How to select features in a feature collection simultaneously row by row in Google Earth Engine?

Question

I try to find a correlation between a linear trend of two NDVI time series before and after a specific day in time. And that I want to repeat (iterate/map?) for a lot of points globally distributed. This is the code @Daniel Well helped me with:

var conflicts = ee.FeatureCollection([
  ee.Feature(ee.Geometry.Point([12.492297, 41.89024]), {conflictDate: '2010-01-01'}),
  ee.Feature(ee.Geometry.Point([12.453361, 41.902216]), {conflictDate: '1990-01-01'})
])

var startDate = '1989-01-01'
var endDate = '2013-12-16'

var timeSeries = ee.ImageCollection('NASA/GIMMS/3GV0')
    .filterDate(startDate, endDate)
    .select(['ndvi'])
    .map(function (image) {
      return image.addBands(
        ee.Image(image.getNumber('system:time_start'))
          .subtract(ee.Date(startDate).millis())
          .divide(1000*3600*24) // days
          .int()
          .rename('t')
      )
    })

var conflictsWithSlopes = conflicts.map(addSlopes)
print(conflictsWithSlopes)
print('slope change stats', conflictsWithSlopes.aggregate_stats('slopeChange'))
// Other aggregate_*() functions...


function addSlopes(conflict) {
  var conflictDate = conflict.getString('conflictDate')
  var beforeSlope = getSlope(timeSeries, conflict, startDate, conflictDate)
  var afterSlope = getSlope(timeSeries, conflict, conflictDate, endDate)
  return conflict
    .set('beforeSlope', beforeSlope)
    .set('afterSlope', afterSlope)
    .set('slopeChange', afterSlope.subtract(beforeSlope))
}

function getSlope(timeSeries, conflict, startDate, endDate) {
  return timeSeries
    .select(['t', 'ndvi'])
    .filterDate(startDate, endDate)
    .reduce(ee.Reducer.linearFit())
    .select('scale')
    .reduceRegion({
      // If your conflict regions aren't points, 
      // you might want to use a mean or median reducer
      reducer: ee.Reducer.first(), 
      geometry: conflict.geometry(),
      scale: 8000
    })
    .getNumber('scale')
}

Now I would like to use conflict points (around 9000) and their corresponding dates (like in the example) stored in a FC Link The conflict coordinates column is called "coordinate" and the date "date_start".

So far I have tried this:

var conflicts = conflicttable.select(['coordinate']);
var conflictDate = conflicttable.select(['date_start']);

and I get this error: FeatureCollection (Error) Error in map(ID=000000000000000005a2): DateRange: Parameter 'start' is required.

Answer 1

The problem (except for just picking up the conflict date with a different property name, conflictDate -> date_start) is that you have conflicts for dates where you have no imagery. You need to have some images both before and after your conflict date to calculate the two slopes. You can filter out conflicts outside of some date range like this:

var startDate = '1989-01-01'
var endDate = '2013-12-16'
var conflicts = ee.FeatureCollection('users/welp1992/Conflicts25_Date')
  .filterMetadata('date_start', 'greater_than', '1990-01-01')
  .filterMetadata('date_start', 'less_than', '2013-01-01')
  .limit(259)

https://code.earthengine.google.com/7f08b9827f13c8ccde7b0c5bc7718525