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Can't get 'var express = require("express")' to work

I am new to js and I am trying to develop a simple node.js-mysql app. No matter what I do I can't get the standard
var express = require("express");
statement to work.
I have installed node.js and express correctly, express is in package.json. I have a local server running. But this simple line will not work.
On the node.js side at Windows command line I have no error but when I go to localhost:3000 on the browser, I get
'Uncaught Error: Module name "express" has not been loaded yet for
context: _. Use require([])' error at js console.
I tried changing it to
require(['express']`, function (express) {}
as suggested at node.js web site but then at the Windows command terminal I get a different error saying like
'expecting a string but received an array....'.
I have tried import instead of require and I have tried every suggestion that I could find on the Internet. I have been blowing my brains for weeks to get this to work with no success. I am so frustrated that I am seriously thinking about giving up all together. If someone can help I will be forever greatfull to him/her.
My main js code is as follows:
var port = 3000;
// Import or load node.js dependency modules.
var express = require("express");
var app = express();
var path = require("path");
var bodyParser = require("body-parser");
app.use(express.urlencoded({ extended: true })); // to support URL-encoded bodies.
app.listen(port, () => {
console.log(`server is running at http://127.0.0.1:8887`);
});
app.get('/', function(req, res){
res.sendFile("D:/Behran's files/Web projects/havuzlusite/index.html");
});

Require.JS is for loading AMD modules (and is, honestly, obsolete in today's JS landscape).
Node.js modules are either ECMAScript modules (which use import and export) or CommonJS modules (which use require and module.exports).
Even though both AMD and CommonJS modules use a function named require they are not compatible.
There are methods you can use to run ES modules and CommonJS modules in the browser however they can't replace APIs that are provided by runtimes.
Express.js needs to be able to listen for incoming HTTP requests. Browsers do not provide any mechanism to make that possible. Node.js does.
If you want to run Express.js you have to run it using Node.js and not a browser.
Express.js creates an HTTP server. A browser can make requests to it (e.g. if you type http://127.0.0.1:3000 into the address bar.
(Your code says server is running at http://127.0.0.1:8887 but the port constant is set to 3000).
All your Express.js code must run through Node.js.
You can't send a copy of that code to the browser and run it there too.

require is not defined on Spotify authorization code

We are doing a web using HTML and JavaScript. We have some problems when trying to do the authorization process to connect to the Spotify API.
var express = require('express'); // Express web server framework
var request = require('request'); // "Request" library
var querystring = require('querystring');
var cookieParser = require('cookie-parser')
This code we found on the internet uses require() to get some data but Chrome says:
Reference error: require is not defined.
We found that the JavaScript node may be the problem so we installed npm and Browserify but is not working.
We also used this in our HTML:
<script type="text/javascript" src="node.js"></script>
<script type="text/javascript" src="js/main.js"></script>
HELP??

What you have is the code that suppose to run on the server side. Node, Express, are server side components, and you can't run them on the clients side (inside of the browser). Server side is using 'require' to manage and load modules, that is why you have require there.
I think what should you do is to send your request to the server and server should process authentication code that you have there and send response back with the status if user authenticated or not.

require() is not an existing method for client-side JS.
Use a <script> tag then call the method after it.
This was already answered here:
Client on node: Uncaught ReferenceError: require is not defined\
Node.JS is a serverside application and you are attempting to run this clientside.
Please try using and referring to this http://requirejs.org/

Using socket.io in multi files

I am using Node.js Express with socket.io.
//app.js
var server = http.createServer(app).listen(app.get('port'), function(){
console.log('Express server listening on port ' + app.get('port'));
});
var io = require('socket.io').listen(server,{'log level': 1});
and if I want to do "io.sockets.emit" only for people watching index page which is provided by index.js how can I call "io.sockets.emit" in index.js other than using "require" for app.js(main file)?
I've googled about it though I couldn't find any sample source code which use socket.io in multiple files.

If your backend is separated in several module files like index.js, it's possible to pass variables to modules upon initialization.
Try importing index.js after initializing socket.io, and pass io to the index.js module being imported:
require('./index.js')(io);
See also this answer.

Socket.IO don't load inside TideSDK app

i created a chat app using Node.JS + Socket.IO + TideSDK, actually my Node.JS+Socket.IO server work perfectly through browser, but when a try load the server with my TideSDK app this error happend:
ReferenceError: Can't find variable: io
What's can be happend?
The code below is how i call the server.
<script src="https://mynodeserver.com/socket.io/socket.io.js"></script>
<script>
var socket = io.connect('https://mynodeserver.com/');
</script>

Where is the Socket.IO client-side .js file located?

I am trying to get socket.io (Node library) to work.
I have the server-side js working, and it is listening. The socket.io website states simply:
<script src="/socket.io/socket.io.js"></script>
<script>
var socket = io.connect('http://localhost');
socket.on('news', function (data) {
console.log(data);
socket.emit('my other event', { my: 'data' });
});
</script>
This is nice, however, what JS file am I importing!?!
I went into the node_modules directory, where I installed socket.io through npm, and inside socket.io/lib/ is socket.io.js file. However, this is server-side (uses the phrase require(), which errors on the client).
I have spent an hour looking around and I can't get any client .js file to work.
What am I missing?

I managed to eventually answer this for myself.
The socket.io getting started page isn't clear on this, but I found that the server side of socket.io automatically hosts the .js file on starting node, in the directory specified in the documentation:
"/socket.io/socket.io.js"
So you literally just point to this url regardless of your web app structure, and it works.

I would suggest checking if your node_modules directory is at the top level of your app directory. Also, I do believe you need to specify a port number; you should write something like var socket = io.connect('http://localhost:1337');, where the port number is 1337.

If you did npm install then the client socket.io file is located at node_modules/socket.io-client/dist/socket.io.js
Source: Socket get-started page

The client is available in a few ways:
supplied by the socket.io server at /socket.io/socket.io.js
included via webpack as the module socket.io-client
via the official CDN https://cdnjs.cloudflare.com/ajax/libs/socket.io/<version>/socket.io.js
For the first one, the server can be configured in a couple of ways:
// standalone
var io = require('socket.io')(port);
// with existing server from e.g. http.createServer or app.listen
var io = require('socket.io')(server);