fix: Add isoWeeksInYear plugin

src/plugin/isoWeeksInYear/index.js

@@ -0,0 +1,12 @@
+export default (o, c) => {
+  const proto = c.prototype
+  proto.isoWeeksInYear = function () {
+    const isLeapYear = this.isLeapYear()
+    const last = this.endOf('y')
+    const day = last.day()
+    if (day === 4 || (isLeapYear && day === 5)) {
+      return 53
+    }
+    return 52
+  }
+}

test/plugin/isoWeeksInYear.test.js

@@ -0,0 +1,30 @@
+import MockDate from 'mockdate'
+import dayjs from '../../src'
+import isoWeeksInYear from '../../src/plugin/isoWeeksInYear'
+import isLeapYear from '../../src/plugin/isLeapYear'
+
+dayjs.extend(isoWeeksInYear)
+dayjs.extend(isLeapYear)
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+})
+
+it('isoWeeksInYear', () => {
+  expect(dayjs('2004').isoWeeksInYear()).toBe(53)
+  expect(dayjs('2005').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2006').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2007').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2008').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2009').isoWeeksInYear()).toBe(53)
+  expect(dayjs('2010').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2011').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2012').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2013').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2014').isoWeeksInYear()).toBe(52)
+  expect(dayjs('2015').isoWeeksInYear()).toBe(53)
+})

types/plugin/isoWeeksInYear.d.ts

@@ -0,0 +1,10 @@
+import { PluginFunc } from 'dayjs'
+
+declare const plugin: PluginFunc
+export = plugin
+
+declare module 'dayjs' {
+  interface Dayjs {
+    isoWeeksInYear(): number
+  }
+}