asynchronous javascript best practices

Question

Using node.js/express.js, here is an example of a typical problem that I face with asynchronous code (function rendering a page on a request with 2 db call):

exports.index = function(req, res){
    res.render('index', {
        data1: getData1(),
        data2: getData2()
    });
};

Here is what one function could look like:

function getData1() {
    var assets = db.get("assets");
    assets.find().on('success', function (docs) {
        // What to do ?
    });
}

I tried using async.js which is a very helpful module, but I'm still missing knowledge on how to do with a case like this.

And can someone suggest good resources for best practice regarding asynchronous javascript coding ?

You may want to also look at jquery's deferred object api.jquery.com/category/deferred-object — crad
– crad, Commented Dec 26, 2013 at 22:27

bevacqua · Accepted Answer · 2013-12-26 22:38:10Z

1

Use an asynchronous workflow library, such as async.

function getData1(done) {
    var assets = db.get('assets');
    assets.find().on('success', done);
}

function getData2(done) {
    var assets2 = db.get('assets2');
    assets2.find().on('success', done);
}

exports.index = function(req, res, next){
    async.parallel({
        d1: getData1,
        d2: getData2
    }, render);

    function render (err, data) {
        if (err) {
            return next(err);
        }

        res.render('index', {
            data1: data.d1,
            data2: data.d2
        });
    });
};

In this way, you can stick to the callback(err, result) style perpetrated throughout Node packages, while keeping your code sane and pretty. async provides methods to simplify your workflows. In this case, I used the async.parallel method which allows you to run any amount of callbacks concurrently, and then runs the function provided as a second argument when the other callbacks is done.

answered Dec 26, 2013 at 22:38

bevacqua

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2 Comments

Begoodpy Over a year ago

Thanks Nico, I like the approach with async. Do it means that in getData I have to call assets.find().on('success', done(err, queyrresult)); ? If so, it doesn't make the functions getData really reusable, right ?

bevacqua Over a year ago

You can just pass the done callback, as in .on('success', done), because it has the (err, result) signature you describe. They're completely reusable because you can use those same queries in other contexts where you need to do getData1 or 2

Andrew Whitaker · Accepted Answer · 2013-12-26 22:34:58Z

One way to solve your problem would be to chain the asynchronous commands by passing a callback function to the functions performing the asynchronous operations:

exports.index = function(req, res){
    getData1(function (data1) {
        getData2(function (data2) {
            res.render('index', {
                data1: data1,
                data2: data2
            });
        });
    });
};

function getData1(callback) {
    var assets = db.get("assets");
    assets.find().on('success', callback);
}
// Similarly for getData2

However, the nested callbacks can become unmanageable quickly. You can name the functions to help with this:

exports.index = function(req, res){
    var data2Success = function (data1, data2) {
        res.render('index', {
            data1: data1,
            data2: data2
        });
    },
    data1Success = function (data1) {
        getData2(data2Success.bind(this, data1));
    };

    getData1(data1Success);
};

Jamis Charles · Accepted Answer · 2013-12-26 22:40:52Z

0

I usually do one of the following:

1) Nest your calls:

exports.index = function(req, res){
  //first call
  assets.getRecord(function (err, docs) {

    // 2nd call
    assets.getSomeOtherRecord(function (err, otherDocs) {
        res.render('index', {
          data1:docs,
          data2: otherDocs
        });
    );

  });

};

If your code is simple, and you don't mind running these one after the other, then this suffices. If those don't apply, I use approach #2:

2) Use async.js or Q as the other answers here show.

Resources for learning about async:
- http://net.tutsplus.com/tutorials/javascript-ajax/managing-the-asynchronous-nature-of-node-js/
- http://www.sebastianseilund.com/nodejs-async-in-practice

answered Dec 26, 2013 at 22:40

Jamis Charles

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1 Comment

Begoodpy Over a year ago

Thanks Jamis, 1) is definitely not a way that I want to go with as my two db query will not be reusable at all. Thanks for the resources!

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