fix[cloneDeep, clone, assign, omit, pick]: preserve own __proto__ property without setting prototype

[TalhaHunter101]

Summary

Fixes #2117.

In JavaScript, setting target[key] = value when key === '__proto__' invokes Object.prototype.__proto__'s setter rather than defining an own property on the target object. This causes two issues:

1. '__proto__' is dropped from the target object's own keys (Object.hasOwn(result, '__proto__') becomes false).
2. The target object's prototype is mutated to the given value (Object.getPrototypeOf(result) is set to the value, causing prototype pollution).

Lodash preserves an own __proto__ property as an own data property without setting or mutating the object's prototype. This PR aligns es-toolkit and es-toolkit/compat with Lodash by using Object.defineProperty whenever assigning '__proto__' keys (matching the existing approach in src/compat/util/toPlainObject.ts).

Lodash difference

const obj = JSON.parse('{"__proto__": {"polluted": true}, "a": 1}');

// Lodash:
const lodashResult = _.cloneDeep(obj);
Object.hasOwn(lodashResult, '__proto__'); // true
Object.getPrototypeOf(lodashResult) === Object.prototype; // true
lodashResult.polluted; // undefined

// es-toolkit (before this fix):
const toolkitResult = cloneDeep(obj);
Object.hasOwn(toolkitResult, '__proto__'); // false
Object.getPrototypeOf(toolkitResult); // { polluted: true }
toolkitResult.polluted; // true (prototype polluted!)

The same behavior difference occurred across clone, cloneDeep, cloneDeepWith, assign, assignIn, assignWith, assignInWith, assignValue, omit, omitBy, pick, pickBy, and mapValues.

Contributing policy checks

1. Compiles against @types/lodash in strict mode:

import { cloneDeep, clone, assign, omit, pick } from 'lodash';

const parsed: Record<string, unknown> = JSON.parse('{"__proto__": {"polluted": true}}');
cloneDeep(parsed);
clone(parsed);
assign({}, parsed);
omit(parsed, []);
pick(parsed, ['__proto__']);

All of these calls typecheck without error under TypeScript strict mode.

2. Real code permalinks:

• In taskcluster/taskcluster (services/secrets/src/api.js#L13), cloneDeep(JSON.parse(...)) is called on decrypted secret payloads.
• In heygen-com/hyperframes (packages/studio/src/player/components/timelineDragDrop.ts#L44), pick(JSON.parse(...) as Record<string, string | undefined>, ...) is called on parsed JSON data.

Changes

• Updated assignValue (src/compat/_internal/assignValue.ts) to use Object.defineProperty when setting '__proto__'.
• Updated clone and cloneDeepWith in src/object/ and src/compat/object/clone.ts to define '__proto__' using Object.defineProperty.
• Updated assignImpl, assignInImpl, assignWithImpl, and assignInWithImpl in src/compat/object/ to safely define '__proto__'.
• Updated omit, omitBy, pick, pickBy, and mapValues in both core and compat to preserve own '__proto__' properties.
• Added regression tests covering own '__proto__' preservation and prototype non-pollution across all 16 corresponding .spec.ts test files.

[vercel]

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

Project	Deployment	Actions	Updated
es-toolkit	Error		Oct 1, 2026 10:20pm UTC