Avoiding Promise anti-patterns with node asynchronous functions

Question

I've become aware of Promise anti-patterns and fear I may have fallen for one here (see code extract from within a function). As can be seen, I have Promises nested within two node asynchronous functions. The only way I have been able to expose the inner Promise is by using an outer one. I'd welcome guidance on how to write this more elegantly.

function xyz() {
    return new Promise(function(resolve, reject) {
        return Resto.find({recommendation: {$gte: 0}}, function (err, data) {
            if (err) return reject("makeSitemap: Error reading database");

            return fs.readFile(__dirname + '/../../views/sitemap.jade', function(err, file) {
                if (err) return reject("makeSitemap: Error reading sitemap template");

                [snip]

                resolve(Promise.all([
                    NLPromise('../m/sitemap.xml', map1),
                    NLPromise('../m/sitemap2.xml', map2)
                ]));
            });
        });
    });
}

I've also caught the issue in this plnkr

Answer 1

The best solution is to create promise versions of callback-using functions. bluebird, an excellent promise implementation (better than Node’s native one), has this built-in.

Bluebird.promisifyAll(Resto); // If you can promisify the prototype, that’s better
Bluebird.promisifyAll(fs);

function xyz() {
    return Resto.findAsync({recommendation: {$gte: 0}}).then(function (data) {
        return fs.readFileAsync(__dirname + '/../../views/sitemap.jade').then(function (file) {
            …

            return Bluebird.all([
                NLPromise('../m/sitemap.xml', map1),
                NLPromise('../m/sitemap2.xml', map2)
            ]);
        });
    });
}

Also, if the fs.readFile doesn’t depend on the Resto.findAsync, you should probably run them together:

return Bluebird.all([
    Resto.findAsync({recommendation: {$gte: 0}}),
    fs.readFileAsync(__dirname + '/../../views/sitemap.jade'),
]).spread(function (data, file) {
    …

    return Bluebird.all([
        NLPromise('../m/sitemap.xml', map1),
        NLPromise('../m/sitemap2.xml', map2),
    ]);
});