().weekday(-7).valueOf()).toBe(moment().weekday(-7).valueOf())
+  expect(dayjs().weekday(7).valueOf()).toBe(moment().weekday(7).valueOf())
+})

types/plugin/weekday.d.ts

@@ -0,0 +1,12 @@
+import { PluginFunc } from 'dayjs'
+
+declare const plugin: PluginFunc
+export = plugin
+
+declare module 'dayjs' {
+  interface Dayjs {
+    weekday(): number
+
+    weekday(value: number): Dayjs
+  }
+}