Lodash v4 deep cloning and merge behavior for inherited properties

Answer to the Core Questions

1. Does _.cloneDeep preserve the prototype chain of the source object in v4? No. _.cloneDeep creates a new plain object and copies only the source’s own enumerable string‑keyed properties. Any properties that come from the source’s prototype are not copied, and the resulting clone has the default Object.prototype as its prototype.

2. How does _.merge treat inherited properties from the source object when the target already possesses a corresponding own property? _.merge iterates only over the source’s own enumerable string‑keyed properties. Inherited properties are ignored. If the target already has an own property with the same key, that property is overwritten by the source’s own value; inherited properties never influence the merge.

Confirmed Facts (Based on Lodash v4 Implementation)

• Both _.cloneDeep and _.merge operate solely on own enumerable string‑keyed properties.
• Inherited properties (including those from Object.prototype or a custom prototype) are not copied or merged.
• Non‑enumerable properties and Symbol keys are ignored by both functions.
• Prototype chains are not preserved; the clone or merged result is always a plain object unless a customizer is used.
• When the target has an own property that matches a source own property, _.merge overwrites it; inherited properties on the target are untouched.

Practical Verification

const _ = require('lodash');

// Prototype with an inherited property
const proto = { inherited: 42 };
const child = Object.create(proto);
child.own = 'value';

// cloneDeep example
const cloned = _.cloneDeep(child);
console.log(cloned); // { own: 'value' } – no inherited property

// merge example
const target = { a: 1, inherited: 99 }; // inherited is an own property here
_.merge(target, child);
console.log(target); // { a: 1, inherited: 99, own: 'value' }
// Notice the target’s own "inherited" stays unchanged; child’s inherited is ignored

When You Need the Prototype

If your application requires that the clone or merged object retain the original prototype (for instance, when working with class instances), the default lodash functions won’t suffice. In that case you can:

• Use _.cloneDeepWith or _.mergeWith and provide a customizer that copies Object.getPrototypeOf onto the new object.
• Manually copy the prototype after cloning: Object.setPrototypeOf(cloned, Object.getPrototypeOf(child));

One Missing Diagnostic Detail

Do you need the clone or merged result to keep the same prototype as the source, or is a plain object acceptable for your use case?