Reaching html file in another subfolder express.js - node.js

Question

My folder hierarchy is as follows;
public
----index.html
src
----server.js
when I run the code
var express = require('express');
var app = express();
var serv = require('http').Server(app);
var net = require('net');
var path = require('path');
var port = 3000;
app.use('/../public/', express.static(path.join(__dirname + '/../public/')));
app.get('/', function(req,res){
res.sendFile(__dirname + '/../public/index.html');
});
serv.listen(3000, function(){
console.log('port ' + port + ' is started to being listened')
});
var io = require('socket.io')(serv,{});
it returns forbidden error. How can I reach a file in another subfile?
First of all, what I thought solved it actually didn't solve because I am running multiple projects to understand the basics and I was confused. But now everything is clear. What I am actually trying to do is trying to use express to divide everything to folders. As I didn't know much about http, I watched a lot of videos and finally, I figured it out after watching official introduction video of express on youtube.
Here again what I am trying to accomplish;
-localhost
--public
---index.html
--src
---all server js files and other files that will concern game programming.
As I was trying to reach above, I couldn't manage to do it. What did actually do the trick is;
creating a variable that specifies 'public' directory which is another sub directory under localhost and use it as static which was simple from the beginning but as I didn't know much, I wasn't able to use it after studying few tutorials. As Michael mentioned above, I didn't even need following lines;
app.get(publicDir, function(req, res){
res.sendFile('index.html');
})
All I did was;
var publicDir = require('path').join(__dirname, '/../public');
app.use(express.static(publicDir));

Answer 1

app.use('/../public/'
doesn't really make sense. This parameter is representing the request URL. You can't listen for request URL's which are in the hierarchy somewhere else but not inside your root folder.
Remember that app.use('/' equals a request URL like http://localhost:3000 - meaning this is the absolute basis of the website - the so called root. So where should '/../public' point to? There is no URL with which you could reach such an address.
If you write app.use('/public' it will listen for requests on http://localhost:3000/public
But of course you can send files located in a file structure like you have. So probably what you wanted is:
app.use(express.static(__dirname + '/../public'));
in order to search the static files in a folder structure like:
public
-styles.css
-script.js
-index.html
server
-currentscript.js
where you could access static files like http://localhost:3000/styles.css
You can also completely remove this controller:
app.get('/', function(req,res){
res.sendFile(__dirname + '/../public/index.html');
});
static files like HTML are already served by the Express static files controller mentioned above.
This is enough to access your index.html by calling http://localhost:3000