Node.js read directory files and write to JSON file

readfiles.js

var fs = require('fs');

fs.readdir([__dirname,'/app/images'].join(''), function(err, files) {
    function filter(file) {
        return /(\.png|\.jpg|\.jpeg)/.test(file);
    }
    var filtered = JSON.stringify(files.filter(filter));
    fs.writeFile([__dirname,'/app/data/images.json'].join(''), filtered, function(err) {
        if (err) throw new Error(err);
        console.log(filtered);
    });
});