Async JS/Node - Node Callbacks - Don't use return values from Callbacks example

example.js

var fs = require('fs')
  , path = 'my_file.txt'
  , file
  
file = fs.readFile(path, function(err, fileData) {
  if(err) {
    console.log(err)
  } else {
    console.log('success')
  }
});

console.log(file) // wtf?