javascript - How to return value from nodejs callback function? - Stack Overflow

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mturk_ops.block = function(callback){


mongodb.collection(collectionName, function(err, collection){

    collection.distinct('workerId',function(err,result){
        var result1 = [];
        console.log(result.length);
        for(var i=0; i< result.length;i++){

            console.log(result[i]);

          result1[result[i]] =  collection.count({
                'workerId':result[i],
                "judgementStat" : "majority"
            },function(err, count){
                //  console.log(count);
              //  globals.push(count);
                return count ;
                // console.log( worker + ' majority : ' + count);

            });

        }

    console.log(result1);
    });


});

}

Here I am trying to print 'result1' but its always printing array with undefined value. 'result1' is an array which is assigned out of the scope of callback function.

mturk_ops.block = function(callback){


mongodb.collection(collectionName, function(err, collection){

    collection.distinct('workerId',function(err,result){
        var result1 = [];
        console.log(result.length);
        for(var i=0; i< result.length;i++){

            console.log(result[i]);

          result1[result[i]] =  collection.count({
                'workerId':result[i],
                "judgementStat" : "majority"
            },function(err, count){
                //  console.log(count);
              //  globals.push(count);
                return count ;
                // console.log( worker + ' majority : ' + count);

            });

        }

    console.log(result1);
    });


});

}

Here I am trying to print 'result1' but its always printing array with undefined value. 'result1' is an array which is assigned out of the scope of callback function.

• javascript
• node.js
• callback

Share Improve this question Follow asked Nov 19, 2012 at 11:17 vcxzvcxz 4,16844 gold badges2020 silver badges1818 bronze badges 1

• 1 You can't return it. It has to be passed forward with a new callback. – Max Commented Nov 19, 2012 at 11:20

Add a ment  | 

1 Answer 1

Sorted by: Reset to default 9

You can't return a value from an asynchronous callback. The callback is usually executed some time after the function in which it was declared has returned (that function will continue execution after calling an asynchronous method). There is nowhere for a callback function to return to. Here's a simple example:

function doSomething() {
    var x = 10;
    doSomethingAsynchronous(function () {
        // This is a callback function
        return 30; // Where would I return to?
    });
    x += 10; // Execution continues here as soon as previous line has executed
    return x; // doSomething function returns, callback may not have run yet
}

If you need to rely on the result of an asynchronous method, you will need to move the code that requires it into the callback.

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