Using promises with Node.js functions on example

Question

I'm started learning JavaScript promises, but I can't understand how apply, for example, Q functions to Node.js callback functions.

In similar question is recommended to use Q.denodeify(), but it works well for fs.readFile() and don't works for fs.exists().

This is a simple functions that returns list of files and their sizes in directory:

function openDir(path, callback) {
    path = __dirname + path;
    fs.exists(path, function (exists) {
        if (exists) {
            fs.readdir(path, function (err, files) {
                if (err) {
                    throw err;
                }
                var filesCounter = 0,
                    filesTotal = files.length,
                    result = [];
                files.forEach(function (filename, index) {
                    fs.stat(path + filename, function (err, stats) {
                        if (err) {
                            throw err;
                        }
                        result[index] = {};
                        result[index].name = filename;
                        result[index].size = stats.size;
                        if (++filesCounter === filesTotal) {
                            callback(result);
                        }
                    });
                });
            });
        }
    });
}

http.createServer(function (req, res) {
    openDir('/', function (data) {
        res.writeHead(200, {
            'Content-Type': 'application/json; charset=utf-8'
        });
        res.end(JSON.stringify(data));
    });
}).listen(1337, '127.0.0.1');

How write this code using promises (with Q or other library)?

You could take a look at my gist, which uses pretty similar tasks and it's written with Q gist.github.com/riston/9022931. — Risto Novik
– Risto Novik, Commented Feb 16, 2014 at 16:19

Scott Puleo · Accepted Answer · 2014-02-18 14:18:12Z

fs.exists is kind of a useless method. It does not return a node style call back and can usually be omitted. In your example you would handle the error from fs.readdir.

fs.exists() is an anachronism and exists only for historical reasons. There should almost never be a reason to use it in your own code. node docs

Here is your example reworked with Q, note that we defer error handling to the caller by returning a promise instead of the stats themselves. I included an example of how to handle fs.exists with Q.defer to show how you can proxy non node style callbacks.

var Q = require('q'); 
var fs = require('fs');
var http = require('http');
var readdir = Q.denodeify(fs.readdir);

function stat(file){
    return Q.nfcall(fs.stat, file)
        .then(function(stats){
            stats.filename = file;
            return stats;
        });
}

function openDir(path) {
    path = __dirname + path;

    return readdir(path)
        .then(function(files) { 
            return files.map(function(file){ 
                return stat(path + file); }); })
        .then(Q.all);
};

// proxy fs.exists to return a promise. Example of Q.defer
function exists(file){
    var deferred = Q.defer();
    fs.exists(file, function(result){
        return result ? deferred.resolve(file) : deferred.reject('invalid file');    
    });
    return deferred.promise;
}

http.createServer(function (req, res) {
    openDir('/').done(
        function (data) {
            res.writeHead(200, {
                'Content-Type': 'application/json; charset=utf-8'});
            res.end(JSON.stringify(data));
        },
        function (err) {
            res.writeHead(500, {
                'Content-Type': 'application/json; charset=utf-8'});
            res.end(JSON.stringify({error: err}));
        });
    }).listen(1337, '127.0.0.1');

Collectives™ on Stack Overflow

Using promises with Node.js functions on example

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related