fix: API .locale() with no argument should return current locale name string

Author: iamkun

src/index.js

@@ -346,6 +346,7 @@ class Dayjs {
   }
 
   locale(preset, object) {
+    if (!preset) return this.$L
     const that = this.clone()
     that.$L = parseLocale(preset, object, true)
     return that

test/locale.test.js

@@ -1,4 +1,5 @@
 import MockDate from 'mockdate'
+import moment from 'moment'
 import dayjs from '../src'
 import es from '../src/locale/es'
 
@@ -44,6 +45,17 @@ it('set global locale', () => {
     .toBe('Saturday 28, April')
 })
 
+it('get instance locale name', () => {
+  expect(dayjs().locale()).toBe('en')
+  expect(dayjs().locale()).toBe(moment().locale())
+  expect(dayjs().locale('es').locale()).toBe('es')
+  expect(dayjs().locale('es').locale()).toBe(moment().locale('es').locale())
+  dayjs.locale(es)
+  moment.locale('es')
+  expect(dayjs().locale()).toBe('es')
+  expect(dayjs().locale()).toBe(moment().locale())
+})
+
 it('immutable instance locale', () => {
   dayjs.locale('en')
   const origin = dayjs('2018-4-28')

types/index.d.ts

@@ -85,6 +85,8 @@ declare namespace dayjs {
 
     isAfter(date: ConfigType, unit?: OpUnitType): boolean
 
+    locale(): string
+
     locale(preset: string | { name: string, [key: string]: any }, object?: { [key: string]: any }): Dayjs
   }