fix: only merge plain objects

src/defu.ts

@@ -1,17 +1,13 @@
 import type { Merger, DefuFn as DefuFunction, DefuInstance } from "./types";
 
-function isObject(value: any) {
-  return value !== null && typeof value === "object";
-}
-
 // Base function to apply defaults
 function _defu<T>(
   baseObject: T,
   defaults: any,
   namespace = ".",
   merger?: Merger,
 ): T {
-  if (!isObject(defaults)) {
+  if (!_isPlainObject(defaults)) {
     return _defu(baseObject, {}, namespace, merger);
   }
 
@@ -34,7 +30,7 @@ function _defu<T>(
 
     if (Array.isArray(value) && Array.isArray(object[key])) {
       object[key] = [...value, ...object[key]];
-    } else if (isObject(value) && isObject(object[key])) {
+    } else if (_isPlainObject(value) && _isPlainObject(object[key])) {
       object[key] = _defu(
         value,
         object[key],
@@ -49,6 +45,23 @@ function _defu<T>(
   return object;
 }
 
+// From sindresorhus/is-plain-obj
+// MIT License
+// Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (https://sindresorhus.com)
+function _isPlainObject(value: unknown): boolean {
+  if (value === null || typeof value !== "object") {
+    return false;
+  }
+  const prototype = Object.getPrototypeOf(value);
+  return (
+    (prototype === null ||
+      prototype === Object.prototype ||
+      Object.getPrototypeOf(prototype) === null) &&
+    !(Symbol.toStringTag in value) &&
+    !(Symbol.iterator in value)
+  );
+}
+
 // Create defu wrapper with optional merger and multi arg support
 export function createDefu(merger?: Merger): DefuFunction {
   return (...arguments_) =>

test/defu.test.ts

@@ -50,6 +50,22 @@ describe("defu", () => {
     }>();
   });
 
+  it.skip("should avoid merging objects with custom constructor", () => {
+    class Test {
+      // eslint-disable-next-line no-useless-constructor
+      constructor(public value: string) {}
+    }
+    const result = defu({ test: new Test("a") }, { test: new Test("b") });
+    expect(result).toEqual({ test: new Test("a") });
+  });
+
+  it.skip("should assign date properly", () => {
+    const date1 = new Date("2020-01-01");
+    const date2 = new Date("2020-01-02");
+    const result = defu({ date: date1 }, { date: date2 });
+    expect(result).toEqual({ date: date2 });
+  });
+
   it("should correctly merge different object types", () => {
     // eslint-disable-next-line unicorn/consistent-function-scoping
     const fn = () => 42;