Express JS Tutorial

Introduction

Is a framework that can be used with NodeJS
You can build a web app and APIs
It supports many features as a plugin which makes it easy
Is server-side framework

const express = require('express');
const app = express();
const port = 3000;
app.get('/', (req, res) => {
    res.send('Hello World!');
});
app.listen(port, () => {
    console.log('Server running at http://localhost:3000');
});
//Output
// Server running at http://localhost:3000
// Hello World!

Basic routing

Routing defines the endpoint for URL
The endpoint is a complete URL, which accepts the client request and sends a response
Every routing will be registered with HTTP methods GET, POST, PUT, DELETE, PATCH etc
Every route has a handler function which will be executed when the endpoint or route is matched

const express = require('express');
const app = express();
const port = 3000;
app.get('/', (req, res) => {
    res.send('Hello World!');
});
app.listen(port, () => {
    console.log(`Server running at http://localhost:3000`);
});
//Output
// Server running at http://localhost:3000
// Hello World!

Routing structure

Syntax: app.METHOD(PATH, HANDLER)
app: is an instance of express
METHOD: is an HTTP request method, in lowercase
PATH: is a path on the server
HANDLER: is the function executed when the route is matched

const express = require('express');
const app = express();
app.get('/getMethod', function (req, res) {
    res.send('Get method');
});
app.post('/postMethod', function (req, res) {
    res.send('Post method');
});
app.put('/putMethod', function (req, res) {
    res.send('Put method');
});
app.delete('/deleteMethod', function (req, res) {
    res.send('Delete method');
})
// output
// Get method
// Post method
// Put method
// Delete method

Serving static files

express.static:\nIs a built-in middleware function in Express
It helps in serving To serve images, CSS files, and JavaScript files
Syntax: express.static(root, [options])
root: It specifies the root directory from which to serve static assets
options: It accepts params for set HTTP headers, redirect, extensions etc
http://localhost:3000/images/profile.jpg

Express routing in-depth

Route methods:\nA route method is derived from one of the HTTP methods
Is attached to an instance of the express class

app.get('/getMethod', function (req, res) {
    res.send('Get method');
});
app.post('/postMethod', function (req, res) {
    res.send('Post method');
});
// output
// Get method
// Post method

Special method

app.all():\nIs a special method
Is used to load middleware functions at a path for all HTTP request methods
Example: Below handler is executed for requests to the route “/api” whether using GET, POST, PUT, DELETE, or any other

app.all('/api', function (req, res, next) {
    console.log('Log API method')
    next()
})

Route paths

Route methods define the endpoints at which requests can be made
Route paths can be strings, string patterns, or regular expressions

app.get('/getMethod', function (req, res) {
    res.send('Get method');
});
app.post('/postMethod', function (req, res) {
    res.send('Post method');
});
// output
// Get method
// Post method

Route parameters

Route parameters are named URL segments
Are used to capture the values specified at their position in the URL
The captured values are populated in the req.params object, with the name of the route parameter specified in the path as their respective keys
We can append a regular expression in parentheses (()) to match/validate/format the params

app.get('/user/:userId', function (req, res) {
    res.send(req.params)
})
//Example:
//Request URL: http://localhost:3000/users/123
//req.params: { "userId": "123" }

Route handlers

You can provide multiple callback functions to handle a request
Route handlers can be in the form of a function, an array of functions

//Example:
const validatePayload = (req, resp) =>{
    //validate
}
const validateLogin = (req, resp) =>{
    // check if valid user
}
app.get('/profile', validatePayload, validateLogin, function (req, res) {
    res.send('Profile info')
})
// output
// Profile info

Response methods

res.download(): Prompt a file to be downloaded
res.end(): End the response process
res.json(): Send a JSON response
res.jsonp(): Send a JSON response with JSONP support
res.redirect(): Redirect a request.
res.render(): Render a view template.
res.send(): Send a response of various types.
res.sendFile(): Send a file as an octet stream.
res.sendStatus(): Set the response status code and send its string representation as the response body

app.route()

You can create chainable route handlers for a route path by using app.route()

//Example:
app.route('/user')
    .get(function (req, res) {
        res.send('Get method')
    })
    .post(function (req, res) {
        res.send('Post method')
    })
    .put(function (req, res) {
        res.send('Put method')
    }).delete(function (req, res) {
        res.send('Delete method')
    })

express.Router

The express.Router class helps to create modular, mountable route handlers
A Router instance is a complete middleware

//Example:
// Create a file userRotes.js and place the code
var express = require('express')
var router = express.Router()
// Get user details
router.get('/details/:userId', function (req, res) {
    res.send('User details')
})
router.get('/create', function (req, res) {
    res.send('Create user')
})
module.exports = router
// create a file index.js
const express = require('express');
const app = express();
var user = require('./user')
app.use('/user', user)