`.castShadow` accepts `boolean` only

src/core/Object3D.d.ts

@@ -135,7 +135,7 @@ export class Object3D extends EventDispatcher {
 	 * Gets rendered into shadow map.
 	 * @default false
 	 */
-	castShadow: undefined | boolean;
+	castShadow: boolean;
 
 	/**
 	 * Material gets baked in shadow receiving.