Deno nuggets: Log listening message

This article is a part of the Deno nuggets series, where each article attempts to suggest a pointed solution to a specific question that can be read in less than a minute. There is no ordering of nuggets.

Problem

How to log the HTTP server listening message that goes to console by default?

~: deno run --no-prompt --allow-net=:8000 app.ts 
Listening on http://localhost:8000/

Solution

The solution is to simply provide a listening callback through the onListen attribute in the options bag of the serve API. The listen callback can take an object as input that contains:

• hostname
• port

The callback function can log the listening data anywhere it likes, such as log files, etc.

Here is the updated server code that logs the listening message in a log file:

import { serve } from "https://deno.land/std/http/mod.ts";
async function logListen(data: any) {
  await Deno.writeTextFile(
    "./httpServer.log",
    `HTTP server listening on ${data.hostname}:${data.port}`,
    { append: true },
  );
}
serve((_) => new Response("Hello from Deno!"), { onListen: logListen });

Let’s run it:

~: deno run --no-prompt --allow-net=:8000 --allow-write=. app.ts
~: cat ./httpServer.log
HTTP server listening on 0.0.0.0:8000