Error: postMessage could not be cloned — DataCloneError in worker_threads

What causes this error

Worker and MessagePort communicate by serializing data with the structured clone algorithm — the exact same rules as structuredClone() and the browser's `postMessage`. Anything non-clonable (functions, class instances with native bindings, live sockets) throws immediately when you try to send it.

The Fix

import { Worker } from 'node:worker_threads';

const worker = new Worker('./worker.js');

// Wrong — sending a class instance with methods
worker.postMessage(new HeavyProcessor(options)); // DataCloneError

// Fix — send plain data, reconstruct inside the worker if needed
worker.postMessage({ options }); // plain object, always clonable

// For large binary data, transfer ownership instead of copying it
const buffer = new ArrayBuffer(1024 * 1024);
worker.postMessage({ buffer }, [buffer]); // second arg = transfer list

Common causes / variations

• Sending an ORM/database model instance instead of its plain field values
• Trying to send a callback function expecting the worker to invoke it (workers can't receive functions — use message-based request/response instead)
• Forgetting the transfer list for large `ArrayBuffer`s, causing a slow full copy instead of a zero-copy transfer

Related errors

See also: DataCloneError from structuredClone(), Only plain objects can be passed to Client Components.