HTTP Server: WebSockets

An example of a HTTP server that handles websocket requests.

To start the server on the default port, call `Deno.serve` with the handler.

Deno.serve((req) => {

First, we verify if the client is negotiating to upgrade to websockets. If not, we can give a status of 501 to specify we don't support plain http requests.

  if (req.headers.get("upgrade") != "websocket") {
    return new Response(null, { status: 501 });
  }

We can then upgrade the request to a websocket

  const { socket, response } = Deno.upgradeWebSocket(req);

We now have access to a standard websocket object. Let's handle the "open" event

  socket.addEventListener("open", () => {
    console.log("a client connected!");
  });

We can also handle messages in a similar way. Here we set up a simple ping / pong example.

  socket.addEventListener("message", (event) => {
    if (event.data === "ping") {
      socket.send("pong");
    }
  });

Lastly we return the response created from upgradeWebSocket.

  return response;
});

Run this example locally using the Deno CLI:

deno run --allow-net https://raw.githubusercontent.com/denoland/deno-docs/main/by-example/http-server-websocket.ts

Additional resources: