Typical node.js callback pattern

gistfile1.js

// include the filesystem module
var fs = require('fs');

// fs.readFile: read a file and all its contents,
//              then call a callback function
fs.readFile('/some/file', function (err, contents) {

    // if any error occurred, throw it
    if (err) throw err;

    // do something when no error occurred
    /* something(); */

});