Add async and mapError to TS definition, #304

index.d.ts

@@ -58,6 +58,13 @@ declare namespace lmdb {
 		**/
 		getMany(ids: K[], callback?: (error: any, values: V[]) => any): Promise<(V | undefined)[]>
 
+		/**
+		 * @experimental Asynchronously get a value by id.
+		 * @param ids
+		 * @param callback
+		 */
+		getAsync(id: K, options?: GetOptions, callback?: (value: V) => any): Promise<(V | undefined)[]>
+
 		/**
 		* Store the provided value, using the provided id/key
 		* @param id The key for the entry
@@ -439,6 +446,7 @@ declare namespace lmdb {
 		filter(callback: (entry: T) => any): RangeIterable<T>
 		[Symbol.iterator]() : Iterator<T>
 		forEach(callback: (entry: T) => any): void
+		mapError<U>(callback: (error: Error) => U): RangeIterable<U>
 		onDone?: Function
 		asArray: T[]
 	}

read.js

@@ -818,7 +818,7 @@ export function addReadMethods(
 
 		getMany(keys, callback) {
 			// this is an asynchronous get for multiple keys. It actually works by prefetching asynchronously,
-			// allowing a separate to absorb the potentially largest cost: hard page faults (and disk I/O).
+			// allowing a separate thread/task to absorb the potentially largest cost: hard page faults (and disk I/O).
 			// And then we just do standard sync gets (to deserialized data) to fulfil the callback/promise
 			// once the prefetch occurs
 			let promise = callback