Errors and exceptions

Review Workers errors and exceptions.

​​ Error pages generated by Workers

When a Worker running in production has an error that prevents it from returning a response, the client will receive an error page with an error code, defined as follows:

Other 11xx errors generally indicate a problem with the Workers runtime itself. Refer to the status page Open external link if you are experiencing an error.

​​ Loop limit

A Worker cannot call itself or another Worker more than 16 times. In order to prevent infinite loops between Workers, the CF-EW-Via header’s value is an integer that indicates how many invocations are left. Every time a Worker is invoked, the integer will decrement by 1. If the count reaches zero, a 1019 error is returned.

​​ “The script will never generate a response” errors

Some requests may return a 1101 error with The script will never generate a response in the error message. This occurs when the Workers runtime detects that all the code associated with the request has executed and no events are left in the event loop, but a Response has not been returned.

​​ Cause 1: Unresolved Promises

This is most commonly caused by relying on a Promise that is never resolved or rejected, which is required to return a Response. To debug, look for Promises within your code or dependencies’ code that block a Response, and ensure they are resolved or rejected.

In browsers and other JavaScript runtimes, equivalent code will hang indefinitely, leading to both bugs and memory leaks. The Workers runtime throws an explicit error to help you debug.

In the example below, the Response relies on a Promise resolution that never happens. Uncommenting the resolve callback solves the issue.

index.js
export default {
  fetch(req) {
    let response = new Response("Example response");
    let { promise, resolve } = Promise.withResolvers();

    // If the promise is not resolved, the Workers runtime will
    // recognize this and throw an error.

    // setTimeout(resolve, 0)

    return promise.then(() => response);
  },
};

​​ Cause 2: WebSocket connections that are never closed

If a WebSocket is missing the proper code to close its server-side connection, the Workers runtime will throw a script will never generate a response error. In the example below, the 'close' event from the client is not properly handled by calling server.close(), and the error is thrown. In order to avoid this, ensure that the WebSocket’s server-side connection is properly closed via an event listener or other server-side logic.

index.js
async function handleRequest(request) {
  let webSocketPair = new WebSocketPair();
  let [client, server] = Object.values(webSocketPair);
  server.accept();

  server.addEventListener('close', () => {
    // This missing line would keep a WebSocket connection open indefinitely
    // and results in "The script will never generate a response" errors

    // server.close();
  });

  return new Response(null, {
    status: 101,
    webSocket: client,
  });
}

​​ Errors on Worker upload

These errors occur when a Worker is uploaded or modified.

​​ Validation Errors (10021)

The 10021 error code includes all errors that occur when you attempt to deploy a Worker, and Cloudflare then attempts to load and run the top-level scope (everything that happens before your Worker’s handler is invoked). For example, if you attempt to deploy a broken Worker with invalid JavaScript that would throw a SyntaxError — Cloudflare will not deploy your Worker.

Specific error cases include but are not limited to:

​​ Script startup exceeded CPU time limit

This means that you are doing work in the top-level scope of your Worker that takes more than the startup time limit (400ms) of CPU time.

This is usually a sign of a bug and/or large performance problem with your code or a dependency you rely on. It’s not typical to use more than 400ms of CPU time when your app starts. The more time your Worker’s code spends parsing and executing top-level scope, the slower your Worker will be when you deploy a code change or a new isolate is created.

This error is most commonly caused by attempting to perform expernsive initialization work directly in top level (global) scope, rather than either at build time or when your Worker’s handler is invoked. For example, attempting to initialize an app by generating or consuming a large schema.

To analyze what is consuming so much CPU time, you should open Chrome DevTools for your Worker and look at the Profiling and/or Performance panels to understand where time is being spent. Is there something glaring that consumes tons of CPU time, especially the first time you make a request to your Worker?

​​ Runtime errors

Runtime errors will occur within the runtime, do not throw up an error page, and are not visible to the end user. Runtime errors are detected by the user with logs.

​​ Identify errors: Workers Metrics

To review whether your application is experiencing any downtime or returning any errors:

1. Log in to the Cloudflare dashboard Open external link and select your account.
2. In Account Home, select Workers & Pages.
3. In Overview, select your Worker and review your Worker’s metrics.

​​ Debug exceptions

After you have identified your Workers application is returning exceptions, use wrangler tail to inspect and fix the exceptions.

Exceptions will show up under the exceptions field in the JSON returned by wrangler tail. After you have identified the exception that is causing errors, redeploy your code with a fix, and continue tailing the logs to confirm that it is fixed.

​​ Set up a logging service

A Worker can make HTTP requests to any HTTP service on the public Internet. You can use a service like Sentry Open external link to collect error logs from your Worker, by making an HTTP request to the service to report the error. Refer to your service’s API documentation for details on what kind of request to make.

When using an external logging strategy, remember that outstanding asynchronous tasks are canceled as soon as a Worker finishes sending its main response body to the client. To ensure that a logging subrequest completes, pass the request promise to event.waitUntil() Open external link . For example:

export default {
  async fetch(request, env, ctx) {
    function postLog(data) {
      return fetch("https://log-service.example.com/", {
        method: "POST",
        body: data,
      });
    }

    // Without ctx.waitUntil(), the `postLog` function may or may not complete.
    ctx.waitUntil(postLog(stack));
    return fetch(request);
  }
}

addEventListener("fetch", (event) => {
  event.respondWith(handleEvent(event));
});

async function handleEvent(event) {
  // ...

  // Without event.waitUntil(), the `postLog` function may or may not complete.
  event.waitUntil(postLog(stack));
  return fetch(event.request);
}

function postLog(data) {
  return fetch("https://log-service.example.com/", {
    method: "POST",
    body: data,
  });
}

​​ Go to origin on error

By using event.passThroughOnException, a Workers application will forward requests to your origin if an exception is thrown during the Worker’s execution. This allows you to add logging, tracking, or other features with Workers, without degrading your application’s functionality.

export default {
  async fetch(request, env, ctx) {
    ctx.passThroughOnException();
    // an error here will return the origin response, as if the Worker wasn't present
    return fetch(request);
  }
}

addEventListener("fetch", (event) => {
  event.passThroughOnException();
  event.respondWith(handleRequest(event.request));
});

async function handleRequest(request) {
  // An error here will return the origin response, as if the Worker wasn’t present.
  // ...
  return fetch(request);
}

​​ Related resources

• Log from Workers - Learn how to log your Workers.
• Logpush - Learn how to push Workers Trace Event Logs to supported destinations.
• RPC error handling - Learn how to handle errors from remote-procedure calls.