Hello! Today, we're unraveling a cornerstone of web application security - User Authentication. User Authentication involves verifying user identities during login attempts. By correctly implementing User Authentication, you can significantly protect your applications from unauthorized access and potential security threats. Our toolkit for today includes Node.js and Express.js for crafting our server-side application, and MongoDB with Mongoose for managing our users' data.

Next, we'll delve into MongoDB, a powerful NoSQL database, and Mongoose, a MongoDB object modeling tool designed to work in an asynchronous environment. They'll assist us in storing and managing user data in a structured manner.

Let's dive straight in and create a User Model, comprising username and password attributes:

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1var mongoose = require('mongoose'); // Importing the Mongoose library
2var Schema = mongoose.Schema; // Retrieving the Schema object from Mongoose
3var UserSchema = new Schema({
4  username: String, // Setting the username attribute as a string
5  password: String, // Setting the password attribute as a string
6});
7var User = mongoose.model('User', UserSchema); // Creating a User model with the above schema

Here, mongoose.model is used to create a User model in our MongoDB database. Each User document in our database will have a username and password field.

Now, let's apply the final touches and create our authentication middleware. Middleware in Express provides a way to work with request and response objects in your application. Middleware functions can perform tasks such as modifying these objects, ending the request-response cycle, or invoking the next middleware function in the stack.

Here's a basic authentication middleware function:

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1var authMiddleware = function (req, res, next) {
2  if (req.query.username === "admin" && req.query.password === "admin") {
3   // If the user is authenticated, the next function is invoked
4    console.log("User authenticated");
5    next();
6  } else {
7   // If credentials are invalid, we respond with 'Invalid credentials'
8    res.send("Invalid credentials");
9  }
10};

This authMiddleware checks whether the submitted username and password match the known credentials. If they match, the next function in the middleware stack is invoked; otherwise, the response 'Invalid credentials' is sent.

We can further improve our authentication middleware by using MongoDB to store and retrieve user credentials, making our method more robust and practical.

Instead of hardcoding a username and password into our middleware function, we can query the user’s data from the MongoDB database using Mongoose. A successful database query will mean the user details are valid, and the user is authenticated.

To handle the asynchronous query calls in an efficient, readable way, we'll use JavaScript's native async/await syntax. This approach leads to cleaner and more structured code, avoiding the potential pitfalls of callback functions.

Let's see how the authentication middleware looks when we use MongoDB and Mongoose:

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1var authMiddleware = async function (req, res, next) {
2  // Search for the user credentials in the MongoDB database
3  var user = await User.findOne({ username: req.query.username, password: req.query.password });
4
5  // If the query returned a user, proceed with the next function
6  if (user) {
7    console.log("User authenticated");
8    next();
9  } else {
10    // If no user returned from the query, authentication fails 
11    res.send("Invalid credentials");
12  }
13};

In this updated version of authMiddleware, we fetch the user data with a Mongoose query inside an async function, and then await the results. The query returns the user's document if the username and password in the request match a document in the MongoDB database. Otherwise, it returns null.

If the user exists, next() is called to proceed with the route handlers. If no user is returned (indicating the credentials don't match any document in the database), a response with 'Invalid credentials' is sent instead.

Fantastic work! We've explored User Authentication, constructed a user model in MongoDB using Mongoose, and established an authentication middleware using Express.js.

It's now time to put our learning into practice! This will not only reinforce your understanding of the new concepts but also demonstrate them in action by simulating real-world web development scenarios. Remember, practice is the cornerstone of mastering any new concept. Best of luck!