Use void instead of undefined to represent lack of value. (#27106)

Using undefined means async functions must have a `return undefined;` rather than omitting the return value altogether, as would be the most natural thing to do in such functions.

types/generic-pool/generic-pool-tests.ts

@@ -11,8 +11,8 @@ const factory = {
             resolve(conn);
         });
     },
-    destroy: (conn: Connection): Promise<undefined> => {
-        return new Promise<undefined>(resolve => {
+    destroy: (conn: Connection): Promise<void> => {
+        return new Promise<void>(resolve => {
             conn.connected = false;
             resolve();
         });
@@ -53,7 +53,7 @@ pool.acquire()
         return pool.destroy(conn);
     }).then(() => {
         return pool.clear();
-    }).then((results: undefined[]) => {
+    }).then(() => {
     });
 
 pool.on('factoryCreateError', (err: Error) => {

types/generic-pool/index.d.ts

@@ -20,14 +20,14 @@ export class Pool<T> extends EventEmitter {
     acquire(priority?: number): PromiseLike<T>;
     release(resource: T): void;
     destroy(resource: T): void;
-    drain(): PromiseLike<undefined>;
-    clear(): PromiseLike<undefined[]>;
+    drain(): PromiseLike<void>;
+    clear(): PromiseLike<void>;
     use<U>(cb: (resource: T) => U): PromiseLike<U>;
 }
 
 export interface Factory<T> {
     create(): PromiseLike<T>;
-    destroy(client: T): PromiseLike<undefined>;
+    destroy(client: T): PromiseLike<void>;
     validate?(client: T): PromiseLike<boolean>;
 }