Let me explain you the server and client in detail.

1. A user sends an HTTP request using either the GET or POST method to a server.
2. The server receives the request and processes it.
3. If the request is a GET request, the server retrieves data from the database based on the parameters specified in the request.
4. If the request is a POST request, the server stores the data sent in the request into the database.
5. Once the data is retrieved or stored, it is formatted into JSON format (if necessary) and sent back to the client in an HTTP response.
6. The client receives the HTTP response and processes the JSON data (if any) to display it to the user.

📃Additional Information

What are req and res?

• req and res are short names for request and response.
• They are objects that help us communicate with the client using HTTP.
• HTTP is a protocol that lets the client and the server exchange data over the internet.

What is the request object (req)?

The request object contains information about what the client wants from the server.

• Some of the information in the request object are:

  • The URL: the address of the resource that the client wants to access.

  • The method: the action that the client wants to perform, such as GET, POST, PUT, DELETE, etc.

  • The headers: additional information about the request, such as the content type, the language, the authorization, etc.

  • The query string: optional data that is added to the URL after a question mark (?), such as /?name=John&age=25.

  • The parameters: optional data that is added to the URL as placeholders, such as /user/:id, where id can be any value.

  • The body: optional data that is sent along with the request, usually in a form or a JSON format.

• The request object also has methods that let us check or modify the request, such as:
  • req.accepts (): checks what kind of data the client can receive, such as text, HTML, JSON, etc.
  • req.is (): checks what kind of data the client has sent, such as text/plain, application/json, etc.
  • req.get (): gets a specific header from the request, such as req.get ('content-type').
  • req.set (): sets a specific header for the request, such as req.set ('content-type', 'application/json').

What is the response object (res)?

The response object contains methods and properties to send back a response to the client.

• Some of the methods and properties in the response object are:
  • res.send (): sends a simple message to the client, such as res.send ('Hello World').
  • res.json (): sends a JSON object to the client, such as res.json ({name: 'John', age: 25}).
  • res.render (): renders a view template to the client, such as res.render ('index', {title: 'Home Page'}).
  • res.status (): sets the status code for the response, such as res.status (200) for success or res.status (404) for not found.
  • res.set (): sets a specific header for the response, such as res.set ('content-type', 'text/plain').
  • res.cookie (): sets a cookie for the response, such as res.cookie ('name', 'John').
  • res.sendFile (): sends a file to the client, such as res.sendFile ('/path/to/file.jpg').
• The response object also has properties that let us access or modify the response, such as:
  • res.statusCode: gets or sets the status code for the response, such as res.statusCode = 200 or console.log (res.statusCode).
  • res.headersSent: checks if the headers have been sent to the client, such as if (res.headersSent) { // do something }.
  • res.locals: stores local variables that are accessible within the view template, such as res.locals.title = 'Home Page'.

How do we use req and res in Express.js?

• Express.js is a web framework for Node.js that makes it easier to create web applications using HTTP.
• In Express.js, we use req and res as parameters in a callback function that handles a specific route or middleware.
• A route is a part of the URL that tells us what to do when a request is made. A middleware is a function that can change or add something to the request or response before or after a route is handled.

Example of using req and res in Express.js

In this example, we will use your code to show how req and res work in Express.js.

const express = require("express");
const app = express();
 
//Home Page
app.get(`/`, (req, res) => {
  console.log("Data send by the client: ", req.query.id);
  res.send("This is our main Page");
});
 
//Server
app.listen(5001);

1. We do

const express = require("express");

This line imports the express module using the require () function and stores it in a variable called express.

2. Then we do

const app = express();

This line creates an Express application using the express () function and stores it in a variable called app.

app.get(`/`, (req, res) => {
  console.log("Data send by the client: ", req.query.id);
  res.send("This is our main Page");
});

• This line defines a route for the home page using app.get (). This means that whenever a client makes a GET request to the root path (/), the callback function will be executed. The callback function takes two arguments: req and res.
• The req argument represents the request object that contains information about the client's request. In this callback function, we use console.log () to print the value of req.query.id to the terminal. This means that if the client sends a request like /?id=123, we will see 123 in the terminal. The req.query property contains an object of key-value pairs that are added to the URL after a question mark (?).
• The res argument represents the response object that contains methods and properties to send back a response to the client. In this callback function, we use res.send () to send a message "This is our main Page" back to the client. This means that the client will see this message in their browser or other HTTP client. The res.send () method can send data in different formats, such as plain text, HTML, JSON, etc.

3. This line starts the server and listens for incoming requests on port 5001. This means that our application will be accessible at http://localhost:5001/ (opens in a new tab).

app.listen(5001);