As mentioned earlier, these are modules that can be used straight-away without any further installation. All you need to do is to require them. There are quite a lot of them, but we will highlight a few that you are likely to come across when building web applications:

For example, the following code reads the content of the lesson-1/temp/sample.txt file using the in-built fs module:

const fs = require('fs');
let file = `${__dirname}/temp/sample.txt`;
fs.readFile(file, 'utf8', (err, data) => 
{
  if (err) throw err;
  console.log(data);
});

The details of this code will be explained when we look at asynchronous programming later in this chapter.

Node Package Manager (npm) is the package manager for JavaScript and the world's largest software registry, enabling developers to discover packages of reusable code.

To install an npm package, you only need to run the command npm install <package-name> within your project directory. We are going to use this a lot in the next two chapters.

Let's look at a simple example. If we wanted to use a package (library) like request in our project, we could run the following command on our Terminal, within our project directory:

npm install request

To use it in our code, we require it, like any other module:

const request = require('request');
request('http://www.example.com', (error, response, body) => 
{
  if (error) console.log('error:', error); // Print the error if one occurred
  else console.log('body:', body); // Print the HTML for the site.
});

It's worth noting how Node.js goes about resolving a particular required module. For example, if a file /home/tony/projects/foo.js has a require call require('bar'), Node.js scans the filesystem for node_modules in the following order. The first bar.js that is found is returned:

Node.js looks for node_moduels/bar in the current folder followed by every parent folder until it reaches the root of the filesystem tree for the current file.

Let's dive a little deeper into npm, by looking at some of the handy npm commands that you will often use:

We have already looked at how local modules are loaded from the previous example that had math.js and index.js.

Since JavaScript Object Notation (JSON) is such an important part of the web, Node.js has fully embraced it as a data format, even locally. You can load a JSON object from the local filesystem the same way you load a JavaScript module. During the module loading sequence, whenever a file.js is not found, Node.js looks for a file.json.

See the example files in lesson-1/b-module-system/1-basics/load-json.js:

const config = require('./config/sample');
console.log(config.foo); // bar

Here, you will notice that once required, the JSON file is transformed into a JavaScript object implicitly. Other languages will have you read the file and perhaps use a different mechanism to convert the content into a data structure such as a map, a dictionary, and so on.