NodeJS Loop trap, callback is called nextTick.

loop_trap.js

var fs = require('fs');
var files = ['a.txt', 'b.txt', 'c.txt'];
for (var i = 0; i < files.length; i++) { 
    fs.readFile(files[i], 'utf-8', function(err, contents) {
    console.log(files[i] + ': ' + contents);
  });
}