fix: keep sign when zero-padding negative years in format('YYYY') (#2722)

src/utils.js

@@ -2,8 +2,10 @@ import * as C from './constant'
 
 const padStart = (string, length, pad) => {
   const s = String(string)
-  if (!s || s.length >= length) return string
-  return `${Array((length + 1) - s.length).join(pad)}${string}`
+  const neg = s[0] === '-'
+  const abs = neg ? s.slice(1) : s
+  if (!abs || abs.length >= length) return string
+  return `${neg ? '-' : ''}${Array((length + 1) - abs.length).join(pad)}${abs}`
 }
 
 const padZoneStr = (instance) => {

test/display.test.js

@@ -263,6 +263,17 @@ it('As ISO 8601 String -> toISOString e.g. 2013-02-04T22:44:30.652Z', () => {
   expect(dayjs().toISOString()).toBe(moment().toISOString())
 })
 
+it('Negative years formatted with YYYY keep the sign and pad with zeroes', () => {
+  [[-1, '-0001'], [-10, '-0010'], [-100, '-0100'], [-1000, '-1000']].forEach(([year, expected]) => {
+    const date = new Date(0)
+    date.setFullYear(year, 5, 15) // mid-year so the local timezone cannot shift the year
+    expect(dayjs(date).format('YYYY')).toBe(expected)
+    expect(dayjs(date).format('YYYY')).toBe(moment(date).format('YYYY'))
+  })
+  expect(dayjs('-000001-06-15').format('YYYY')).toBe('-0001')
+  expect(dayjs('-000001-06-15').year()).toBe(-1)
+})
+
 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

test/utils.test.js

@@ -41,4 +41,14 @@ it('PadZoneStr', () => {
 it('PadStart', () => {
   expect(padStart(1, 2, '0')).toBe('01')
   expect(padStart(0, 2, '0')).toBe('00')
+  expect(padStart(123, 2, '0')).toBe(123)
+  expect(padStart('', 2, '0')).toBe('')
+})
+
+it('PadStart keeps the sign of negative numbers', () => {
+  expect(padStart(-1, 4, '0')).toBe('-0001')
+  expect(padStart(-10, 4, '0')).toBe('-0010')
+  expect(padStart(-100, 4, '0')).toBe('-0100')
+  expect(padStart(-1000, 4, '0')).toBe(-1000)
+  expect(padStart(-10000, 4, '0')).toBe(-10000)
 })