_.cloneDeep behavior with custom class instances and prototype preservation

Answer to the Question

1. Does _.cloneDeep preserve the prototype of custom class instances?

• Not by default. _.cloneDeep creates a new plain object that shares the same prototype as the source, but it does not invoke the class constructor or preserve private fields, non‑enumerable properties, or symbol keys. As a result, instanceof checks against the original class will fail, and any invariants established by the constructor are lost.
• There is no flag or option that forces a true re‑instantiation of the custom class.

2. When was the behavior for custom constructor preservation finalized?

• From the earliest 4.x releases (4.0.0, 2015) onward, _.cloneDeep has always used baseCreate(getPrototype(value)) for generic objects. The prototype chain is copied, but the constructor is never called. There is no later version that changed this fundamental behavior.

Why the Prototype Is Usually Intact but the Class Is Not

The internal baseClone implementation creates a new object with:

var result = baseCreate(getPrototype(value));

This keeps the prototype chain but skips any constructor logic. Methods defined on the prototype remain available, yet the instance is no longer an instance of the original class.

How to Preserve a Custom Class Instance

Use _.cloneDeepWith and provide a customizer that explicitly constructs the class:

function cloneWithCtor(value) {
  if (value instanceof MyClass) {
    // Create a new instance and let lodash clone its properties
    const ctor = Object.getPrototypeOf(value).constructor;
    return new ctor(); // constructor arguments can be added if needed
  }
  return undefined; // fall back to default cloning
}

const cloned = _.cloneDeepWith(original, cloneWithCtor);

Alternatively, if you only need the prototype chain, simply verify that Object.getPrototypeOf(cloned) === Object.getPrototypeOf(original) after a plain _.cloneDeep call.

Verification Steps

1. class Test { constructor(x) { this.x = x; } method() { return this.x; } }
2. Instantiate: const a = new Test(5);
3. Clone: const b = _.cloneDeep(a);
4. Check prototype: Object.getPrototypeOf(b) === Object.getPrototypeOf(a) (true).
5. Check instanceof: b instanceof Test (false).
6. Using _.cloneDeepWith as above restores b instanceof Test to true.

Potential Edge Cases

If your class relies on non‑enumerable properties, getters/setters, or private fields, _.cloneDeep will not copy them. In those situations, a custom cloning strategy (for example, using Object.assign after constructing a new instance) is required.

Missing Diagnostic Detail

Do your class instances use non‑enumerable properties, symbols, or getters/setters that need to be preserved? If so, a custom clone approach is necessary.