如何从nodejs回调函数返回值？

Question

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);
    });


});

}

在这里,我试图打印'result1',但它总是打印数组未定义的值.'result1'是一个在回调函数范围之外分配的数组.

Answer 1

您无法从异步回调中返回值.回调通常在声明它的函数返回后执行一段时间(该函数将在调用异步方法后继续执行).有无处回调函数返回来.这是一个简单的例子:

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
}

如果需要依赖异步方法的结果,则需要将需要它的代码移动到回调中.