函数返回值的问题。javascript [重复]

我将数据传递给函数进行处理，以便随后将处理结果写入文本文件。从日志触发的顺序来看，我明白问题是值没有时间返回，但我不知道如何处理。我真的需要建议。实际功能本身：

function convertMe(polygonArray){
var toWrite = '';
var dataToReturn = '';

var polygon = JSON.parse(polygonArray);
var isFirst = true;
var printFeature = function (name, poly) {
  var out = {
    "type": "Feature",
    "properties": {
      'name': name,
    },
    "geometry": {
      "type": "Polygon",
      "coordinates": poly
    }
  };
  out = JSON.stringify(out);
  if(!isFirst) {
    out = ',' + out
  }
  isFirst = false;

  toWrite = toWrite + out;
}
toWrite = toWrite + '{"type": "FeatureCollection","features": [';
async.mapSeries([polygon], function (poly, cb) {

  var options = {
    coords: poly,
    precision: 6,
    rowMode: true,
    hashMode: 'extent',
    threshold: 0.2
  };
  printFeature('shape', poly);
  var rowStream = hasher.stream(options),
    a = 0;

  rowStream
    .on('end', cb)
    .pipe(through2.obj(function (arr, enc, callback) {
      for(var i = 0; i < arr.length; i++) {
        var bb = ngeohash.decode_bbox(arr[i]);
        printFeature(arr[i], [
              [
                [bb[1], bb[2]],
                [bb[3], bb[2]],
                [bb[3], bb[0]],
                [bb[1], bb[0]],
                [bb[1], bb[2]]
              ]]);
      }
      callback();
    }));
}, function () {
  toWrite = toWrite + ']}';
  console.log('Внутри вызова '+toWrite);
  return toWrite;
});

};

通话地点：

worldData.features.forEach(function(feature){
q.defer(function(done){
  var dataToConvert = JSON.stringify(feature.geometry.coordinates);
  console.log('Место вызова '+convertMe(dataToConvert));
  //fs.writeFileSync(currDir+'/'+outputDir+'/'+'cell_'+i+'.s2', convertMe(dataToConvert));

  i++;
  done()
   });
});

控制台输出：

Место вызова undefined
Внутри вызова {"type": "FeatureCollection","features": [{"type":"Feature","properties":{"name":"shape"},"geometry":{"type":"Polygon","coordinates":[[[[-56.0828202,46.8785071],[-56.0832065,46.8780553],[-56.0840219,46.877938],[-56.0843308,46.8776271],[-56.0848973,46.8777385],[-56.0852407,46.8776564],[-56.0858329,46.8777385],[-56.0865109,46.8773278],[-56.0874293,46.8769875],[-56.0878327,46.877146],[-56.0880559,46.8770697],[-56.0886396,46.8767118],[-56.0895236,46.8764771],[-56.0898154,46.8763011],[-56.0900643,46.8759197],[-56.0904506,46.8756733],[-56.0915664,46.8757554],[-56.0923045,46.8757437],[-56.0929568,46.8755618],[-56.0933002,46.8758024],[-56.0939696,46.8763656],[-56.0944245,46.8769054],[-56.0951026,46.8773689],[-56.0945619,46.8778089],[-56.0939868,46.8784015],[-56.0929483,46.8789295],[-56.0917295,46.8794399],[-56.0907767,46.8796981],[-56.0896781,46.8799797],[-56.0895408,46.8801029],[-56.0891803,46.8802613],[-56.0892404,46.8804549],[-56.0888112,46.88076],[-56.0883048,46.880801],[-56.0880044,46.8810533],[-56.0866397,46.8813232],[-56.0853265,46.8811354],[-56.0846227,46.8810357],[-56.0850347,46.8808069],[-56.084163,46.8808322],[-56.083258,46.8804314],[-56.0830348,46.8801791],[-56.0830434,46.8797333],[-56.0830863,46.8794399],[-56.0837043,46.8792757],[-56.0834468,46.8791994],[-56.0835841,46.8790586],[-56.0833266,46.8789706],[-56.0832665,46.8788239],[-56.0835498,46.8784308],[-56.0830348,46.8786948],[-56.0828202,46.8785071]]]]}},{"type":"Feature","properties":{"name":"fb24ew"},"geometry":{"type":"Polygon","coordinates":[[[-56.09619140625,46.8841552734375],[-56.085205078125,46.8841552734375],[-56.085205078125,46.878662109375],[-56.09619140625,46.878662109375],[-56.09619140625,46.8841552734375]]]}},{"type":"Feature","properties":{"name":"fb24ey"},"geometry":{"type":"Polygon","coordinates":[[[-56.085205078125,46.8841552734375],[-56.07421875,46.8841552734375],[-56.07421875,46.878662109375],[-56.085205078125,46.878662109375],[-56.085205078125,46.8841552734375]]]}},{"type":"Feature","properties":{"name":"fb24et"},"geometry":{"type":"Polygon","coordinates":[[[-56.09619140625,46.878662109375],[-56.085205078125,46.878662109375],[-56.085205078125,46.8731689453125],[-56.09619140625,46.8731689453125],[-56.09619140625,46.878662109375]]]}},{"type":"Feature","properties":{"name":"fb24ev"},"geometry":{"type":"Polygon","coordinates":[[[-56.085205078125,46.878662109375],[-56.07421875,46.878662109375],[-56.07421875,46.8731689453125],[-56.085205078125,46.8731689453125],[-56.085205078125,46.878662109375]]]}}]}

也就是说，首先我从调用的地方得到 Undefined，然后才在这个地方得到工作的结果（顺便说一句，正确的和我需要的）