fix: Fix set Month Year error in last day of the month

Author: iamkun

src/index.js

@@ -218,7 +218,12 @@ class Dayjs {
     }[unit]
     const arg = unit === C.D ? this.$D + (int - this.$W) : int
 
-    if (this.$d[name]) this.$d[name](arg)
+    if (unit === C.M || unit === C.Y) {
+      const date = this.clone().set(C.DATE, 1)
+      date.$d[name](arg)
+      date.init()
+      this.$d = date.set(C.DATE, Math.min(this.$D, date.daysInMonth())).toDate()
+    } else if (name) this.$d[name](arg)
 
     this.init()
     return this
@@ -231,21 +236,16 @@ class Dayjs {
   add(number, units) {
     number = Number(number) // eslint-disable-line no-param-reassign
     const unit = Utils.p(units)
-    const instanceFactory = (u, n) => {
-      // clone is for badMutable plugin
-      const date = this.clone().set(C.DATE, 1).set(u, n + number)
-      return date.set(C.DATE, Math.min(this.$D, date.daysInMonth()))
-    }
     const instanceFactorySet = (n) => {
       const date = new Date(this.$d)
       date.setDate(date.getDate() + (n * number))
       return Utils.w(date, this)
     }
     if (unit === C.M) {
-      return instanceFactory(C.M, this.$M)
+      return this.set(C.M, this.$M + number)
     }
     if (unit === C.Y) {
-      return instanceFactory(C.Y, this.$y)
+      return this.set(C.Y, this.$y + number)
     }
     if (unit === C.D) {
       return instanceFactorySet(1)

test/get-set.test.js

@@ -90,6 +90,21 @@ it('Set Millisecond', () => {
   expect(dayjs().set('millisecond', 999).valueOf()).toBe(moment().set('millisecond', 999).valueOf())
 })
 
+it('Set Month and Year in last day of month', () => {
+  // 2011-07-31 -> 2011-02-28
+  const origin = dayjs('2011-07-31T14:48:00.000Z')
+  const setMonth = origin.set('month', 1)
+  expect(setMonth.month()).toBe(1)
+  expect(origin.date()).toBe(31)
+  expect(setMonth.date()).toBe(28)
+  // 2000-02-29 -> 2001-02-28
+  const origin2 = dayjs('2000-02-29T14:48:00.000Z')
+  const setYear = origin2.set('year', 2001)
+  expect(setYear.month()).toBe(1)
+  expect(origin2.date()).toBe(29)
+  expect(setYear.date()).toBe(28)
+})
+
 it('Set Unknown String', () => {
   const newDate = dayjs().set('Unknown String', 1)
   expect(newDate.valueOf())