What is the 'next' function in Express middleware and how can it help redirect to another function?

I am new in Nodejs and working on Express js, Right now i am working on "middleware function" for specific routes,I want to know that "what is use of next",means after autheticate what "next" function can do ? if we want to move/redirect to another function then how we can do ? And what is "checkAuthentication" ? Here is my current code

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

// Custom middleware function
const authMiddleware = (req, res, next) => {
  // Check if user is authenticated
  const isAuthenticated = checkAuthentication(req);
  
  if (isAuthenticated) {
    next();
  } else {
    // User is not authenticated, send an unauthorized response
    res.status(401).send('Unauthorized');
  }
};

// Middleware function is applied to specific routes
app.get('/protected', authMiddleware, (req, res) => {
  res.send('Protected Route');
});

// Route handler
app.get('/', (req, res) => {
  res.send('Home Page');
});

// Start the server
app.listen(3000, () => {
  console.log('Server is listening on port 3000');
});

Next is a callback function that is passed to a middleware function. You can find different names of it in different frameworks but, the concept remains the same.

I'll try to explain the middleware through your code itself.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

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

function checkAuthentication(req) {
  /*
  I am considering this as my authentication function. 
  When the user logged in, server has sent him a token, which now will act as a validator. 
  */

  if (req.headers.token) {
    const user = someFunctionToFetchUser(req.header.token)
    return user
  } else {
    return false
  }
}

/*
Now I want a function to be there on each protected api routes
to make user if user has a valid token then only allow them to interact with the data base otherwise throw an error 
*/


const authMiddleware = (req, res, next) => {
  // Check if user is authenticated
  const isAuthenticated = checkAuthentication(req);

  if (isAuthenticated) {
    // Now you have verified the has valid a token. So allow user 
    // to reach to the controller or any other middleware, if any.
    next();
  } else {
    // User is not authenticated, send an unauthorized response
    res.status(401).send('Unauthorized');
  }
};

// Middleware function is applied to specific routes
app.get('/protected', authMiddleware, (req, res) => {
  res.send('Protected Route');
});

// You can have any number of middlewares
app.get('/protected', authMiddleware, someOtherMiddleware, (req, res) => {
  res.send('Protected Route');
});

// And also you can pass data from middleware to next middleware/controller also by attahching it with request

function newMiddleware(req, res, next) {
  req.foo = "bar" //Now you can access this foo variable in next function.
}

<!-- end snippet -->

When you are writing a middleware handler, you have several choices at the end of whatever you are doing.

1. You can send a response to the request and finish the routing for that request. You send a response and do NOT call next(). In your example, that's what happens in the else branch of your if (isAuthentcated) that does res.status(401).send('Unauthorized');. It sends a response without calling next().

2. You can send Express to its error handler by calling next(err) where you pass next an error object. This will terminate further routing and control flow will move to whatever error handler is installed for Express.

3. You can tell Express that you want to continue routing to other route handlers that may also match this request. You do that by calling next() not passing any arguments to next. In your code example, this is what happens in the if (isAuthentcated) branch where it calls next().

Summary - So, calling next() with no arguments, continue routing to other request handlers that match the current route. Calling next(err) passes control the the Express error handler.

For your other questions:

> if we want to move/redirect to another function then how we can do ?

It's not clear what exactly you mean here. next() just continues routing. It doesn't call a specific function. If you want to call a specific function, you can just do so with a function call in your middleware.

If you want to redirect the client to a new URL, you can send a response with res.redirect(someURL) and not call next().

> And what is "checkAuthentication" ?

Since you don't disclose the code for that, we don't know for sure what it does. I would guess that it's checking something in a cookie or a header to see if the client appears to already be authenticated or if credentials were included with this request.

I hope this helps you to understand it better.
<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const express = require('express');
const app = express();
const port = 3000;
const path = require('path');

function firstMiddleware(req, res, next) {
  console.log('You are in first middleware');
  req.firstMiddleware = true;
  next(); // I am deliberately asking to pass the control to the next immediate function
}

function secondMiddleware(req, res, next) {
  console.log('You are in second middleware');
  req.secondMiddleware = true;
  next(); // same here, sending control to next immediate function
}


app.get('/', firstMiddleware, secondMiddleware, function(req, res) {
  console.log('Now I am in a controller');
  console.log(
    `Checking if the control was in firstMiddleware ${req.firstMiddleware}`
  );
  console.log(
    `Checking if the control was in secondMiddleware ${req.secondMiddleware}`
  );
  //here i do not have next paramater, this is what makes it different from a middleware. 
  //So if next is not there then you need to send the control back to frontend when everything is done
  res.status(200).send('API worked');
});
app.listen(port, () => {
  // Code.....
})

<!-- end snippet -->