Callbacks are functions that are executed asynchronously, or at a later time. Instead of the code reading top to bottom procedurally, asynchronous programs may execute different functions at different times based on the order and speed of earlier functions.

Since JavaScript treats functions like any other object, we can pass a function as an argument in another function and alter execute that passed-in function or even return it to be executed later.

We saw such a function previously when we were looking at the fs module in The Module System section. Let's revisit it:

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

On line 3, we use a variable part of the globals, _ _dirname, which basically gives us the absolute path of the directory (folder) in which our current file (read-file.js) is, from which we can access the temp/sample.txt file.

Our main point of discussion is the chunk of code between lines 5 and 8. Just like most of the methods you will come across in Node.js, they mostly take in a callback function as the last argument.

Most callback functions will take in two parameters, the first being the error object and the second, the results. For the preceding case, if file reading is successful, the error object, err, will be null and the contents of the file will be returned in the data object.

Let's break down this code for it to make more sense:

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

Now, let's look at the asynchronous part. Let's add an extra line to the preceding code:

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

See what we get as a print out:

Print out last!
  hello,
  world

How come Print out last! comes first? This is the whole essence of asynchronous programming. Node.js still runs on a single thread, line 10 executes in a non-blocking manner and moves on to the next line, which is console.log('Print out last!'). Since the previous line takes a long time, the next one will print first. Once the readFile process is done, it then prints out the content of file through the callback.