express js Logger

const express = require("express");
const fs = require("fs");
const PORT = 5000;
const app = express();


const getActualRequestDurationInMilliseconds = start => {
  const NS_PER_SEC = 1e9; //  convert to nanoseconds
  const NS_TO_MS = 1e6; // convert to milliseconds
  const diff = process.hrtime(start);
  return (diff[0] * NS_PER_SEC + diff[1]) / NS_TO_MS;
};


let demoLogger = (req, res, next) => { //middleware function
  let current_datetime = new Date();
  let formatted_date =
    current_datetime.getFullYear() +
    "-" +
    (current_datetime.getMonth() + 1) +
    "-" +
    current_datetime.getDate() +
    " " +
    current_datetime.getHours() +
    ":" +
    current_datetime.getMinutes() +
    ":" +
    current_datetime.getSeconds();
  let method = req.method;
  let url = req.url;
  let status = res.statusCode;
  const start = process.hrtime();
  const durationInMilliseconds = getActualRequestDurationInMilliseconds(start);
  let log = `[${formatted_date}] ${method}:${url} ${status} ${durationInMilliseconds.toLocaleString()} ms`;
  console.log(log);
  fs.appendFile("request_logs.txt", log + "\n", err => {
    if (err) {
      console.log(err);
    }
  });
  next();
};


app.use(demoLogger);


app.get("/", (req, res) => {
  res.send("This is the home Page");
});
app.get("/about", (req, res) => {
  res.send("This is the About page");
});
app.post("/add", (req, res) => {
  res.send("Post request");
});
app.listen(PORT, () => {
  console.log(`App is listening to port ${PORT}`);
});