Handle incoming request cancellation in Workers with Request.signal

In Cloudflare Workers, you can now attach an event listener to Request objects, using the signal property ↗. This allows you to perform tasks when the request to your Worker is canceled by the client. To use this feature, you must set the enable_request_signal compatibility flag.

You can use a listener to perform cleanup tasks or write to logs before your Worker's invocation ends. For example, if you run the Worker below, and then abort the request from the client, a log will be written:

index.jsjs

export default {
	async fetch(request, env, ctx) {
		// This sets up an event listener that will be called if the client disconnects from your
		// worker.
		request.signal.addEventListener("abort", () => {
			console.log("The request was aborted!");
		});

		const { readable, writable } = new IdentityTransformStream();
		sendPing(writable);
		return new Response(readable, {
			headers: { "Content-Type": "text/plain" },
		});
	},
};

async function sendPing(writable) {
	const writer = writable.getWriter();
	const enc = new TextEncoder();

	for (;;) {
		// Send 'ping' every second to keep the connection alive
		await writer.write(enc.encode("ping\r\n"));
		await scheduler.wait(1000);
	}
}

index.tsts

export default {
  async fetch(request, env, ctx): Promise<Response> {
    // This sets up an event listener that will be called if the client disconnects from your
    // worker.
    request.signal.addEventListener('abort', () => {
      console.log('The request was aborted!');
    });

    const { readable, writable } = new IdentityTransformStream();
    sendPing(writable);
    return new Response(readable, { headers: { 'Content-Type': 'text/plain' } });
  },
} satisfies ExportedHandler<Env>;

async function sendPing(writable: WritableStream): Promise<void> {
	const writer = writable.getWriter();
	const enc = new TextEncoder();

	for (;;) {
		// Send 'ping' every second to keep the connection alive
		await writer.write(enc.encode('ping\r\n'));
		await scheduler.wait(1000);
	}
}

For more information see the Request documentation.