diff --git a/types/mongodb/index.d.ts b/types/mongodb/index.d.ts
index 230f593094..5111e4d9ca 100644
--- a/types/mongodb/index.d.ts
+++ b/types/mongodb/index.d.ts
@@ -955,9 +955,9 @@ export interface Collection<TSchema = Default> {
     /** http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#listIndexes */
     listIndexes(options?: { batchSize?: number, readPreference?: ReadPreference | string, session?: ClientSession }): CommandCursor;
     /** http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#mapReduce */
-    mapReduce(map: CollectionMapFunction | string, reduce: CollectionReduceFunction | string, callback: MongoCallback<any>): void;
-    mapReduce(map: CollectionMapFunction | string, reduce: CollectionReduceFunction | string, options?: MapReduceOptions): Promise<any>;
-    mapReduce(map: CollectionMapFunction | string, reduce: CollectionReduceFunction | string, options: MapReduceOptions, callback: MongoCallback<any>): void;
+    mapReduce<TKey, TValue>(map: CollectionMapFunction<TSchema> | string, reduce: CollectionReduceFunction<TKey, TValue> | string, callback: MongoCallback<any>): void;
+    mapReduce<TKey, TValue>(map: CollectionMapFunction<TSchema> | string, reduce: CollectionReduceFunction<TKey, TValue> | string, options?: MapReduceOptions): Promise<any>;
+    mapReduce<TKey, TValue>(map: CollectionMapFunction<TSchema> | string, reduce: CollectionReduceFunction<TKey, TValue> | string, options: MapReduceOptions, callback: MongoCallback<any>): void;
     /** http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#options */
     options(options?: { session: ClientSession }): Promise<any>;
     options(callback: MongoCallback<any>): void;
@@ -1663,9 +1663,9 @@ export interface MapReduceOptions {
     session?: ClientSession;
 }
 
-export type CollectionMapFunction = () => void;
+export type CollectionMapFunction<TSchema> = (this: TSchema) => void;
 
-export type CollectionReduceFunction = (key: string, values: any) => any;
+export type CollectionReduceFunction<TKey, TValue> = (key: TKey, values: TValue[]) => TValue;
 
 /** http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#~WriteOpResult */
 export interface WriteOpResult {
diff --git a/types/mongodb/mongodb-tests.ts b/types/mongodb/mongodb-tests.ts
index d8eb287964..f34e2e5c29 100644
--- a/types/mongodb/mongodb-tests.ts
+++ b/types/mongodb/mongodb-tests.ts
@@ -317,3 +317,29 @@ mongodb.connect(connectionString).then((client) => {
         runTransactionWithRetry(updateEmployeeInfo, client, session)
     );
 });
+
+// https://docs.mongodb.com/manual/core/map-reduce/
+
+// Declare emit function to be called inside map function
+declare function emit(key: any, value: any): void;
+
+interface ITestMapReduceSchema {
+    cust_id: string;
+    amount: number;
+    status: string;
+}
+
+function testCollectionMapFunction(this: ITestMapReduceSchema) {
+    emit(this.cust_id, this.amount);
+}
+
+function testCollectionReduceFunction(_key: string, values: number[]): number {
+    return values.reduce((a, v) => a + v, 0);
+}
+
+mongodb.connect(connectionString).then((client) => {
+    client.db("test").collection<ITestMapReduceSchema>('test-mapReduce-collection').mapReduce(
+        testCollectionMapFunction,
+        testCollectionReduceFunction
+    );
+});