diff --git a/index.d.ts b/index.d.ts
index 962d16a5c..996d000e1 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -3,23 +3,95 @@ export interface Skus {
   android: string[];
 }
 
-export interface ItemDescription {
-  type?: string; // only on Android
+export type ItemDescription = ItemDescriptionAndroid | ItemDescriptionIos 
+
+export type SkuTypeAndroid = 'INAPP' | 'SUBS'
+
+export interface ItemDescriptionAndroid {
+  type: SkuTypeAndroid;
+  productId: string;
+  title: string;
+  description: string;
+  price: string;
+  currency: string;
+  price_currency: string;
+  localizedPrice: string;
+}
+
+export interface ItemDescriptionIos {
   productId: string;
   title: string;
   description: string;
-  price: number | string; // number on iOS, string on Android
+  price: number;
   currency: string;
-  localizedPrice: string; // only on Android
 }
 
+/**
+ * Get purchasable items in array.
+ * @param {Skus} skus
+ * @returns {Promise<ItemDescription[]>} Promise of the array of purchasable items
+ */
 export function getItems(skus: Skus) : Promise<ItemDescription[]>;
+
+/**
+ * Get subscription items.
+ * @param {Skus} skus
+ * @returns {Promise<ItemDescription[]>} Promise of the array of subscription items
+ */
 export function getSubscribeItems(skus: Skus) : Promise<ItemDescription[]>;
+
+/**
+ * Purchase item.
+ * @param {string} item
+ * @returns {Promise<string>} Promise of the ...
+ */
 export function buyItem(item: string) : Promise<string>;
+
+/**
+ * Buy subscription item.
+ * @param {string} item
+ * @returns {Promise<any>} Promise of the ...
+ */
 export function buySubscribeItem(item: string) : Promise<any>;
+
+/** 
+ * Refresh all items to make them available to buy again.
+ * @returns {Promise<any>} Promise of the refreshed items.
+*/
 export function refreshAllItems() : Promise<any>;
 
-export function prepareAndroid() : Promise<string> | void;
-export function refreshPurchaseItemsAndroid(type: string | null) : void;
-export function getPurchasedItemsAndroid(type: string | null) : Promise<any> | void;
-export function consumeItemAndroid(token: string) : Promise<string> | void;
\ No newline at end of file
+/**
+ * Prepare IAP module for android. Should be called in android before using any methods in RNIap.
+ * @returns {Promise<string>|null} Promise of the messages, or null if the Platform is not Android.
+ */
+export function prepareAndroid() : Promise<string> | null;
+
+/**
+ * Refresh purchased items for android.
+ * What is different from refreshAllItems is that this method can get parameter to refresh INAPP items or SUBS items.
+ * @param {SkuTypeAndroid} [type] `'INAPP'` or `'SUBS'`
+ */
+export function refreshPurchaseItemsAndroid(type?: SkuTypeAndroid) : void;
+
+/**
+ * Get purchased items for android.
+ * This method also gets parameter to refresh INAPP items or SUBS items.
+ * @param {SkuTypeAndroid} [type] `'INAPP'` or `'SUBS'`
+ * @returns {Promise<any>|null} Promise of the purchased items, or null if the Platform is not Android.
+ */
+export function getPurchasedItemsAndroid(type?: SkuTypeAndroid) : Promise<any> | null;
+
+/**
+ * Consume item for android.
+ * After buying some item from consumable item in android, you can use this method to consume it.
+ * Therefore you can purchase the item again.
+ * @param {string} token
+ * @returns {boolean|null} Promise of the status of billing response, or null if the Platform is not Android.
+ */
+export function consumeItemAndroid(token: string) : Promise<boolean> | null;
+
+/**
+ * TODO: Add description here.
+ * @returns {Promise<Object>|null} Promise of somthing, or null if the Platform is not iOS
+ */
+export function restoreIosNonConsumableProducts() : Promise<Object> | null;
diff --git a/index.js b/index.js
index 161282aee..79934e5e0 100644
--- a/index.js
+++ b/index.js
@@ -178,6 +178,8 @@ export const prepareAndroid = () => {
         resolve(msg);
       });
     });
+  } else {
+    return null;
   }
 };
 
@@ -209,6 +211,8 @@ export const getPurchasedItemsAndroid = (type: string | null) => {
         }
       );
     });
+  } else {
+    return null;
   }
 };
 
@@ -226,6 +230,8 @@ export const consumeItemAndroid = (token: string) => {
         resolve(success);
       });
     });
+  } else {
+    return null;
   }
 };