In today’s lesson, we will learn how to ensure the data we work with is correct and how to avoid having the same data more than once in our MongoDB database using Mongoose. This is crucial because making sure our data is accurate and unique helps our applications run smoothly without errors or confusion.

In this lesson, you'll learn:

• How to validate data in Mongoose schemas.
• How to handle and prevent duplicate entries.
• Practical examples to implement these concepts in a simple web application.

Let's start by setting up an Express.js server and connecting it to our MongoDB instance. This allows us to serve our application and communicate with the database.

In this part of the code, we set up an Express.js server and connect to a MongoDB database named todo-app running on localhost. The useNewUrlParser and useUnifiedTopology options are included to avoid deprecation warnings. We also use the middleware to parse incoming JSON requests, which is essential for handling JSON payloads in API requests.