CODEWITHSUNDEEP
javascriptnode.jsasynchronous
Julien DES
Julien DES

Reputation: 113

Return object into async.each function in node.js

I want to understand one thing about async module in node.js.

I have created a function that map an object from a form to a model object and return this object. This object is a video with an array of tags.

My question is where can I return the video ? I know normally it is inside the async callback function but if I do that, the object returned is undefined. Whereas If i return the video object at the end of the whole function, it works but it's not safe as I'm not sure, my async is finished...

By the way, I don't understand the callback function passed in argument to async.each and called after video.products.push(tag); . What does this function do?

Regards

in my mapping.js : 

 exports.video = function(object) {

        var video = new Video();            
        video.name = object.name;   
        video.products = [];

        async.each(object.tags, function(tago, callback) {

                            tag = {
                            "name" : tago.name
                                 }


                            video.products.push(tag);
                            callback();

                        } ,
                   function(err) {
                            if( err ) {
                        console.log('Error' + error);
                                throw err;  
                            }

                          logger.debug("into async" + video);

                    }

                    );

           logger.debug("end function " );
            **//return video;**

     }

in my video.js : 

 var video = mapping.video(object); 
 logger.debug(video);  // return undefined

Upvotes: 2

Views: 2190

Answers (2)

Sunday Ironfoot
Sunday Ironfoot

Reputation: 13050

You'd need to define a callback for your exports.video function e.g..

exports.video = function(object, callback) {
    // video code (snip)...

   async.each(object.tags,
       function eachTag(tag, done) {
           // code run for each tag object (snip)...
           done();
       },
       function finished(err) {
           // code run at the end (snip)...

           callback(thingThatsReturned);
       });
};

...and call it like this:

var videoUtils = require('videoUtils');
var tags = getTags();

videoUtils.video({ tags: tags }, function(thingThatsReturned) {
   // do something with 'thingThatsReturned'
});

By the way, I don't understand the callback function passed in argument to async.each and called after video.products.push(tag); . What does this function do?

The async.each function will call the 'eachTag' function above (2nd argument) for each item in your array. But because it's done asynchronously, and you might do something else async in the function (hit a database/api etc.), it needs to know when that function for that particular array item has finished. Calling done() tells async.each that the function has finished processing. Once all the functions are finished processing (they've all called done()), async.each will run the 'finished' function above (3rd argument).

This is pretty standard async stuff for Node.js, but it can be tricky to get ones head around it at first. Hang in there :-)

Edit: It looks like your code isn't doing anything asynchronous. If it was, then the above code would be the way to do it, otherwise the following code would work better:

exports.video = function(object) {
    // video code (snip)...

    if (Array.isArray(object.tags)) {
        object.tags.forEach(function eachTag(tag) {
            // code run for each tag object (snip)...
        });
    }

    return thingThatsReturned;
};

...and call it...

var videoUtils = require('videoUtils');
var tags = getTags();

var thingThatsReturned = videoUtils.video({ tags: tags });

Upvotes: 2

Owen
Owen

Reputation: 1662

The simple answer is that you can't - at least not via easy or obvious approach. As its name suggests, async is a library for queuing up asynchronous function calls into the event loop. So your exports.video function simply kicks off a bunch of asynchronous functions, which execute one after the other on an unpredictable time-frame, and then returns immediately. No matter where you try to return your video object within the scope of your function calls which are instantiated by async, the exports.video function will already have returned.

In this case it doesn't really seem like you need asynchronous function calls for what you're doing. I'd suggest that you replace your use of async with something like Underscore's each method, which executes synchronously, instead.

http://documentcloud.github.io/underscore/#each

Upvotes: 2

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