08 - Building APIs with Express.js

• nodejs
• by Sn0wAlice

Introduction

Express.js is widely used for building RESTful APIs in Node.js applications. This chapter covers API creation, route handling, request validation, error handling, and best practices.

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1. Setting Up an Express API

Install Express.js

npm install express

Create a Basic API Server

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

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

app.get("/", (req, res) => {
    res.send("Welcome to our API");
});

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

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2. Defining API Routes

APIs typically use different HTTP methods:

• GET – Retrieve data
• POST – Create new data
• PUT – Update existing data
• DELETE – Remove data

Example: CRUD Routes

const users = [];

// Create User (POST)
app.post("/users", (req, res) => {
    const user = { id: users.length + 1, name: req.body.name };
    users.push(user);
    res.status(201).json(user);
});

// Get All Users (GET)
app.get("/users", (req, res) => {
    res.json(users);
});

// Get Single User (GET)
app.get("/users/:id", (req, res) => {
    const user = users.find(u => u.id == req.params.id);
    if (!user) return res.status(404).send("User not found");
    res.json(user);
});

// Update User (PUT)
app.put("/users/:id", (req, res) => {
    const user = users.find(u => u.id == req.params.id);
    if (!user) return res.status(404).send("User not found");
    user.name = req.body.name;
    res.json(user);
});

// Delete User (DELETE)
app.delete("/users/:id", (req, res) => {
    const index = users.findIndex(u => u.id == req.params.id);
    if (index === -1) return res.status(404).send("User not found");
    users.splice(index, 1);
    res.send("User deleted");
});

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3. Validating Requests with Express Validator

Validation ensures incoming data is correct before processing.

Install express-validator

npm install express-validator

Example: Validating User Input

const { body, validationResult } = require("express-validator");

app.post("/users",
    [body("name").isLength({ min: 3 }).withMessage("Name must be at least 3 characters long")],
    (req, res) => {
        const errors = validationResult(req);
        if (!errors.isEmpty()) {
            return res.status(400).json({ errors: errors.array() });
        }
        const user = { id: users.length + 1, name: req.body.name };
        users.push(user);
        res.status(201).json(user);
    }
);

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4. Error Handling in APIs

Example: Centralized Error Handling Middleware

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send("Something went wrong!");
});

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5. Best Practices for Building APIs

• Use proper status codes (200 for success, 400 for bad requests, etc.)
• Implement authentication & authorization (JWT, OAuth, etc.)
• Validate user input using express-validator
• Use environment variables for sensitive data
• Rate-limit requests to prevent abuse (express-rate-limit)
• Enable CORS for cross-origin API access

Enable CORS

npm install cors

const cors = require("cors");
app.use(cors());

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🏆 Exercises

• Create an Express API with CRUD operations for managing products.
• Add request validation for user registration (minimum username length, valid email).
• Implement error handling for missing routes.
• Secure your API with JWT authentication.
• Enable CORS and implement rate-limiting to prevent API abuse.

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Conclusion

This chapter covered building APIs with Express.js, handling routes, validation, and best practices. Next, we will explore connecting APIs to databases.

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