Welcome back! In the previous lesson, you set up a basic chat interface using Node.js, Express.js, and HTML. This laid the foundation for creating a user-friendly web application. Now, we will take the next step by connecting this interface to our backend API. This connection is crucial for transforming our static interface into a dynamic, interactive chat application. By the end of this lesson, you will understand how to integrate the frontend with the backend, enabling real-time communication between the user and the server.

In this lesson, we will enhance our chat interface by connecting it to the backend API using the Fetch API. This tool will allow us to capture user input and send it to the server, transforming our chat into a dynamic application.

The Fetch API is a modern interface that allows you to make network requests from your web page. It provides a more powerful and flexible feature set for handling HTTP requests and responses. With Fetch, you can send and receive data from the server in the background, creating a smoother and more interactive user experience.

Here's how it works:

1. Making a Request: When you want to get or send data, you use the fetch() function. You tell it where to send the request (the URL) and what kind of request it is.

2. Handling the Response: After the request is sent, the server will reply. The Fetch API lets you handle this reply using .then(). You can think of this as opening the letter your friend sent back.

3. Dealing with Errors: Sometimes things go wrong, like if the server is down. The Fetch API lets you handle these problems using .catch(), so your web page can show a message or try again.

Here's a simple example of how the Fetch API works within a function:

Now that we have a basic understanding of the Fetch API, let's prepare our chat application by initializing some variables to store the current chat and user IDs. These variables will help us manage and track each chat session as we implement dynamic functionalities.

These variables are initialized to null and will be used to store the chat and user IDs once a new chat session is created. With this setup in place, we can now move on to updating the startNewChat and sendMessage functions to incorporate Fetch API functionalities for dynamic communication.

Previously, the startNewChat function simply cleared the chat history. Now, we will update it to initialize a new chat session by making a Fetch request to the backend.

In this updated version, the startNewChat function uses the Fetch API to send a request to the server. Here's how it works:

• url: This is the endpoint on the server where we want to send our request. In this case, it's /api/create_chat.
• method: This specifies the type of request we're making. We use POST here because we're sending data to the server to create something new.
• then: This method runs if the request is successful. The server sends back a response, and we use the chatId and userId from this response to set up our chat session.
• catch: This method runs if something goes wrong with the request. We display an alert to inform the user of the error.

The sendMessage function will now send the user's message to the server and display the server's response in the chat interface.

In this enhanced version, the sendMessage function uses the Fetch API to send a request to the /api/send_message endpoint. Here's a breakdown of the key parts:

• url: We specify the server endpoint /api/send_message where the message should be sent.
• method: We use POST again because we're sending data to the server.
• headers: This tells the server what kind of data we're sending. Here, it's application/json, which means we're sending JSON data.
• body: We use JSON.stringify() to convert our data into a JSON string. This includes the userId, chatId, and the message itself.

In this lesson, you learned how to connect the chat interface to a backend API using Node.js and Express.js. We covered the setup of API endpoints, the integration of the frontend with the backend, and the implementation of chat functionality. This connection is a crucial step in creating a dynamic and interactive chat application.

As you move on to the practice exercises, focus on reinforcing these concepts and experimenting with the code. This hands-on practice will deepen your understanding and prepare you for the next unit, where we will continue to enhance the chatbot's capabilities. Keep up the great work, and let's continue building this exciting application together!