node.js async call inside other function

Question

node.js async call inside other function

I've got quite silly questions, i'm trying to make redirection service using node.js based on conditions pulled from mongodb... so code lookis like that:

var http = require('http'),
util = require('util'),
url = require('url');

var server = http.createServer(function(req, res) {
    domain = req.headers['host'];
    var url = getRedirect(domain);

    res.writeHead(301, { 'Location': url});
    res.end();
});

server.listen(8080);

var getRedirect = function(domain) {

    Domain.findOne({ 'name': domain }, 'url', function (err, domain) {
        if (err) return handleError(err);
        console.log('redirect url %s.', domain.url) 
        return domain.url
    })
}

Of course - doesn't work as it should be, because Domain.findOne is finding results with delay, and return doesn't assign resuluts to url... I tried to pass "res" object into getRedirect function but i'm pretty sure that it's wrong.

Any clues?

node.js
redirect
asynchronous

Answer 1

You'll need to use a callback for this, since (as you've pointed out), the domain/url fetching is asynchronous. The node convention is to use an "error first callback" signature.

Try this:

var http = require('http'),
    util = require('util'),
    url = require('url');

var server = http.createServer(function(req, res) {
    var domain = req.headers['host'];
    // provide a callback to getRedirect so we can get the value asynchronously
    getRedirect(domain, function(err, url) {
        if (err) return handleError(err);
        res.writeHead(301, { 'Location': url });
        res.end();
    });
});

server.listen(8080);

var getRedirect = function(domain, cb) {
    Domain.findOne({ 'name': domain }, 'url', function (err, domain) {
        if (err) return cb(err);
        console.log('redirect url %s.', domain.url);
        // instead of returning, pass the result to the callback
        cb(null, domain.url);
    });
};

Answer 2

I used async.series but I your solutions looks better

async.series([
    function (callback) {
        Domain.findOne({ 'name': domain }, 'url', function (err, domain) {
            if (err) return handleError(err);
            console.log('looking for domain' + domain);
            callback(null, domain.url);
        })
    }
    ], function (err, results) {
        res.writeHead(301, { 'Location': results[0]});
        res.end();
});