Routes ; Help : function(request,Response)

Express uses series of "Middleware function" to handle and manipulate requests to the server as they move through the "Request-Response Cycle." Middleware functions have three arguments (four if it's a custom error handling middleware function), request, response, and next. Note these are just parameters so you can name them whatever you want, this or their short hand req, res, next is what you'll see most often. req represents the request object TO THE SERVER. (https://expressjs.com/en/4x/api.html#req). res represents the HTTP response send BACK TO THE CLIENT (https://expressjs.com/en/4x/api.html#res). next is used to denote this middleware function has completed and to move on to the next middleware function. The idea of middleware for Express can be confusing but whether you knew it or not I'm sure you've already used it!

Let's take this example,

var express = require('express');
var app = express();

app.get('/heyshay', function(req, res){
  res.send('hello world');
});

app.listen(3000);

In the above example, we tell Express to route all requests to the path '/heyshay' to this middleware function. We could access properties about the request, such as query string or path parameters from req.query.name and req.param.name. We can them set properties for the response back the client such as a status code or a response string with res.status(200).send("HEY SHAY")

For more information checkout the Express docs, they're really really good docs. https://expressjs.com/en/4x/api.html

That is correct. The route handler (which is the type of method you are asking about) get called when the route is accessed by the client. The request from the client and the response that will be returned to the client are then passed in the the route handler. Once the response is closed, it is sent to the client. Hope that makes sense!