Fetch Handler

​​ Background

Incoming HTTP requests to a Worker are passed to the fetch() handler as a request Open external link object. To respond to the request with a response, return a Response object:

export default {
	async fetch(request, env, ctx) {
		return new Response('Hello World!');
	},
};

​​ Parameters

​​ Lifecycle methods

When responding to a HTTP request, the fetch handler may use any of the following methods to augment or control how the request is handled.

​​ context.waitUntil()

The waitUntil() method extends the lifetime of the fetch event. It accepts a Promise-based task which the Workers runtime will execute before the handler terminates but without blocking the response. For example, this is ideal for caching responses or handling logging.

You can call waitUntil() multiple times. Similar to Promise.allSettled, even if a promise passed to one waitUntil call is rejected, promises passed to other waitUntil() calls will still continue to execute.

export default {
  async fetch(request, env, context) {
    // Forward / Proxy original request
    let res = await fetch(request);

    // Add custom header(s)
    res = new Response(res.body, res);
    res.headers.set('x-foo', 'bar');

    // Cache the response
    // NOTE: Does NOT block / wait
    context.waitUntil(caches.default.put(request, res.clone()));

    // Done
    return res;
  },
};

​​ context.passThroughOnException()

The passThroughOnException method allows a Worker to fail open Open external link , and pass a request through to an origin server when a Worker throws an unhandled exception. This can be useful when using Workers as a layer in front of an existing service, allowing the service behind the Worker to handle any unexpected error cases that arise in your Worker.

export default {
  async fetch(request, env, context) {
    // Proxy to origin on unhandled/uncaught exceptions
    context.passThroughOnException();
    throw new Error('Oops');
  },
};

​​ Related resources

• Write your Worker code in ES modules syntax for an optimized experience.