Node.js：async.series内部的forEach

Question

I started developing my node.js app for work but i can't "think asynchronous". In my function i have to do a query and for every rows i have to do another query to compute the final result and create my json. I try to do this:

async.series([
    function(callback) {
            db.myCannedResponseList(req.params.websiteid, function(err,questions) {
                if (err) return callback(err);
                ...
                callback();
            });
        },
    function(callback) {
            async.forEachSeries(temp,function(quest,callback) {
                    db.getCannedAnswer(quest.id, function(err,answers) {
                         console.log(answers);
                    });
            }, function(err) {
                if (err) return next(err);
            });
            callback();
        }
    ],
        function(err) { 
            if (err) return next(err);
            res.json(output);
    });

But there is a problem: the result of getCannedAnswer is show after res.json(output) . How can i solve it?

Answer 1

You need to call the callback when the forEachSeries is done. You also need to call the callback that is passed to the forEachSeries handler functions:

async.forEachSeries(temp, function(quest, callback2) {
  db.getCannedAnswer(quest.id, function(err, answers) {
    console.log(answers);
    // call the callback for forEachSeries so it can proceed:
    callback2(); 
  });
}, callback); // forEachSeries is done here, call the callback for async.series

Answer 2

Your async functions are nested and so need to be your callbacks.

Here's an approach:

    async.series([
        //async.apply(db.myCannedResponseList,req.params.websiteid), // This is equivalent to the first function below
        //db.myCannedResponseList(req.params.websiteid,callback), // This is equivalent too
        function(callback) {
            db.myCannedResponseList(req.params.websiteid, callback);
        },    
        function(callback) {
            // I'm not sure forEach is the best choice here
            // since it's callback isn't called with all the results so it won't be able to be forwarded...
            // Personally, I use async.times for something like this.
            async.times(temp.length,function(i,next) {
                db.getCannedAnswer(temp[i].id,next);
            },callback)
        }
     ],function(err) { 
          if (err) return next(err);
          res.json(output);
    });

I'm not sure what this function should do but if you encounter an error in async.series you should either call res.json(500,error) or not call res on success.

A good approach would be to separete this function altogether and have it call another callback when it's done. You would then call res there.

Also, instead of passing an array to async.series you can pass an object so the results will be namespaced.

Full example:

    function getCannedStuff(data,callback){
        async.series({
            list : async.apply(db.myCannedResponseList,data.websiteid),
            answer : function(cb) {
                async.times(data.temp.length,function(i,next) {
                db.getCannedAnswer(data.temp[i].id,next);
                },cb)
            }
        },callback);  
    }

Then call the function from another module:

    getCannedStuff({
        websiteid: req.params.websiteid,
        temp:temp
    },function(error,results) {
        console.log(results.list);
        console.log(results.answer);
        if(err) return res.json(500,{error : error});
        res.json(results)
    });

You should decide if you want to control the error inside or outside getCannedStuff but it's a good approach to pass them along and not do all the time if(err) to see if they occur.