DOMException: Could not be cloned — DataCloneError from structuredClone()

What causes this error

structuredClone() is a global built into Node.js that deep-clones data using the same algorithm browsers use for `postMessage`. It supports plain objects, arrays, Maps, Sets, Dates, and typed arrays — but not functions, DOM-like objects, class instances with private fields tied to native resources, or anything holding an open file/socket handle.

The Fix

// Wrong — class instance wraps a live database connection
class UserSession {
  constructor(dbConnection) { this.dbConnection = dbConnection; }
}
structuredClone(new UserSession(conn)); // DataCloneError

// Fix — clone only the plain data you actually need
const plain = { id: session.id, userId: session.userId, createdAt: session.createdAt };
const copy = structuredClone(plain);

Common causes / variations

• Cloning an ORM model instance instead of a plain object built from its fields
• Cloning an object that (directly or nested) contains a function, Promise, or WeakMap
• Passing a class instance across a `worker_threads` boundary without first converting it to plain data

Related errors

See also: postMessage DataCloneError in worker_threads, Only plain objects can be passed to Client Components.