Improve typings for 'toJSONDeep' function (#1995)

Author: IvanGoncharov

src/language/__tests__/toJSONDeep.js

@@ -6,7 +6,7 @@ import isObjectLike from '../../jsutils/isObjectLike';
  * Deeply transforms an arbitrary value to a JSON-safe value by calling toJSON
  * on any nested value which defines it.
  */
-export default function toJSONDeep<T>(value: T): T {
+export default function toJSONDeep(value: mixed): mixed {
   if (!isObjectLike(value)) {
     return value;
   }
@@ -20,7 +20,7 @@ export default function toJSONDeep<T>(value: T): T {
     return value.map(toJSONDeep);
   }
 
-  const result: any = {};
+  const result = Object.create(null);
   for (const prop of Object.keys(value)) {
     result[prop] = toJSONDeep(value[prop]);
   }