---
title: FetchEvent
description: Extend the lifetime of a fetch event with waitUntil and passThroughOnException.
---

The `cloudflare` package exposes two [FetchEvent](https://developers.cloudflare.com/workers/runtime-apis/fetch-event/) lifecycle methods.

## WaitUntil

`cloudflare.WaitUntil` keeps the worker alive after the response has been sent. Use it for logging, analytics, cache writes, and other work that should not delay the client.

```go
func handler(w http.ResponseWriter, req *http.Request) {
	cloudflare.WaitUntil(func() {
		// runs after the response is returned
		writeAnalytics(req)
	})
	w.Write([]byte("ok"))
}
```

`WaitUntil` accepts a synchronous `func()` — run slow work inside it, and the Workers runtime waits for it to finish before terminating the invocation.

## PassThroughOnException

`cloudflare.PassThroughOnException` forwards the request to the origin server when the worker throws an unhandled exception, instead of returning a runtime error response. It needs a route or custom domain configured on the worker.

```go
func handler(w http.ResponseWriter, req *http.Request) {
	cloudflare.PassThroughOnException()
	// if this handler panics, the request goes to the origin
}
```

## Example project

[_examples/fetch-event](https://github.com/syumai/workers-go/tree/main/_examples/fetch-event) combines both methods with a reverse proxy.