Explain passport in Node.js

// Telling node to include the following
// external modules
var express = require('express');
var app = express();

// Mongoose for connecting to our database
const mongoose = require("mongoose");

// Body parser to fetch HTML form data later on
const bodyParser = require("body-parser");

// Connecting mongoose to our database
// named "userDatabase"
mongoose.connect(
'mongodb://localhost:27017/userDatabase', {
  useNewUrlParser: true,
  useUnifiedTopology: true
});

// Handling get request on home route.
app.get("/", function (req, res) {
    res.send("This is the home route");
});

// Allowing app to listen on port 3000
app.listen(3000, function () {
    console.log("server started successfully");
})

/*We are going to add simple authentication to our website 
We are going to collect data(email and password) entered by 
user in the HTML form, created in the INDEX.HTML file, and 
we are going to store that data in our database 
this is how we can simply register any new user */

/* if we want to log in our already registered user, 
then we collect email and password from HTML 
form created in LOGIN.HTML file, and 
we can find data(if any) associated with this 
email, and return it to user */
const mongoose = require('mongoose')
var express = require('express'); 
var app = express(); 
const bodyParser = require("body-parser"); 

// Allowing app to use body parser 
app.use(bodyParser.urlencoded({extended:true})); 

// Connecting mongoose to our database 
// named "userDatabase" 
mongoose.connect( 
'mongodb://localhost:27017/todo',{ 
useNewUrlParser: true, 
useUnifiedTopology: true
}); 

const userSchema = new mongoose.Schema({ 
email: String, 
password: String 
}); 

// Creating the User model. 
const User = new mongoose.model("User", userSchema); 


/* setting a simple get request on the home route, 
and sending our index.html file containing a form 
which will allow user to enter his details and 
register. */
app.get("/", function (req, res) { 
res.sendFile(__dirname + "/index.html"); 
}) 
app.get("/login", function(req, res) { 
res.sendFile(__dirname + "/login.html"); 
}) 

// Handling the post request on /register route. 
app.post("/register", function(req, res){ 
console.log(req.body); 
	
// Getting the email and password entered 
// by the user 
var email = req.body.username; 
var password = req.body.password; 
	
// Creating a new user with entered credentials. 
var newuser = new User({ 
	email : email, 
	password : password 
}) 
	
// Saving the newuser. 
newuser.save(); 
console.log("saved successfully"); 
	
// Sending the response that user 
// is saved successfully 
res.send("saved successfully"); 
}) 

app.post("/login", function(req, res) { 
	console.log(req.body); 
	
	// Getting the email and password entered 
	// by the user 
	var emailEntered = req.body.username; 
	var passwordEntered = req.body.password; 
	
	// Checking if the email entered exists 
	// in database or not. 
	User.findOne({email : emailEntered}, 
				function(err, data){ 
	if(data) { 

		// The email exists in the database. 
		console.log(data); 
			
		/* checking if the password entered 
		is matching the original password */
		if(data.password == passwordEntered){ 
			res.send("login successful!"); 
		} 
		else { 

			// Password is incorrect. 
			res.send("Incorrect Password"); 
			} 
	} 
	else { 

		// The email does not exist in the database 
		console.log(err); 
	} 
	}); 
}) 

// Allowing app to listen on port 3000 
app.listen(3000, function () { 
console.log("server started successfully"); 
})

const session = require("express-session");
const passport = require("passport");
const passportLocalMongoose = require("passport-local-mongoose");

app.use(express.static("public"));
app.use(bodyParser.urlencoded({extended:true}));

// Below all the app.use methods
app.use(session({
    secret : "any long secret key",
    resave : false,
    saveUninitialized : false
}));

app.use(session({
  secret: "any long secret key",
  resave: false,
  saveUninitialized: false
}));

// Initializing Passport
app.use(passport.initialize());

// Starting the session
app.use(passport.session());

// Creating user schema and adding a plugin to it

const userSchema = new mongoose.Schema({
  email: String,
  password: String
});
userSchema.plugin(passportLocalMongoose);

const User = new mongoose.model("User", userSchema);
passport.use(User.createStrategy());

// Serializing and deserializing
passport.serializeUser(User.serializeUser());
passport.deserializeUser(User.deserializeUser());

// Handling get request on the home and login route
app.get("/", function (req, res) {

  /* req.isAuthentcated() returns true or 
  false depending upon whether a session 
  is already running or not.*/
  if(req.isAuthenticated()) {
 
    /* if the request is already authenticated, 
    i.e. the user has already logged in and 
    there is no need to login again. Or we 
    can say, the session is running. */  
    res.send("
     You have already logged in. No need to login again");
  }
  
  else{

    // If the user is new and no session
    // is Running already 
    res.sendFile(__dirname + "/index.html");
  }
})

// Handling get request on login route
app.get("/login", function(req, res) {
    if(req.isAuthenticated()){
        /* if request is already authenticated, 
        i.e. user has already logged in and 
        there is no need to login again. */ 
        res.send("
You have already logged in. No need to login again");
     }
     else{
       res.sendFile(__dirname + "/login.html");
   }
})

/* The index.html file will be same as that
used in the earlier method of authentication*/ 
app.post("/register", function(req, res){
  console.log(req.body);
  
  // Getting Email and PAssword Entered by user
  var email = req.body.username;
  var password = req.body.password;
  
  /* Registering the user with email and
  password in our database  
  and the model used is "User" */
  User.register({ username : email }, 
  req.body.password, function (err, user) {      
    if (err) {
    
      // if some error is occurring, log that error
      console.log(err);
    }
    else {
      passport.authenticate("local")
      (req, res, function() {
        res.send("successfully saved!"); 
      })
    }
  })
})

// All handling related to login is done below.
// Here we are handling the post request on
// /login route
app.post("/login", function (req, res) {
  console.log(req.body);

  const userToBeChecked = new User({
    username: req.body.username,
    password: req.body.password,
  });

  // Checking if user if correct or not
  req.login(userToBeChecked, function (err) {
    if (err) {

      console.log(err);
      
      // If authentication fails, then coming
      // back to login.html page
      res.redirect("/login");
    } else {
      passport.authenticate("local")(
        req, res, function () {
        User.find({ email: req.user.username }, 
          function (err, docs) {
          if (err) {
            console.log(err);
          } else {
            //login is successful
            console.log("credentials are correct");
            res.send("login successful");
          }
        });
      });
    }
  });
});

var express = require('express');
var app = express();
const mongoose = require("mongoose");
 
/* Requiring body-parser package  
to fetch the data that is entered 
by the user in the HTML form.*/
const bodyParser = require("body-parser");
 
// Telling our Node app to include all these modules
const session = require("express-session");
const passport = require("passport");
const passportLocalMongoose = 
       require("passport-local-mongoose");
 
// Allowing app to use body parser
app.use(bodyParser.urlencoded({ extended: true }));
 
app.use(session({
    secret: "long secret key",
    resave: false,
    saveUninitialized: false
}));
 
// Initializing Passport
app.use(passport.initialize());

// Starting the session
app.use(passport.session());
 
// Connecting mongoose to our database 
mongoose.connect(
'mongodb://localhost:27017/userDatabase', {
    useNewUrlParser: true,
    useUnifiedTopology: true
}); 
 
/* Creating the schema of user which now 
include only email and password for 
simplicity.*/
const userSchema = new mongoose.Schema({
    email: String,
    password: String
});
 
/* Just after the creation of userSchema, 
we add passportLocalMongoose plugin 
to our Schema */
userSchema.plugin(passportLocalMongoose);
 
// Creating the User model
const User = new mongoose.model("User", userSchema);
 
/* After the creation of mongoose model, 
we have to write the following code */
passport.use(User.createStrategy());
 
// Serializing and deserializing
passport.serializeUser(User.serializeUser());
passport.deserializeUser(User.deserializeUser());
 
// Handling get request on the home route
app.get("/", function (req, res) {

    /* req.isAuthentcated() returns true or 
    false depending upon whether a session is 
    already running or not. */
    if (req.isAuthenticated()) {
 
        /* if request is already authenticated, 
        i.e. user has already logged in and 
        there is no need to login again or 
        we can say, session is running.*/
        res.send(
"You have already logged in. No need to login again");
    }
 
    else {

        // If the user is new and 
        // no session is Running already
        res.sendFile(__dirname + "/index.html");
    }
})
 
 
// Handling get request on login route
app.get("/login", function (req, res) {
    if (req.isAuthenticated()) {

        /* If request is already authenticated, 
        i.e. user has already logged in and 
        there is no need to login again. */
        res.send(
"You have already logged in. No need to login again");
    }
    else {
 
        /* if session has expired, then user 
         need to login back again and 
         we will send the Login.html */
        res.sendFile(__dirname + "/login.html");
    }
})
 
/* Registering the user for the first time
handling the post request on /register route.*/
app.post("/register", function (req, res) {
    console.log(req.body);
    var email = req.body.username;
    var password = req.body.password;
    User.register({ username: email },
        req.body.password, function (err, user) {
            if (err) {
                console.log(err);
            }
            else {
                passport.authenticate("local")
                 (req, res, function () {
                    res.send("successfully saved!");
                })
            }
        })
})
 
// Handling the post request on /login route
app.post("/login", function (req, res) {
    console.log(req.body);

    const userToBeChecked = new User({
        username: req.body.username,
        password: req.body.password
    });
 
    // Checking if user if correct or not
    req.login(userToBeChecked, function (err) {
        if (err) {
            console.log(err);
            res.redirect("/login");
        }
        else {
            passport.authenticate("local")
                (req, res,function () {
                User.find({ email: req.user.username },
                    function (err, docs) {
                      if (err) {
                         console.log(err);
                      }
                     else {

                       //login is successful
                       console.log("credentials are correct");
                     res.send("login successful");
                        }
                    });
            });
        }
    })
})
 
// Allowing app to listen on port 3000
app.listen(3000, function () {
    console.log("server started successfully");
})