fix: .format add padding to 'YYYY' (#2231)

src/index.js

@@ -280,7 +280,7 @@ class Dayjs {
 
     const matches = {
       YY: String(this.$y).slice(-2),
-      YYYY: this.$y,
+      YYYY: Utils.s(this.$y, 4, '0'),
       M: $M + 1,
       MM: Utils.s($M + 1, 2, '0'),
       MMM: getShort(locale.monthsShort, $M, months, 3),

test/display.test.js

@@ -260,3 +260,15 @@ it('As JSON -> toJSON', () => {
 it('As ISO 8601 String -> toISOString e.g. 2013-02-04T22:44:30.652Z', () => {
   expect(dayjs().toISOString()).toBe(moment().toISOString())
 })
+
+it('Year 1 formatted with YYYY should pad with zeroes', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1) // Required because 0-99 are parsed as 19xx in JS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date#year
+  expect(dayjs(date).format('YYYY')).toBe('0001')
+})
+
+it('Year 1 formatting matches moment format', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1)
+  expect(dayjs(date).format('YYYY')).toBe(moment(date).format('YYYY'))
+})