How to return value from nodejs callback function?

问题:

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.

回答1:

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.