Where to write code in nodejs server next?

Question

Node.js

I have created a nodejs server. Where to write js code next?
I have app.js:

// Подключение всех модулей к программе
const fs = require("fs");
const express = require("express");
const app = express();
const server = require("http").createServer(app);

// Отслеживание порта
server.listen(3000);

// Отслеживание url адреса и отображение нужной HTML страницы
app.get("/", function(req, res) {
  res.sendFile(__dirname + "/index.html");
});

const fileOutput = document.querySelector("#fileOutput");

fs.readFile("index.txt", "utf-8", function(err, data) {
  if (err) throw err;
  
  let fileData = data;
  fileOutput.innerText = fileOutput;
});

It gives me an error document is not definedon the command line.
And then I realized that the server cannot receive my document! How it is? I connected the server to index.html.
I tried to write everything on index.html in the script tag, but then the passage where I read the file gave me an error require is not defined

. What should I do? Where to write js code? How to use all nodejs features and modules with document?

Answer 1

in html