Hello World

Make sure that you have installed the express framework:

npm install --save express

We require the express package:

const express = require('express');

We create an express application instance:

const app = express();

A web application essentially handles incoming requests at different (URL) endpoints and returns data (in form of HTML/text/JSON etc.) as response. To achieve this, we define different request handlers. A request handler is a function with two arguments (request and response). The request object captures all information about incoming HTTP request. The response object provides methods for sending response HTTP headers and data.

Here is a simple request handler function:

const index = function (request, response) {
    response.send('Hello World!');
}

We now tell the express framework about the endpoint for which this request handler will be used:

app.get('/', index);

We choose port number for the express web application:

const port = 3000;

We now setup the express application to listen at the specified port. If the listening starts successfully, then a callback will get called informing us about it:

app.listen(port, function() {
    console.log(`Example app listening on port ${port}!`);
});

Here is the complete code. Please save it in a file named helloworld.js.

const express = require('express');
const app = express();
const port = 3000;

const index = function (request, response) {
    response.send('Hello World!');
}

app.get('/', index);

app.listen(port, function() {
    console.log(`Example app listening on port ${port}!`);
});

We can start it by running:

node helloworld.js

Time to head to http://localhost:3000 and see the result.