Datatype errors when exporting a CSV in EarthEngine

Question

I'm using Google Earth Engine's web-based code editor to grab a bunch of evapotranspiration snapshots, get a zonal mean, create a table of the results by date, and export to CSV. Code is below.

If I try to export using the raw date (from image.date()) for the date-data pair, I get "Error: Error: Feature: Invalid property name: **ee.Date**({" followed by a whole pile of attributes. See image below.

If I try to convert the date to a string (curDate.format('YYYYMMDD')), I get the same error, but it's "Error: Error: Feature: Invalid property name: **ee.String**({" followed by the same. See image below.

Is there a different way I should be doing this?

// Get Data
var dataset = ee.ImageCollection('MODIS/NTSG/MOD16A2/105') // ET
                  .filter(ee.Filter.date('2000-01-01', '2015-01-01'))
                  .filterBounds(AOI);
var etDataset = dataset.select('ET');
var etDatasetList = etDataset.toList(etDataset.size());
var etDatasetSize = etDatasetList.size().getInfo();

// Process
var image;
var curDate;
var curDateString;
var meanDictionary
var data = [];
for(var i = 0; i <etDatasetSize; i++){
  image = ee.Image(etDatasetList.get(i)).clip(AOI);
  curDate = image.date();
  curDateString = curDate.format('YYYYMMDD');
  meanDictionary = image.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: AOI
  });
  data[curDate] = meanDictionary; //For if I try to use the date itself
  // data[curDateString] = meanDictionary; // For if I try to use the string
}

//Export
var feature = ee.Feature(null, data);
var featureCollection = ee.FeatureCollection([feature]);
Export.table.toDrive({
  collection: featureCollection,
  description: 'ET_Sample',
  fileFormat: 'CSV'
});

Error if I use a Date Error if I use a String

Answer 1

It is never good practice to use a for loop (client-side), especially if you want to upscale your process tot large imagecollections like you are trying here. This is an alternative .map() function (server side):

var mappedFunction = etDataset.limit(3).map(function(img){
    
  var test= etDataset.first().reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: AOI
  });
  
  // return img.set({'ET': test.get('ET')}) // append the property to the existing 
  return ee.Feature(null, test) // or if you perse want a featureCollection
  
})

print(mappedFunction, 'mappedFunction')

see additional information on the developers page: https://developers.google.com/earth-engine/guides/client_server

https://code.earthengine.google.com/1b65b50dce8bd4c87ba3c89010b35507