Fix issues with adding/subtracting durations from Dayjs objects

src/plugin/duration/index.js

@@ -179,11 +179,13 @@ export default (option, Dayjs, dayjs) => {
   dayjs.isDuration = isDuration
 
   const oldAdd = Dayjs.prototype.add
-  Dayjs.prototype.add = function (addition, units) {
-    if (isDuration(addition)) {
-      addition = addition.asMilliseconds()
-      units = 'ms'
-    }
-    return oldAdd.bind(this)(addition, units)
+  const oldSubtract = Dayjs.prototype.subtract
+  Dayjs.prototype.add = function (value, unit) {
+    if (isDuration(value)) value = value.asMilliseconds()
+    return oldAdd.bind(this)(value, unit)
+  }
+  Dayjs.prototype.subtract = function (value, unit) {
+    if (isDuration(value)) value = value.asMilliseconds()
+    return oldSubtract.bind(this)(value, unit)
   }
 }

test/plugin/duration.test.js

@@ -139,7 +139,7 @@ describe('Add', () => {
   expect(a.add({ days: 5 }).days()).toBe(6)
 })
 
-describe('Add duration', () => {
+test('Add duration', () => {
   const a = dayjs('2020-10-01')
   const days = dayjs.duration(2, 'days')
   expect(a.add(days).format('YYYY-MM-DD')).toBe('2020-10-03')
@@ -151,6 +151,11 @@ describe('Subtract', () => {
   expect(a.subtract(b).days()).toBe(1)
 })
 
+test('Subtract duration', () => {
+  const a = dayjs('2020-10-20')
+  const days = dayjs.duration(2, 'days')
+  expect(a.subtract(days).format('YYYY-MM-DD')).toBe('2020-10-18')
+})
 
 describe('Seconds', () => {
   expect(dayjs.duration(500).seconds()).toBe(0)

types/index.d.ts

@@ -226,7 +226,7 @@ declare namespace dayjs {
      *
      * Docs: https://day.js.org/docs/en/manipulate/add
      */
-    add(value: number, unit: OpUnitType): Dayjs
+    add(value: number, unit?: OpUnitType): Dayjs
     /**
      * Returns a cloned Day.js object with a specified amount of time subtracted.
      * ```
@@ -236,7 +236,7 @@ declare namespace dayjs {
      *
      * Docs: https://day.js.org/docs/en/manipulate/subtract
      */
-    subtract(value: number, unit: OpUnitType): Dayjs
+    subtract(value: number, unit?: OpUnitType): Dayjs
     /**
      * Returns a cloned Day.js object and set it to the start of a unit of time.
      * ```

types/plugin/duration.d.ts

@@ -56,6 +56,11 @@ declare namespace plugin {
 }
 
 declare module 'dayjs' {
+  interface Dayjs {
+    add(value: plugin.Duration): Dayjs
+    subtract(value: plugin.Duration): Dayjs
+  }
+
   export function duration(input?: plugin.DurationInputType , unit?: string): plugin.Duration
   export function isDuration(d: any): d is plugin.Duration
 }