Error when extracting specific field of shapefile in Google Earth Engine?

Question

I'm trying to extract specific field data which is in shapefile. There was an error while executing the following code in GEE as argument invalid type. the screenshot shows the error message.

Can you help to solve the problem?

var startYear = 2018;
var endYear = 2019;
print(countries)
for(var i=0;i<countries.size().getInfo();i++){
  var startMonth =countries.filter(ee.Filter.eq('startMonth',i));
  for(var j=0;j<countries.size().getInfo();j++){
    var startDay = countries.filter(ee.Filter.eq('startDay',j));
    var start = ee.Date.fromYMD(startYear, startMonth,startDay);

  }
}
for(var i=0;i<countries.size().getInfo();i++){
  var endMonth =countries.filter(ee.Filter.eq('endMonth',i));
  for(var j=0;j<countries.size().getInfo();j++){
    var endDay = countries.filter(ee.Filter.eq('endDay',j));
    var end  = ee.Date.fromYMD(endYear, endMonth, endDay);
    //print(typeof endMonth)
  }
}
print(typeof startDay)
print(start)
print(end)

Here startMonth,startDay and endMonth,endDate are the fields in attribute table(in shapefile) I want to extract these field data based on the country.special thing is there is another column country as 'ADM0_NAME'.

Answer 1

The filter function does not give you a value but instead is used to filter your larger collection to narrow it down to the datasets that you want. If you know your filter is going to give you one feature and you want to get a value out of it, you first need to extract a feature out of it and get the value of that property.

In your case it would be

var countriesWithStartDayj = countries.filter(ee.Filter.eq('startDay',j));
// you can get the first feature with
var country = countriesWithStartDayj.first();
// now you can get the property value
var startDay = country.get('startDay');

Since it looks like you want to get start month and end month of each feature you can use the aggregate_array function

var startDays = countries.aggregate_array('startDay');
var endDays = countries.aggregate_array('endDay');

This would give you ee.List objects with values of field startDay and endDay of all features.