Fix cloneDeepWith to match lodash behavior for null prototype objects

[Copilot]

cloneDeepWith in es-toolkit/compat preserves null prototype when cloning objects created with Object.create(null), while lodash returns an object with Object.prototype.

import { cloneDeepWith } from 'es-toolkit/compat';

const obj = Object.create(null);
obj.a = 1;

const cloned = cloneDeepWith(obj);
// Before: Object.getPrototypeOf(cloned) === null, cloned.toString === undefined
// After:  Object.getPrototypeOf(cloned) === Object.prototype, cloned.toString === [Function]

Changes

• Added objectTag case in compat customizer to detect plain objects with null prototype
• Create new object with Object.prototype instead of preserving null prototype
• Handle circular references for null prototype objects
• Added tests for basic, nested, customizer, and circular reference cases

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Minor behaviour difference in cloneDeepWith compared to lodash-es</issue_title>
<issue_description>Given the following code:

const data = cloneDeepWith( Object.create( null ) );
console.log( data.toString );

The object returned by lodash-es has an Object prototype, even though the original object was created using Object.create( null ), so the output to the console is [Function: toString]. However, the same function in es-toolkit/compat returns an object without a prototype, so the output to the console is undefined.

While the es-toolkit behavior makes more sense to me, you may consider this a bug if you care about full backward compatibility with lodash.

---

Reproduction: https://stackblitz.com/edit/node-e1e3y8ws?file=index.js

In the console, run node index.js.</issue_description>

<agent_instructions>Could you update cloneDeepWith to match lodash's behavior? Currently, when cloning an object created with Object.create(null), es-toolkit preserves the null prototype, while lodash returns an object with the standard Object prototype.
For full backward compatibility with lodash, the cloned object should have the Object prototype regardless of the source object's prototype.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Minor behaviour difference in cloneDeepWith compared to lodash-es #953

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
es-toolkit	Ready	Preview	Comment	Nov 26, 2025 2:59pm