Why Lodash _.assign Prevents Deep Cloning DOM Objects

Lodash’s _.assign avoids accidentally deep cloning complex Document Object Model (DOM) objects primarily because it is fundamentally designed as a shallow-copy operation that only iterates over own enumerable properties. By restricting property traversal to a single level and copying references rather than recursively duplicating object graphs, _.assign inherently avoids the circular references, prototype chains, and massive property trees typical of browser host objects.

Shallow Copying by Specification

Unlike _.cloneDeep or _.merge, which recursively traverse nested objects and arrays, _.assign is strictly a shallow copy function equivalent to ECMAScript's native Object.assign. When copying properties from a source to a target:

_.assign(target, { element: document.getElementById('my-div') });

The function performs direct property assignment (target[key] = source[key]). It copies the memory address of the DOM node reference rather than constructing a new node or traversing its children. Because there is zero recursion in the algorithm, it is structurally impossible for _.assign to initiate a deep clone of any object, DOM elements included.

Enumeration of Own Properties Only

DOM elements are complex host objects with deep prototype chains extending through HTMLElement, Element, Node, and EventTarget. The vast majority of DOM properties—such as childNodes, parentNode, innerHTML, and event handlers—are non-enumerable getters and setters defined on these prototypes, not own properties of the instance.

Lodash’s _.assign relies internally on retrieving only own enumerable properties (similar to Object.keys()). As a result, passing a raw DOM element directly as a source object yields almost no properties to copy:

const div = document.createElement('div');
const result = _.assign({}, div); 
// result contains almost nothing because DOM properties are inherited and non-enumerable

Because _.assign completely ignores inherited prototype properties, the massive surface area of the DOM interface is systematically excluded from the operation.

Immunity to Circular References

DOM structures are cyclic graphs by default; an element references its parent via parentNode, and the parent references the child via children or childNodes. Recursive cloning utilities must maintain a hash map of visited objects to prevent infinite call stack errors.

Because _.assign does not inspect or traverse nested values, it never follows these cyclic relationships into deeper layers. It treats an HTMLElement simply as a primitive object reference, effectively rendering circular references a non-issue.