Alchemy? Random success exporting lot of images to Drive

Question

I have been confused by a strange phenomenon.

I use the code below to export a batch of images to Drive, but I cannot export successfully every time, sometimes here comes an Error message Cannot read property 'length' of undefined after almost 10 min awaiting, so I want to test on whether the list of imageCollection is empty or not, then I print the list out, then it works! But sometimes it still dosen't work, and I turn to comment all 'print', it works in some cases! Finally, I just try to export images to Drive with or without 'print'. It works randomly!

My Internet connection is available and I think 'print' may have no contribution to my code.

what happened?

export_imgCollection_raw2(L4578_delta, roi_boundingBox, stationName);

function export_imgCollection_raw2(imgCol, region, stationName){
  var indexList = imgCol.reduceColumns(ee.Reducer.toList(), ["id_info"]).get("list");
  // print('indexList',indexList)
  indexList.evaluate(function(indexs) { 
    for (var i=0; i<indexs.length; i++) { 
      var image = ee.Image(imgCol.filter(ee.Filter.eq("id_info", indexs[i])).first());
      var id ='delta_' + stationName + '_' + String(indexs[i]);
      print(i);
      Export.image.toDrive({ 
        image: image,
        folder: 'delta',
        description: id,
        fileNamePrefix: id,
        region: region, 
        scale: 30, 
        crs: "EPSG:4326", 
        maxPixels: 1e13, 
        fileFormat: 'GeoTiff',
      }); 
    }
  }); 
} 

Answer 1

It sounds like you are encountering an error of some sort when computing indexList. When this happens, evaluate will call your function(indexs) with indexs undefined, and a second argument (which your code is currently ignoring) containing the error message.

You can check for an error with a little extra code in the function:

  indexList.evaluate(function(indexs, error) { 
    if (error) {
      print("Failed to evaluate indexList", error);
      return;
    }
    
    for (var i=0; i<indexs.length; i++) { 
      // ... rest of the code you already had ...
    }
  });

In this way, you won't get the unhelpful "Cannot read property 'length' of undefined" message (which came from i<indexs.length trying to work while indexs isn't an array), but instead see the original error message from the server.