Creating an Express App

Creating an Express App

Now that you understand HTTP methods, it is time to build a real API server. Express.js is a minimal, flexible web framework for Node.js that makes creating APIs simple and clean. In this episode you will install Express and set up the server.

Step 1: Install Express

npm install express

Step 2: Create the Server

Replace the contents of index.js:

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

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

// Basic route
app.get('/', (req, res) => {
    res.json({ message: 'Welcome to the REST API' });
});

// Start the server
const PORT = 3000;
app.listen(PORT, () => {
    console.log(`Server running on port ${PORT}`);
});

Understanding the Code

Line	Purpose
require('express')	Imports the Express library
express()	Creates an Express application instance
app.use(express.json())	Middleware that parses JSON request bodies automatically
app.get('/', callback)	Defines a route that responds to GET requests on /
res.json()	Sends a JSON response (sets Content-Type automatically)
app.listen(PORT)	Starts the server on the specified port

Route Handler Basics

app.get('/api/ninjas', (req, res) => {
    res.json({ message: 'GET all ninjas' });
});

app.post('/api/ninjas', (req, res) => {
    res.json({ message: 'POST a new ninja' });
});

app.get('/api/ninjas/:id', (req, res) => {
    res.json({ message: 'GET ninja ' + req.params.id });
});

Method	What It Handles
app.get()	GET requests
app.post()	POST requests
app.put()	PUT requests
app.delete()	DELETE requests
app.patch()	PATCH requests

The Request Object (req)

app.get('/api/ninjas/:id', (req, res) => {
    console.log(req.params);   // { id: '123' }
    console.log(req.query);    // { sort: 'name' }  from ?sort=name
    console.log(req.body);     // POST/PUT body data
    console.log(req.method);   // 'GET'
    console.log(req.url);      // '/api/ninjas/123?sort=name'
});

Property	Contains	Example
req.params	URL parameters (from :id)	{ id: '123' }
req.query	Query string parameters	{ sort: 'name' }
req.body	Parsed request body (JSON)	{ name: 'Ryu' }
req.method	HTTP method used	'GET'

The Response Object (res)

// Send JSON
res.json({ name: 'Ryu' });

// Send with status code
res.status(201).json({ name: 'Ryu' });

// Send a string
res.send('Hello World');

// Send a status only
res.sendStatus(204);  // 204 No Content

Testing the Server

npm run dev

Open http://localhost:3000 in a browser and you should see:

{ "message": "Welcome to the REST API" }

Key Takeaways

• express() creates an app instance; app.listen() starts the server
• express.json() middleware parses JSON request bodies automatically
• app.get(), app.post(), app.put(), app.delete() handle specific HTTP methods
• req.params contains URL parameters, req.query has query strings, req.body has parsed JSON
• res.json() sends a JSON response and sets the Content-Type header automatically