diff --git a/types/co/co-tests.ts b/types/co/co-tests.ts
index b610b280aa..7348f77394 100644
--- a/types/co/co-tests.ts
+++ b/types/co/co-tests.ts
@@ -1,21 +1,21 @@
 import co = require('co');
 
-function* gen(num: number, str: string, arr: number[], obj: object, fun: () => void){
+function* gen(num: number, str: string, arr: number[], obj: object, fun: () => void) {
     return num;
 }
 
-co(gen, 42, 'forty-two', [42], { value: 42 }, function () {})
+co(gen, 42, 'forty-two', [42], { value: 42 }, () => {})
     .then((num: number) => {}, (err: Error) => {})
     .catch((err: Error) => {});
 
-co.default(gen, 42, 'forty-two', [42], { value: 42 }, function () {})
+co.default(gen, 42, 'forty-two', [42], { value: 42 }, () => {})
     .then((num: number) => {}, (err: Error) => {})
     .catch((err: Error) => {});
 
-co.co(gen, 42, 'forty-two', [42], { value: 42 }, function () {})
+co.co(gen, 42, 'forty-two', [42], { value: 42 }, () => {})
     .then((num: number) => {}, (err: Error) => {})
     .catch((err: Error) => {});
 
-co.wrap(gen)(42, 'forty-two', [42], { value: 42 }, function () {})
+co.wrap(gen)(42, 'forty-two', [42], { value: 42 }, () => {})
     .then((num: number) => {}, (err: Error) => {})
     .catch((err: Error) => {});
diff --git a/types/co/index.d.ts b/types/co/index.d.ts
index 0be8976a5f..59fafd2495 100644
--- a/types/co/index.d.ts
+++ b/types/co/index.d.ts
@@ -2,6 +2,7 @@
 // Project: https://github.com/tj/co#readme
 // Definitions by: My Self <https://github.com/doniyor2109>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 3.1
 
 type ExtractType<T> = T extends IterableIterator<infer R> ? R : never;