Express is a Nodejs web application framework
It is used to assign routes (or paths) that are to be exposed by a server and add middlewares
Middlewares are added to a web app, so that every request passes through these middlewares allowing a number of functionalities such as request body parsing, authentication etc
Middlewares can be also imported from external packages apart from custom implementations
Some middlewares like authentication might return a response and stop further processing of a request
Middlewares can be added as generic middlewares or for specific routes
Provided example sets up a nodejs express server
Check NodeJs Installation
Create a new directory
mkdir express_testThe name of a directory should not match that of any (nodejs) dependency to be used
Change directory to newly created directory
cd express_test Run npm init command to create a package.json file so that node packages can be installed for current package
npm init -y -y is added so that it doesn't prompt for setting some common info about package
Run following command from project directory
npm install express --save package.js is created on npm init command
Save following code in index.js
// import express
const express = require('express');
// import external module to be used as middleware
const bodyParser = require('body-parser');
// declare an express app
const app = express();
// add bodyParser.json() middleware to application to parse body of each request to json
// so that any further middleware or method can access request body as json
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());
// assign a function to the route(or path) '/base', which only accepts Http Get requests (for Post request use app.post())
app.get('/base', function(req, res, next) {
// Send a string as response, can be changed to html or json content
res.send('Message from Express server');
});
// send a json response for route '/json'
app.get('/json', function(req, res, next) {
// Send a json as response
res.send({name: "test", id: 243});
});
// receives a post request and returns value of 'message' received from request body
app.post('/echo', function(req, res, next) {
// Send the 'message' field of request body
res.send(req.body.message);
});
// start server at port 8800
const server = app.listen(8800, function () {
// function called after server is started
console.log('Server listening on 8800');
});nextnext is a part of connect middleware
Middlewares are added as a stack
Each middleware calls next() which invokes the next middleware
Middlewares are executed one by one for each request to the app, until a middleware does not call next() within it
This flow is also stopped if next is called with an argument next(err) which is considered as an error
These can be tested to check if they behave differently, but maybe prefer altering a request req variable to send data to next middleware than trying with next(some_object)
Run following command from project directory
nodejs index.jsTo make it as accessible over a local network such as wifi, enter local ip address with command
nodejs index.js 192.168.43.12Here, 192.168.43.12 is ip address
Enter url in a browser to test the Apis such as http://127.0.0.1:8800/json
Save following code in a file named test.html (or some other name with .html extension) in a directory
Replace url in case server is run in some ip address other that localhost
Open the file in a browser (double-click should do that)
<!doctype html>
<html>
<meta charset="utf-8">
<body>
<div>
<div>
<h3>Test echo</h3>
<form action="http://127.0.0.1:8800/echo" method="post">
<table>
<tr>
<td><label for="id_message">Enter some text</label></td>
<td><input type="text" id="id_message" name="message"></td>
</tr>
<tr>
<td></td>
<td><input type="submit" value="submit"></td>
</tr>
</table>
</form>
</div>
</body>
</html>