How Lodash Deep Clone Handles Circular References

This article explains how the Lodash JavaScript library successfully handles circular references when creating deep copies of objects using functions like _.cloneDeep. While naive recursive cloning algorithms crash with stack overflow errors and JSON.parse(JSON.stringify()) throws a TypeError, Lodash relies on an internal tracking mechanism to preserve memory references and gracefully resolve cyclical data structures.

The Problem with Circular References

A circular reference occurs when an object references itself directly or indirectly through a chain of properties. For example:

const obj = { name: "Root" };
obj.self = obj;

A standard recursive cloning function will attempt to clone obj, then clone obj.self, which points back to obj, continuing infinitely until the JavaScript engine throws a RangeError: Maximum call stack size exceeded.

Lodash's Solution: The Internal Stack Cache

Lodash resolves this issue inside its core internal cloning function, baseClone. The key to handling circular dependencies is an internal tracking cache known as Stack.

Whenever _.cloneDeep is invoked, Lodash initializes an internal Stack instance. In modern JavaScript environments, this stack acts similarly to a WeakMap or Map, storing associations between original source objects and their newly created cloned counterparts.

Step-by-Step Execution

When traversing an object graph, Lodash follows a strict sequence for every object or array it encounters:

  1. Cache Lookup: Before creating a copy of an object, Lodash checks if the object already exists in the stack cache using stack.get(value).
  2. Cycle Interruption: If the object is found in the cache, Lodash halts further traversal down that path and immediately returns the previously created clone associated with that object. This eliminates infinite recursion.
  3. Cache Registration: If the object is not in the cache, Lodash initializes a new cloned target (such as an empty object or array) and immediately registers the pair with stack.set(value, result) before copying any child properties.
  4. Recursive Traversal: Lodash then recursively clones the properties or elements of the object, passing the same stack instance through every subsequent call.

Code Demonstration

The following example demonstrates how _.cloneDeep preserves the structure of a cyclical graph:

const _ = require('lodash');

const user = { name: "Alex" };
user.profile = { owner: user };

const clonedUser = _.cloneDeep(user);

// The clone is completely decoupled from the original
console.log(clonedUser !== user); // true

// The circular structure is preserved within the cloned graph
console.log(clonedUser.profile.owner === clonedUser); // true

By registering newly instantiated objects in the stack before descending into their nested properties, Lodash ensures that any cyclical path finds the already-allocated reference, effectively maintaining identical graph topologies without crashing execution.