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.

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.