diff --git a/typescript/core/base.ts b/typescript/core/base.ts
index 79dc317af..e681f2ac4 100644
--- a/typescript/core/base.ts
+++ b/typescript/core/base.ts
@@ -177,7 +177,8 @@ export interface _base {
 
     functions: (object: object) => string[];
 
-    extend: (object: object, ...sources: any[]) => object;
+    extend<T extends object>(object: T): T;
+    extend<T extends object, U extends object>(object: T, ...sources: U[]): T & UnionToIntersection<U>;
 
     defaults: (object: object, ...sources: any[]) => object;
 
@@ -399,3 +400,11 @@ type merge = {
 
     deepExtend(object: any, ...otherArgs: any[]): any;
 }
+
+type UnionToIntersection<Union> = (
+	Union extends unknown
+		? (x: Union) => void
+		: never
+) extends ((x: infer Intersection) => void)
+	? Intersection
+	: never;