gistfile1.js

var Rx = require('rx');
var fs = require('fs');

var Observable = Rx.Observable;

// helper function for converting callback APIs to Observables
var toObservableFunction = Observable.fromNodeCallback;

// convert each callback function to a function that returns Observables
var readdir = toObservableFunction(fs.readdir)
var stat = toObservableFunction(fs.stat);

var path = process.argv[2];

// Now that the callback APIs are gone, we're off to the races with functional composition!

// get observable of array of files ex. {......["file1", "file2"]}
readdir(path).
    flatMap(function(list) { 
        // convert array of files to Observable. ["file1","file2"] -> {...."file1"...."file2"}
        return Observable.fromArray(list).
            // call stat for each file and creates a tuple of file and time from the result.
            // This creates a two dimensional collection, which is then flattened using merge (the default flattening strategy)
            // {...."file1"...."file2"} -> 
            // 
            // -> {.....{file: "file1", mtime:23432}....{file:"file2", mtime:65465}}
            flatMap(function(file) {
                // call stat and create a tuple of file and time. "file1" -> {.....{file: "file1", mtime: 23432}}
                return stat(path + "/" + file).
                    map(function(stat) {
                        return {file: file, mtime: stat.mtime};
                    }).
                    // if a call to stat fails, filter the file from the results by returning an empty stream.
                    // Remember, empty streams disappear during flatten operations.
                    onErrorResumeNext(Observable.empty());
            });
    }).
    // convert observable of file and time pairs back to an array
    // {...{file: "file1", mtime:23432}....{file:"file2", mtime:65465}} -> [{file: "file1", mtime:23432}, {file:"file2", mtime:65465}]
    toArray().
    map(function(fileDescriptors) {
        // sort the array by time.
        // [{file: "file2",mtime: 45384}, {file: "file1",mtime:23432}] -> [{file: "file1",mtime:23432}, {file: "file2",mtime: 45384}]
        return fileDescriptors.
            sort(function(left, right) {
                return left.mtime - right.mtime
            }).
            // pull out just the file
            // [{file: "file1",mtime:23432}, {file: "file2",mtime: 45384}] -> ["file1","file2"]
            map(function(fileAndTime) {
                return fileAndTime.file;
            })
    }).
    forEach(
        function(files) {
            console.log(files);
        },
        function(e) {
            console.log("Unable to get file list!" + e);
        });