diff --git a/test/index.d.test.ts b/test/index.d.test.ts
index 6a2f2eb2b..47ec60b5e 100644
--- a/test/index.d.test.ts
+++ b/test/index.d.test.ts
@@ -71,3 +71,9 @@ dayjs().isSame(dayjs())
 dayjs().isAfter(dayjs())
 
 dayjs('2000-01-01').isLeapYear()
+
+dayjs.extend((o, c, d) => {
+  o.locale.trim()
+  new c().unix() // eslint-disable-line new-cap
+  d().unix()
+})
diff --git a/types/index.d.ts b/types/index.d.ts
index b568b5656..cec7a4be8 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -1,9 +1,9 @@
 export = dayjs;
 
-declare function dayjs (config?: dayjs.ConfigType, option?: dayjs.OptionType): dayjs.Dayjs
+declare function dayjs (date?: dayjs.DateType, option?: dayjs.OptionType): dayjs.Dayjs
 
 declare namespace dayjs {
-  export type ConfigType = string | number | Date | Dayjs
+  export type DateType = string | number | Date | Dayjs
 
   export type OptionType = { locale: string }
 
@@ -20,7 +20,7 @@ declare namespace dayjs {
   }
 
   class Dayjs {
-    constructor (config?: ConfigType)
+    constructor (date?: DateType)
 
     clone(): Dayjs
 
@@ -54,7 +54,7 @@ declare namespace dayjs {
 
     format(template?: string): string
 
-    diff(dayjs: Dayjs, unit: UnitType, float?: boolean): number
+    diff(date: DateType, unit: UnitType, float?: boolean): number
 
     valueOf(): number
 
@@ -74,22 +74,22 @@ declare namespace dayjs {
 
     toString(): string
 
-    isBefore(dayjs: Dayjs): boolean
+    isBefore(date: DateType): boolean
 
-    isSame(dayjs: Dayjs): boolean
+    isSame(date: DateType): boolean
 
-    isAfter(dayjs: Dayjs): boolean
+    isAfter(date: DateType): boolean
 
     isLeapYear(): boolean
 
     locale(arg1: any, arg2?: any): Dayjs
   }
 
-  export type PluginFunc = (option: ConfigType, d1: Dayjs, d2: Dayjs) => void
+  export type PluginFunc = (option: any, c: typeof Dayjs, d: typeof dayjs) => void
 
-  export function extend(plugin: PluginFunc, option?: ConfigType): Dayjs
+  export function extend(plugin: PluginFunc, option?: any): Dayjs
 
-  export function locale(arg1: any, arg2?: any): string
+  export function locale(preset: string | { name: string, [key: string]: any }, object?: { [key: string]: any }, isLocal?: boolean): string
 
   export function isDayjs(d: any): d is Dayjs
 }