diff --git a/src/locale.ts b/src/locale.ts
index d5efe7987..5aaf6ed0b 100644
--- a/src/locale.ts
+++ b/src/locale.ts
@@ -52,6 +52,16 @@ export interface BooleanLocale {
   isValue?: Message;
 }
 
+export interface LocaleObject {
+  mixed?: MixedLocale;
+  string?: StringLocale;
+  number?: NumberLocale;
+  date?: DateLocale;
+  boolean?: BooleanLocale;
+  object?: ObjectLocale;
+  array?: ArrayLocale;
+}
+
 export let mixed: Required<MixedLocale> = {
   default: '${path} is invalid',
   required: '${path} is a required field',
diff --git a/src/setLocale.js b/src/setLocale.ts
similarity index 59%
rename from src/setLocale.js
rename to src/setLocale.ts
index ce0c3850e..171ea95ab 100644
--- a/src/setLocale.js
+++ b/src/setLocale.ts
@@ -1,6 +1,6 @@
-import locale from './locale';
+import locale, { LocaleObject } from './locale';
 
-export default function setLocale(custom) {
+export default function setLocale(custom: LocaleObject) {
   Object.keys(custom).forEach(type => {
     Object.keys(custom[type]).forEach(method => {
       locale[type][method] = custom[type][method];