sse.ts

// Server-Sent Events
type RequestEvent = Deno.RequestEvent;
type Connection = Deno.Conn;
type StreamController = ReadableStreamDefaultController<Uint8Array>;

const encoder = new TextEncoder();

export class EventConnection {
    request: RequestEvent;
    controller: StreamController | undefined;

    constructor(request: RequestEvent) {
        this.request = request;
    }

    write(text: string) {
        try {
            if (this.controller === undefined) {
                console.log("dropped write", text);
                return;
            }
            this.controller.enqueue(encoder.encode(text));
        } catch (e) {
            // TODO
            console.log(e);
            throw e;
        }
    }

    connect_(controller: StreamController) {
        console.log("connect_");
        this.controller = controller;
    }

    cancel_(reason: string) {
        console.log("cancel_");
    }

    async connect() {
        const self = this;
        const body = new ReadableStream<Uint8Array>({
            start: (controller) => {
                self.connect_(controller);
            },
            // pull(controller) {
            //     console.log("PULL");
            // },
            cancel: (reason) => {
                self.cancel_(reason);
            }
        });

        const headers = {
            "Connection": "keep-alive",
            "Cache-control": "no-cache",
            "Content-type": "text/event-stream",
            "Keep-Alive": `timeout=${Number.MAX_SAFE_INTEGER}`
        };

        const response = new Response(body, { headers, status: 200, statusText: "OK" });
        await this.request.respondWith(Promise.resolve(response));
    }

    async sendMessage(
        data: string | object,
        eventName?: string | undefined,
        id?: string | undefined
    ) {
        const text = (typeof data === "string") ? data : JSON.stringify(data, null, 2);

        if (eventName !== undefined) {
            this.write(`event: ${eventName}\n`);
        }

        if (id !== undefined) {
            this.write(`id: ${eventName}\n`);
        }

        this.write("data: ");
        this.write(text.replaceAll("\n", "\ndata: ")); // TODO - efficiency
        this.write("\n\n");
    }
}

const ec = new EventConnection(requestEvent);
ec.connect();

ec.sendMessage("any data");

In the browser:
const source = new EventSource('');
source.addEventListener('message', function(e) {
console.log(e.data);
}, false);