Understanding function (err, data) callbacks

Question

So I am confused about how the function(err, data) callback works, is the first argument always an error handler?

What about the remaining arguments if you had something like function (x, y, z, a, b, c)?

How does the data from fs.readFile pass from the top line of code to the bottom line of code? Or in other words, how does the output of fs.readFile get put into the data argument?

fs.readFile(pathToFile, function (err, **data**) {
    bufferString = **data**.toString();

I could replace function (err, data) with function (x, y) and function (x, y, z, a, b, c)

But only the second argument works (data and y), is this just the syntax of javascript callbacks?

For example, this is working code to asynchronously read a file and print out the number of lines given a file:

var fs = require('fs');
var pathToFile = process.argv[2];
var bufferString, bufferStringSplit;

function counter(callback) {
  fs.readFile(pathToFile, function (err, data) {
    bufferString = data.toString();
    bufferStringSplit = bufferString.split('\n');
    callback();
  });
}

function logMyNumber() {
  console.log(bufferStringSplit.length-1);
}

counter(logMyNumber);

Answer 1

The caller of the callback (which is the readFile method in this case) decides what arguments are passed to the callback. You need to declare your callback to match what readFile says that it will pass to the callback. You can name the arguments anything you want (the names you use do not matter), but they will get values in the order that readFile decides.

In this case, fs.readFile() calls the callback with the two arguments you have in your code as in callback(err, data).

Here's an example from the node.js docs:

fs.readFile('/etc/passwd', function (err, data) {
  if (err) throw err;
  console.log(data);
});