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:

• assert: Provides a set of assertion tests to be used during unit testing
• buffer: To handle binary data
• child_process: To run a child process
• crypto: To handle OpenSSL cryptographic functions
• dns: To do DNS lookups and name resolution functions
• events: To handle events
• fs: To handle the filesystem
• http or https: For creating HTTP(s) servers
• stream: To handle streaming data
• util: To access utility functions like deprecate (for marking functions as deprecated), format (for string formatting), inspect (for object debugging), and so on

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.