Building CRUD Applications with Express.js

# CHAPTER 10

Building CRUD Applications with Express.js #

1. Introduction #

2. Learning Objectives #

• Understand how to structure an Express app using Controllers and modular Routers.

• Build the 5 standard REST endpoints for a resource.

• Simulate a database using an in-memory JavaScript array.

• Implement full CRUD operations handling JSON requests and responses.

3. Beginner-Friendly Explanation #

• Routes (routes/bookRoutes.js): The hallway signs. "Go to the kitchen."

• Controllers (controllers/bookController.js): The actual rooms where the work happens.

4. Project Setup & Simulated Database #

5. Step 1: The Controller (The Logic) #

// Simulated Database
let books = [
    { id: 1, title: '1984', author: 'George Orwell' },
    { id: 2, title: 'Dune', author: 'Frank Herbert' }
];

// READ ALL (GET /api/books)
const getBooks = (req, res) => {
    res.status(200).json({ status: 'success', data: books });
};

// READ ONE (GET /api/books/:id)
const getBookById = (req, res) => {
    const bookId = parseInt(req.params.id);
    const book = books.find(b => b.id === bookId);

    if (!book) return res.status(404).json({ error: 'Book not found' });
    res.status(200).json({ status: 'success', data: book });
};

// CREATE (POST /api/books)
const createBook = (req, res) => {
    const { title, author } = req.body;
    if (!title || !author) {
        return res.status(400).json({ error: 'Title and author required' });
    }

    const newBook = { id: books.length + 1, title, author };
    books.push(newBook);
    
    res.status(201).json({ status: 'success', data: newBook });
};

// UPDATE (PUT /api/books/:id)
const updateBook = (req, res) => {
    const bookId = parseInt(req.params.id);
    const book = books.find(b => b.id === bookId);

    if (!book) return res.status(404).json({ error: 'Book not found' });

    // Update fields if provided in req.body
    book.title = req.body.title || book.title;
    book.author = req.body.author || book.author;

    res.status(200).json({ status: 'success', data: book });
};

// DELETE (DELETE /api/books/:id)
const deleteBook = (req, res) => {
    const bookId = parseInt(req.params.id);
    books = books.filter(b => b.id !== bookId); // Remove the book

    // 204 means "No Content" (Successful deletion, nothing to return)
    res.status(204).send(); 
};

// Export all the functions
module.exports = { getBooks, getBookById, createBook, updateBook, deleteBook };

6. Step 2: The Router (The Hallway Signs) #

const express = require('express');
const router = express.Router(); // Create a modular router

// Import the Controller functions
const bookController = require('../controllers/bookController');

// Define the standard REST endpoints
router.get('/', bookController.getBooks);
router.post('/', bookController.createBook);
router.get('/:id', bookController.getBookById);
router.put('/:id', bookController.updateBook);
router.delete('/:id', bookController.deleteBook);

module.exports = router; // Export the router

7. Step 3: Wiring it to index.js #

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

// Import the Router file
const bookRoutes = require('./routes/bookRoutes');

// Essential Middleware!
app.use(express.json());

// Tell Express to use the bookRoutes for any URL starting with /api/books
app.use('/api/books', bookRoutes);

app.listen(3000, () => {
    console.log("Server running on port 3000");
});

8. Backend Workflow: Reviewing the Flow #

1. 1. Mobile App sends POST /api/books with JSON data.

1. 2. The Request hits index.js.

1. 3. express.json() middleware parses the data into req.body.

1. 4. index.js sees the URL starts with /api/books, so it hands traffic control to bookRoutes.

1. 5. bookRoutes sees it is a POST request to /, so it triggers bookController.createBook.

1. 6. The Controller function extracts the data, validates it, adds it to the array, and returns a 201 Status Code with the new JSON object.

9. Best Practices #

• Consistent File Structure: Always separate your concerns. index.js configures the server. routes/ defines the URLs. controllers/ contains the logic. This architecture is universally recognized by professional Node developers and makes scaling massive applications possible.

10. Common Mistakes #

• parseInt vs String: In the URL /api/books/1, the 1 is captured in req.params.id as a String ("1"). In our array, the ID is an Integer (1). If you try to find a match using strict equality (===) without converting the string to an integer using parseInt(), JavaScript will fail to find the book.

11. Exercises #

1. 1. Explain the architectural advantages of moving business logic out of the Router files and into dedicated Controller files.

12. Coding Challenges #

• Challenge: Using Postman, craft the JSON payload required to test the updateBook (PUT) route. What URL do you use? What body format do you select? What HTTP method do you select?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the Separation of Concerns in an Express.js API regarding the Router layer and the Controller layer.

• Q: Walk me through the 5 standard RESTful endpoints for a resource named "articles", including the HTTP verb and the URL path for each.

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #