EC
ERR_WORKER_NOT_RUNNING
Node.jsERRORNotableWorkerHIGH confidence

Operation failed because worker thread is not running

Production Risk

Low — track worker state; implement restart logic for long-running worker pools.

What this means

Thrown when an operation is attempted on a Worker instance that is no longer running. Once a worker has exited (either normally or due to an error), operations such as postMessage() are no longer valid.

Why it happens
  1. 1Calling worker.postMessage() after the worker has exited
  2. 2Sending a message to a worker that crashed and emitted the error event
  3. 3Not tracking worker exit state before sending messages
How to reproduce

Triggered when postMessage() or other operations are attempted on an already-terminated Worker.

trigger — this will error
trigger — this will error
const { Worker } = require('worker_threads');
const w = new Worker('./worker.js');
w.on('exit', () => {
  w.postMessage('hello'); // throws — worker is no longer running
});

expected output

Error [ERR_WORKER_NOT_RUNNING]: Worker is not running

Fix

Track worker running state and guard before posting messages

WHEN When workers may exit during operation

Track worker running state and guard before posting messages
const { Worker } = require('worker_threads');
let running = true;
const w = new Worker('./worker.js');
w.on('exit', () => { running = false; });

function sendToWorker(msg) {
  if (running) w.postMessage(msg);
}

Why this works

A boolean flag updated on the exit event prevents postMessage from being called after termination.

Code examples
Triggerjs
const { Worker } = require('worker_threads');
const w = new Worker('./worker.js');
w.on('exit', () => {
  w.postMessage('hello'); // throws — worker is no longer running
});  // this triggers ERR_WORKER_NOT_RUNNING
Handle in try/catchjs
try {
  // operation that may throw ERR_WORKER_NOT_RUNNING
  riskyOperation()
} catch (err) {
  if (err.code === 'ERR_WORKER_NOT_RUNNING') {
    console.error('ERR_WORKER_NOT_RUNNING:', err.message)
  } else {
    throw err
  }
}
Defensive pattern to avoid itjs
let running = true
worker.on('exit', () => { running = false })
function send(msg) { if (running) worker.postMessage(msg) }
What not to do

✕ Post messages to workers in exit handlers

The worker is already terminated at exit time; messages cannot be delivered.

Same error in other languages
RuntimeErrorin Python→
Sources
Official documentation ↗

Node.js Error Codes Documentation

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