Building a REST API

APIs (Application Programming Interfaces) are the backbone of modern web development. They allow different applications to communicate with each other—whether it’s your frontend talking to your backend, or third-party services integrating with your app. Think of a REST API like a restaurant menu. The menu (API documentation) tells you what dishes (resources) are available and how to order them (HTTP methods). You do not need to know how the kitchen works internally—you just place an order (request) and receive your dish (response). The waiter (HTTP) carries messages back and forth using a standardized process (the REST constraints).

What is REST?

REST (Representational State Transfer) is an architectural style for designing networked applications. It’s not a protocol or standard, but a set of constraints that, when followed, create scalable and maintainable APIs.

Core Principles of REST

Principle	Description
Stateless	Each request contains all the information needed to process it. The server doesn’t store client state between requests.
Client-Server	The client and server are separate, allowing them to evolve independently.
Uniform Interface	Resources are accessed using standard HTTP methods (GET, POST, PUT, DELETE).
Resource-Based	Everything is a resource (users, products, orders) identified by URLs.

HTTP Methods and Their Meanings

Method	Purpose	Example
`GET`	Retrieve data	Get all users, get user by ID
`POST`	Create new data	Create a new user
`PUT`	Update existing data (replace)	Update a user’s information
`PATCH`	Partial update	Update only user’s email
`DELETE`	Remove data	Delete a user

Why REST?

Universal: Works with any programming language that can make HTTP requests
Scalable: Statelessness allows easy horizontal scaling
Cacheable: GET requests can be cached for better performance
Simple: Uses familiar HTTP concepts developers already know

In this chapter, we will build a simple REST API to manage a list of items, demonstrating these principles in action.

Setup

Create a new file server.js and install express if you haven’t.

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

app.use(express.json()); // Middleware to parse JSON bodies

const PORT = process.env.PORT || 5000;

// Mock Database -- In a real application, this would be a database like
// PostgreSQL or MongoDB. Using an in-memory array means all data is lost
// when the server restarts. This is fine for learning, but never for production.
let items = [
  { id: 1, name: 'Item One' },
  { id: 2, name: 'Item Two' }
];

app.listen(PORT, () => console.log(`Server started on port ${PORT}`));

GET: Read Data

// Get all items
app.get('/api/items', (req, res) => {
  res.json(items);
});

// Get single item
app.get('/api/items/:id', (req, res) => {
  const found = items.some(item => item.id === parseInt(req.params.id));

  if (found) {
    res.json(items.filter(item => item.id === parseInt(req.params.id)));
  } else {
    res.status(404).json({ msg: `No item with the id of ${req.params.id}` });
  }
});

POST: Create Data

app.post('/api/items', (req, res) => {
  const newItem = {
    id: items.length + 1, // Simple ID generation -- WARNING: this breaks after deletions!
                           // If you delete item 2 from a 3-item list and add a new one,
                           // you get id=3 which collides with the existing item 3.
                           // In production, use UUIDs or database auto-increment.
    name: req.body.name
  };

  if (!newItem.name) {
    return res.status(400).json({ msg: 'Please include a name' });
  }

  items.push(newItem);
  res.status(201).json(newItem); // Return the created item with 201 status
});

PUT: Update Data

app.put('/api/items/:id', (req, res) => {
  const found = items.some(item => item.id === parseInt(req.params.id));

  if (found) {
    const updItem = req.body;
    items.forEach(item => {
      if (item.id === parseInt(req.params.id)) {
        item.name = updItem.name ? updItem.name : item.name;
        res.json({ msg: 'Item updated', item });
      }
    });
  } else {
    res.status(404).json({ msg: `No item with the id of ${req.params.id}` });
  }
});

DELETE: Remove Data

app.delete('/api/items/:id', (req, res) => {
  const found = items.some(item => item.id === parseInt(req.params.id));

  if (found) {
    items = items.filter(item => item.id !== parseInt(req.params.id));
    res.json({ msg: 'Item deleted', items });
  } else {
    res.status(404).json({ msg: `No item with the id of ${req.params.id}` });
  }
});

Testing with Postman / Insomnia

Since we don’t have a frontend yet, use tools like Postman or Insomnia to test your API.

GET http://localhost:5000/api/items -> Should return list.
POST http://localhost:5000/api/items with JSON body {"name": "New Item"} -> Should add item.
PUT http://localhost:5000/api/items/1 with JSON body {"name": "Updated Item"} -> Should update item 1.
DELETE http://localhost:5000/api/items/1 -> Should delete item 1.

Summary

REST APIs use standard HTTP methods for CRUD operations
GET for retrieving data (should never modify server state)
POST for creating data (return 201, not 200)
PUT for full replacement, PATCH for partial update
DELETE for removing data (return 204 No Content for success)
Always validate input and handle errors—never trust data from the client
Use proper HTTP status codes: they are part of the API contract, not decoration

Request Validation with Joi

Never trust client input! Validate all requests:

npm install joi

const Joi = require('joi');

// Define schema
const itemSchema = Joi.object({
  name: Joi.string().min(3).max(50).required(),
  description: Joi.string().max(500),
  price: Joi.number().positive().precision(2),
  category: Joi.string().valid('electronics', 'clothing', 'food'),
  tags: Joi.array().items(Joi.string()),
  inStock: Joi.boolean().default(true)
});

// Validation middleware
const validate = (schema) => (req, res, next) => {
  const { error, value } = schema.validate(req.body, { abortEarly: false });
  
  if (error) {
    const errors = error.details.map(detail => ({
      field: detail.path.join('.'),
      message: detail.message
    }));
    return res.status(400).json({ success: false, errors });
  }
  
  req.body = value; // Use validated/sanitized values
  next();
};

// Usage
app.post('/api/items', validate(itemSchema), (req, res) => {
  // req.body is now validated and sanitized
  res.status(201).json(req.body);
});

API Response Standards

Maintain consistent response formats:

// Success responses
const sendSuccess = (res, data, statusCode = 200) => {
  res.status(statusCode).json({
    success: true,
    data
  });
};

// Paginated responses
const sendPaginated = (res, data, page, limit, total) => {
  res.json({
    success: true,
    data,
    pagination: {
      page,
      limit,
      total,
      pages: Math.ceil(total / limit),
      hasNext: page * limit < total,
      hasPrev: page > 1
    }
  });
};

// Error responses
const sendError = (res, message, statusCode = 400) => {
  res.status(statusCode).json({
    success: false,
    error: message
  });
};

Pagination

app.get('/api/items', async (req, res) => {
  const page = parseInt(req.query.page) || 1;
  const limit = parseInt(req.query.limit) || 10;
  const skip = (page - 1) * limit;
  
  // Database query with pagination
  const items = await Item.find()
    .skip(skip)
    .limit(limit)
    .sort({ createdAt: -1 });
  
  const total = await Item.countDocuments();
  
  res.json({
    success: true,
    data: items,
    pagination: {
      page,
      limit,
      total,
      pages: Math.ceil(total / limit)
    }
  });
});

Filtering and Sorting

app.get('/api/items', async (req, res) => {
  // Build filter object
  const filter = {};
  
  if (req.query.category) {
    filter.category = req.query.category;
  }
  if (req.query.minPrice) {
    filter.price = { ...filter.price, $gte: parseFloat(req.query.minPrice) };
  }
  if (req.query.maxPrice) {
    filter.price = { ...filter.price, $lte: parseFloat(req.query.maxPrice) };
  }
  if (req.query.search) {
    filter.name = { $regex: req.query.search, $options: 'i' };
  }
  
  // Build sort object
  const sort = {};
  if (req.query.sortBy) {
    const order = req.query.order === 'desc' ? -1 : 1;
    sort[req.query.sortBy] = order;
  }
  
  const items = await Item.find(filter).sort(sort);
  res.json({ success: true, data: items });
});

// Example: GET /api/items?category=electronics&minPrice=100&sortBy=price&order=desc

HTTP Status Codes Reference

Code	Name	When to Use
200	OK	Successful GET, PUT, PATCH
201	Created	Successful POST (resource created)
204	No Content	Successful DELETE
400	Bad Request	Invalid input, validation error
401	Unauthorized	Missing or invalid authentication
403	Forbidden	Authenticated but not authorized
404	Not Found	Resource doesn’t exist
409	Conflict	Duplicate entry, version conflict
422	Unprocessable Entity	Valid JSON but semantic errors
429	Too Many Requests	Rate limit exceeded
500	Internal Server Error	Server-side error

API Versioning

const v1Router = require('./routes/v1');
const v2Router = require('./routes/v2');

app.use('/api/v1', v1Router);
app.use('/api/v2', v2Router);

// Or using headers
app.use('/api', (req, res, next) => {
  const version = req.headers['api-version'] || 'v1';
  req.apiVersion = version;
  next();
});

Complete REST Controller Example

// controllers/itemController.js
class ItemController {
  // GET /api/items
  async getAll(req, res, next) {
    try {
      const { page = 1, limit = 10, sort = '-createdAt' } = req.query;
      
      const items = await Item.find()
        .sort(sort)
        .skip((page - 1) * limit)
        .limit(parseInt(limit));
        
      const total = await Item.countDocuments();
      
      res.json({
        success: true,
        count: items.length,
        data: items,
        pagination: { page: parseInt(page), limit: parseInt(limit), total }
      });
    } catch (error) {
      next(error);
    }
  }

  // GET /api/items/:id
  async getById(req, res, next) {
    try {
      const item = await Item.findById(req.params.id);
      
      if (!item) {
        return res.status(404).json({
          success: false,
          error: 'Item not found'
        });
      }
      
      res.json({ success: true, data: item });
    } catch (error) {
      next(error);
    }
  }

  // POST /api/items
  async create(req, res, next) {
    try {
      const item = await Item.create(req.body);
      res.status(201).json({ success: true, data: item });
    } catch (error) {
      next(error);
    }
  }

  // PUT /api/items/:id
  async update(req, res, next) {
    try {
      const item = await Item.findByIdAndUpdate(
        req.params.id,
        req.body,
        { new: true, runValidators: true }
      );
      
      if (!item) {
        return res.status(404).json({
          success: false,
          error: 'Item not found'
        });
      }
      
      res.json({ success: true, data: item });
    } catch (error) {
      next(error);
    }
  }

  // DELETE /api/items/:id
  async delete(req, res, next) {
    try {
      const item = await Item.findByIdAndDelete(req.params.id);
      
      if (!item) {
        return res.status(404).json({
          success: false,
          error: 'Item not found'
        });
      }
      
      res.status(204).send();
    } catch (error) {
      next(error);
    }
  }
}

module.exports = new ItemController();

08. Express.js Basics 10. Deployment & Best Practices

​Building a REST API

​What is REST?

​Core Principles of REST

​HTTP Methods and Their Meanings

​Why REST?

​Setup

​GET: Read Data

​POST: Create Data

​PUT: Update Data

​DELETE: Remove Data

​Testing with Postman / Insomnia

​Summary

​Request Validation with Joi

​API Response Standards

​Pagination

​Filtering and Sorting

​HTTP Status Codes Reference

​API Versioning

​Complete REST Controller Example