MAKE IT MODULAR

caller.js

var filterListModule = require('./filterListModule');

var dir = process.argv[2];
var postFix = process.argv[3];

filterListModule(dir, postFix, function (err, filteredList) {
    filteredList.forEach(function (fileName) {
        console.log(fileName);
    });
});

filterListModule.js

var fs = require('fs');

module.exports = function (dir, postFix, callBack) {

    String.prototype.endsWith = function (suffix) {
        return this.indexOf(suffix, this.length - suffix.length) !== -1;
    };

    fs.readdir(dir, function (err, list) {
        if (err) {
            return callBack(err);
        }

        var filteredList = list.filter(function (fileName) {
            return fileName.endsWith("." + postFix);
        });
        callBack(null, filteredList);
    });
}