docs: Add comprehensive project documentation in changes-made.md

Author: Iswanna

chat-app/changes-made.md

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+## Changes Made
+
+### Backend Implementation (`backend/`)
+
+#### `server.js` (New)
+- Set up Express server with CORS middleware
+- Implemented **POST `/messages`** endpoint with input validation
+  - Validates sender name and message text
+  - Creates message objects with id, sender, text, likes, and dislikes
+  - Returns 201 status on success, 400 on validation error
+  - Triggers long polling callbacks to notify waiting clients
+  
+- Implemented **GET `/messages`** endpoint with long polling
+  - Accepts `?since=` query parameter for incremental message loading
+  - Returns only messages with id > sinceId
+  - Holds client requests until new messages arrive (long polling)
+  - Handles edge case where since=0 correctly
+  
+- Implemented **POST `/messages/:id/like`** endpoint
+  - Increments like count for specified message
+  - Notifies all waiting clients via long polling
+  - Returns 200 on success, 404 if message not found
+
+#### `package.json` (New)
+- Configured as ES module project (`"type": "module"`)
+- Added Express 5.2.1 dependency
+- Added CORS 2.8.6 dependency
+
+### Frontend Implementation (`frontend/`)
+
+#### `index.html` (New)
+- Semantic HTML structure with proper meta tags
+- Chat form with sender name and message inputs
+- Message container div for displaying chat history
+- Deferred JavaScript execution for proper DOM loading
+
+#### `script.js` (New)
+- **`getAllMessages()`** async function
+  - Fetches messages from backend with `?since=` parameter
+  - Implements incremental message loading via `lastIdSeen` tracking
+  - Updates existing message likes without re-rendering
+  - Creates DOM elements for new messages with text, like count, and like button
+  - Uses long polling with `setTimeout(getAllMessages, 0)` for real-time updates
+  - Includes error handling with automatic retry on fetch failure
+
+- **Form submission handler**
+  - Validates sender name and message text
+  - Sends POST request with JSON payload
+  - Clears input fields after successful submission
+  - Includes try-catch error handling
+
+- **Like button functionality**
+  - Sends POST request to `/messages/:id/like` endpoint
+  - Extracts current like count from DOM
+  - Provides immediate UI feedback (optimistic update)
+  - Updates display instantly without waiting for server response
+
+#### `styles.css` (New)
+- Styled message containers with border, padding, and rounded corners
+- Light gray background (#f9f9f9) for message boxes
+- Styled like buttons with blue background (#007bff) and white text
+- Proper spacing and cursor pointer for better UX
+
+## Testing Instructions
+
+### Setup
+```bash
+# Install backend dependencies
+cd backend
+npm install
+
+# Start backend server
+node server.js
+