Creating loop in Google Earth Engine?

Question

I have written a code that is taking data from MODIS about NDVI, filter for March only , calculates the mean value for the whole month and clip it with geometry. The NDVI data is for period of years, for example, from 2000-2003.

The problem is that I need to run this process for few periods, 2000-2003, 2004-2007 and more and I dont want to use copy paste. So I want to create a loop but I'm coming from python and the exmples I have seen here in this forum confused me a little because I dont understand what the loop contains.

If someone can explain me the logic or how to put it inside a loop it will be very helpful.

This is the code I have worked on untill now:

var dataset = ee.ImageCollection('MODIS/006/MOD13A1')
.filter(ee.Filter.date('2000-03-01', '2003-03-31'))
.select('NDVI');

// filter the image collection to contain only images from March

var onlyMarch = dataset.filter(ee.Filter.calendarRange(3, 3,'month'));
print(onlyMarch);
// make a composite (mean image) of the images of March and clip to the geometry

var meanImage = onlyMarch.mean().clip(table).divide(10000);
print(meanImage);

var mean20002003 = meanImage.reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: table,
  scale:500
});

//var ndvi = dataset.select('NDVI')
var ndviParams = {min: 0, max: 1, palette: [
    'FFFFFF', 'CE7E45', 'DF923D', 'F1B555', 'FCD163', '99B718', '74A901',
    '66A000', '529400', '3E8601', '207401', '056201', '004C00', '023B01',
    '012E01', '011D01', '011301'
  ]};

Map.centerObject(table, 7);
Map.addLayer(meanImage,ndviParams,'NDVI20002003');
print(mean20002003,'mean NDVI 2000-2003')

Answer 1

First, you should make a list of dates to map over:

// settings for the years to filter on
var interval = 3;
var increment = 'year';
var start = '2000-01-01';
// make a list of start years
var startDate = ee.Date(start);
var secondDate = startDate.advance(interval, increment).millis();
var increase = secondDate.subtract(startDate.millis());
var list = ee.List.sequence(startDate.millis(), ee.Date('2020-01-01').millis(), increase);

Then you can map over that list and filter the image collection on the date range of 3 year. Note that I filtered the images on beforehand on March, as you did in your example:

// make a composite (mean image) of the images of March and clip to the geometry
var composites = ee.ImageCollection.fromImages(list.map(function(startYear){
  var filtCol = onlyMarch.filterDate(ee.Date(startYear), ee.Date(startYear).advance(interval, increment));
  var meanImage = filtCol.mean().clip(table).divide(10000);
  // add the mean to every image
  var meanVal = meanImage.reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: table,
    scale: 500
  });
  return ee.Image(meanImage.setMulti(meanVal)).set('system:time_start', ee.Date(startYear).millis());
}));

link code