Earth Engine sample regions functions after a loop returns different formatting than individual output within the loop

Question

I have a feature collection of points which has a date column within it. I want to loop through the unique dates in the feature collection and then match corresponding dates within an image collection. After matching the features and images I want to extract values from the images using the sampleRegions function.

I have successfully done this manually with one date at the top of my script, and then tried to implement the same thing in a loop which seems to return seemingly empty output.

Here is a link to my code: https://code.earthengine.google.com/ea5f56d6c7b98619fbeffa373c9d95ed

And here is the code:

var bui = ee.ImageCollection("users/spotter/GFWED_v25/MERRA2_BUI"),
    pts = ee.FeatureCollection("users/spotter/ak_ca_combustion/raw/sol_pts");

//print a single pts feature to see what the first Date column item is
print('First Feature', pts.first()); //date is '2001-04-24'


//------------------------this works for extracting a single date without a loop
//the same date in the first feature
var date = ee.Date('2001-04-24');

//filter the pts by this date
var sub_pts = pts.filter(ee.Filter.eq("Date", '2001-04-24'));

//filter the image collection by this date
var bui_sub = bui.filterDate(ee.Date(date), ee.Date(date).advance(1,'day')).mean();

//extract the pts that match the image collection by date
var extract_test = bui_sub.sampleRegions({collection: sub_pts,
                                scale: 460,
                                projection: 'SR-ORG:6974',
                                tileScale: 16,
                                geometries: false
  });

print('Extracted Data', extract_test);


//---------Now I want to do the same thing over many dates at onc

//select specific dates of interest
var all_dates = ['2001-04-27', '2001-04-28', '2001-04-29'];


//loop many dates at once

var feat_iteration = ee.List(all_dates.map(function(date){

  //get sub image collection matching date
  var bui_sub = bui.filterDate(ee.Date(date), ee.Date(date).advance(1,'day')).mean();

  //get sub pts matching date
  var sub_pts = pts.filter(ee.Filter.eq("Date", date));

  //get the values for each point feature in sub_pts to be extracted from bui_sub
  var extract = bui_sub.sampleRegions({collection: sub_pts,
                                scale: 460,
                                projection: 'SR-ORG:6974',
                                tileScale: 16,
                                geometries: false
  });


  //this looks fine
  print('Each Extract', extract);
  return(extract);
}));

//the output data here is formatted much differen't than individual print statements
print(ee.FeatureCollection(feat_iteration));

Answer 1

SampleRegions returns a feature collection. Thus, we have a featureCollection at every index of a List (can be client-side or server-side). Therefore, cast the list to a featureCollection and flatten the collection of collections

//loop through all the unique dates in pts, I want this to work across all pts
var feat_iteration = ee.FeatureCollection(all_dates.map(function(date){

  //get sub image collection
  var bui_sub = bui.filterDate(ee.Date(date), ee.Date(date).advance(1,'day')).mean();

  //get sub pts
  var sub_pts = pts.filter(ee.Filter.eq("Date", date));

  //get the values for each point feature in sub_pts to be extracted from bui_sub
  var extract = bui_sub.sampleRegions({collection: sub_pts,
                                scale: 460,
                                projection: 'SR-ORG:6974',
                                tileScale: 16,
                                geometries: false
  });

  return extract;
})).flatten();

link code