Node.js - undefined variable after assignment in fs functions - node.js

Question

stackoverflow!
I am fairly new to node.js, and this seems to be node-specific. I keep defining a variable and I get errors that fileName is undefined. I have no idea why this is happening because, from my perspective, I'm just assigning a global variable inside a function, and that should work in all other programming languages I've worked in. Does the argument function in fs.readFile() somehow differ from a normal function? I honestly have no idea. Anyways, this is my full code:
var fs = require('fs');
var dateObject = new Date();
var fileName;
function Start() {
fs.readFile('./counter.txt', function (err, data) {
if (err) throw err;
var current = parseInt(data);
current++;
fs.writeFile('./counter.txt', current.toString(), function(err) {
if (err) throw err;
console.log("Written!");
});
var fullDate = `${dateObject.getDate()}-${dateObject.getMonth() + 1}-${dateObject.getFullYear()}`;
fileName = `./logs/${fullDate} ${current}.txt`;
console.log(fileName);
fs.appendFile(fileName, "Logger Initiated!", function(err){
if (err) throw err;
})
});
}
function PAL (text) {
if (fileName === undefined) {
console.log("...");
return "500";
}
console.log(fileName);
fs.appendFile(fileName, text, function(err) {
if (err) throw err;
})
}
module.exports = {Start, PAL};
Another file:
const logger = require('./logger')
// ....
app.listen(port, () => {
logger.Start();
logger.PAL("test");
})

Answer 1

You got an asynchronous trap in code, fs.readFile is an async function, so when you run .PAL, I think you expected .Start function done, but it's not true. Start function:
async function Start() {
return new Promise((resolve, reject) => {
fs.readFile('./counter.txt', function (err, data) {
if (err) throw reject(err);
var current = parseInt(data);
current++;
fs.writeFile('./counter.txt', current.toString(), function(err) {
if (err) reject(err);
console.log("Written!");
var fullDate = `${dateObject.getDate()}-${dateObject.getMonth() + 1}-${dateObject.getFullYear()}`;
fileName = `./logs/${fullDate} ${current}.txt`;
console.log("FILENAME: ", fileName);
fs.appendFile(fileName, "Logger Initiated!", function(err){
if (err) reject(err);
resolve();
})
});
});
})
}
Call: logger.Start().then(() => logger.PAL("test"));