Stratified random sample a FeatureCollection in GEE

Question

I want to randomly select a feature from each group in a FeatureCollection. My actual data has many groups, so filtering each group and choosing one from each group is not an option. I tried using reduceColumns() with a grouped reducer to choose a random item from each group. My idea was to extract the random values from that dictionary as a list, and then use that to filter the original feature collection, but got an error when trying to convert the dictionary to a list.

How can I get a list of the random values from my dictionary (see below)?

var fc = ee.FeatureCollection([
  ee.Feature(null, {group: 'A'}),
  ee.Feature(null, {group: 'A'}),
  ee.Feature(null, {group: 'B'}),
  ee.Feature(null, {group: 'B'}),
  ee.Feature(null, {group: 'C'}),
  ee.Feature(null, {group: 'C'})
]);

// Add random column.
var fc = fc
  .randomColumn()

print(fc)

// Get a random value for each group
//  (the random value is nested deep in a dictionary)
var randomDict = fc
  .reduceColumns({
    selectors: ['random', 'group'],
    reducer: ee.Reducer.min().group({
      groupField: 1
    })
  })
  
print(randomDict)

// Extract the random value from dictionary, return list
// BREAKS HERE:
var randomList = randomDict
  .get('groups')
  .map(function(e){
    return e.get('min')
  })
print(randomList) // randomDict.get(...).map is not a function

// Use list of random values to filter the original fc
var randomFC = fc
  .filter(ee.Filter.inList('random', randomList))

print(randomFC)

Answer 1

You will need to correctly cast the objects to an ee.List() and ee.Dictionary():

// Extract the random value from dictionary, return list
var randomList = ee.List(randomDict
  .get('groups'))
  .map(function(e){
    return ee.Dictionary(e).get('min')
  })
print(randomList) 

link code