Node.js provides a way to enter a run JavaScript code from the command line called the Read-Eval-Print Loop or REPL. You can start the REPL simply by typing node at the command line in a console. It's a great way to begin exploring some of the possibilities with Node.js, as shown in the following command:

$ node
> var sum = 1 + 2;
undefined
> sum
3
console.log(sum);
3
undefined

Here, it might appear that there are two return values, the first being 3 and the second being undefined. In this case, console.log() is a function that is used to write things out to the screen, but the function actually returns undefined. This will be useful in writing Node.js code where you want to log things to the screen, similar to print statements in other programming languages.

To quit out of the REPL, hit Ctrl + C twice.

Using the REPL on its own will not be incredibly useful. Node.js allows developers to create programs by writing them out and saving them as text files with a .js file extension.

One of the advantages of this is that you can use any text editor and practically any IDE to write Node.js programs.

To show you how powerful Node.js can be, let's start by building a simple web server. It will not do much, other than handling HTTP requests, but it's a great start.

Let's create a new file, call it hello.js, and open it in your favorite text editor or text-based IDE. Then, type the following lines of code:

var http = require('http'); 
var serverResponse = function(req, res){ 
    res.end("Hello Node"); 
} 
 
var server = http.createServer(serverResponse); 
server.listen(3000); 

Save the file and navigate it to the directory where it is stored with a console program. Then, type the following command:

$ node hello

At this point, if you typed the code correctly, there will be no output in the console other than a new line. Open your favorite browser and type localhost:3000 in your address bar.

You should see Hello Node in your browser's main window. And just like that, you have written a web server.

You can stop the server by typing Ctrl + C in the console.

There's a lot going on in a small piece of code, so let's walk through what the program is doing:

As Node.js projects become larger, having a tool to mange it properly is important. Concerns such as consistent dependency management, version management, and environment management are made easier using Node Package Manager and the package.json file.

Fortunately, the smart people who created Node.js created a method of doing that using NPM. Running npm init from the command line will set up a Node.js project and build out a package.json file, which is used to manage your node projects. Let's try it with your hello Node project. Note that some of the prompts are optional and can be left blank, as shown in the following commands:

$ npm init
This utility will walk you through creating a package.json file.
It only covers the most common items, and tries to guess sensible defaults.
See `npm help json` for definitive documentation on these fields
and exactly what they do.
Use `npm install <pkg> --save` afterwards to install a package and
save it as a dependency in the package.json file.
Press ^C at any time to quit.
name: (SPA-js) hello-node
version: (1.0.0) 
description: my new program
entry point: (hello.js) 
test command: 
git repository: 
keywords: 
author: Your Name
license: (ISC) 
About to write to /Users/your-home/helloNode/package.json:
{
  "name": "hello-node",
  "version": "1.0.0",
  "description": "my new program",
  "main": "hello.js",
  "scripts": {
    "test": "echo "Error: no test specified" && exit 1"
  },
  "author": "John Moore",
  "license": "ISC"
}
Is this ok? (yes) 

Pressing Enter at this point will create the package.json file in the directory you're in. Open it in your IDE or text editor and take a look. While you're in there, let's make the following small change to the scripts section:

"scripts": { 
  "test": "echo "Error: no test specified" && exit 1", 
  "start": "node hello" 
} 

The scripts section of package.json allows you to run code using NPM. In this case, typing npm start will run the node hello command and start your web server. It's not a super efficient shortcut at this point, but you can create efficient and useful aliases for lots of commands this way.

One of the very important things that NPM does is manage dependencies. In the next section, on Express, you'll see how to store references to NPM modules and their versions in the package.json file. This is important when working as part of a group or team. By taking a copy of a project's package.json file, a developer can recreate the environment for the project just by running NPM installation.