nestjs-sse/src/index.ts

Middleware for Nest.js to broadcast Server-Sent Events (SSE)

import { Injectable, NestMiddleware } from "@nestjs/common";
import { Request, Response } from "express";

@Injectable()
export class SSEMiddleware implements NestMiddleware {
    use(req: Request, res: Response, next: () => void) {
        req.socket.setKeepAlive(true);
        req.socket.setTimeout(0);

        res.setHeader("Content-Type", "text/event-stream");
        res.setHeader("Cache-Control", "no-cache");
        res.setHeader("Connection", "keep-alive");
        res.status(200);

        // export a function to send server-side-events
        res.sse = function sse(data) {
            res.write(data);

            // support running within the compression middleware
            // @ts-ignore
            if (res.flushHeaders() && data.match(/\n\n$/)) {
                res.flushHeaders();
            }
        };

        // write 2kB of padding (for IE) and a reconnection timeout
        // then use res.sse to send to the client
        res.write(":" + Array(2049).join(" ") + "\n");
        res.sse("retry: 2000\n\n");

        // keep the connection open by sending a comment
        const keepAlive = setInterval(() => {
            res.sse(":keep-alive\n\n");
        }, 20000);

        // cleanup on close
        res.on("close", function close() {
            clearInterval(keepAlive);
        });

        next();
    }
}