Date objects are destroyed by _deepClone via JSON.parse(JSON.stringify())

[monteiz]

Date objects are destroyed by _deepClone via JSON.parse(JSON.stringify())

Description

The internal _deepClone method relies on JSON.parse(JSON.stringify(obj)) for object cloning. Because JSON.stringify automatically invokes Date.prototype.toJSON(), any Date instance in the source object is permanently cast into an ISO string during the cloning phase. This mutates the original data types in memory, breaking time zone calculations and native Date operations downstream.

Reproduction

const jsonpatch = require('fast-json-patch');

const document = {
  createdAt: new Date("2026-06-30T09:48:00Z")
};

const patch = [{ op: "add", path: "/updatedAt", value: new Date() }];

// _deepClone internally destroys the Date object
const result = jsonpatch.applyPatch(document, patch, false, false);

console.log(typeof result.newDocument.createdAt); 
// Expected: "object" (Date instance)
// Actual: "string"

Proposed Solution

I believe the core cloning mechanism should be updated to utilize the native structuredClone() algorithm, which is fully supported across Node.js 17+ and all modern browsers.

export function _deepClone(obj) {
    return structuredClone(obj);
}

This prevents silent data mutation while retaining performance. I can submit a PR with this update if the team agrees with this direction.

[monteiz]

I believe this cloning bug might also be the root cause affecting #278, #318, #325. The destructive JSON serialization in _deepClone would trigger the exact same data mutation side effects described there.