File handling on NodeJS

// countLines.js
let fs = require('fs');
let filename = process.argv[2]; // parametro
let buffer = fs.readFileSync(filename);
let str = buffer.toString();
console.log(str.split('\n').length - 1);

// countLinesAsync.js
var fs = require('fs');
var filename = process.argv[2];
fs.readFile(filename, function(err, data) {
    var str = data.toString();
    console.log(str.split('\n').length - 1);
});

// ls.js
var fs = require('js');
var path = require('path');
var dirPath = process.argv[2];
var ext = '.' + process.argv[3];

fs.readdir(dirPath, function (err, list) {
    if (err) {
        return;
    }
    var files = list.filter(function(f) {
        return path.extname(f) === ext;
    });
    console.log(files.join('\n'));
});