Fix for ts definition

src/core/BufferGeometry.d.ts

@@ -99,7 +99,7 @@ export class BufferGeometry extends EventDispatcher {
 	 */
 	computeVertexNormals(): void;
 
-	merge( geometry: BufferGeometry, offset: number ): BufferGeometry;
+	merge( geometry: BufferGeometry, offset?: number ): BufferGeometry;
 	normalizeNormals(): void;
 
 	toNonIndexed(): BufferGeometry;