Complete GEE Code Editor app inclusing BestNDVIMosaic core

BestNDVIMosaic-full.js

/* 
 *  @notice fully functional GEE JavaScript for Code Editor
 *  @notice Generate a mosaic using pixels of the best NDVI of all images of a month
 *  @notice Do it over all same months of a period of time, for example March (or August, or December)
 *  @notice ... useful to remove clouds and calculate monthly NDVI stats over several years
 *  @author Vu Tien Khang, licence MIT, Jan 2021
 */
// prepare the refrence set of images, here it's Sentinel-2 MSI: MultiSpectral Instrument, Level-1C
var myPoint = ee.Geometry.Point(78.2875, 10.4156);  // Sendurai, Tamil Nadu, India
Map.centerObject(myPoint, 13);    // close zoom around central point, zoom scale 13
var roi = myPoint.buffer(10000);  // radius 10 km to limit image Collection
var refSetS2 = ee.ImageCollection('COPERNICUS/S2').filterBounds(roi);
var studyMonth = 4;               // month over which the composite mosaic is done

var years = ee.List.sequence(2017, 2020);
var bestNDVICollection = years.map(makeBestImage(studyMonth, refSetS2));
print('bestNDVICollection', bestNDVICollection);
var visuVeg = {bands:['B11', 'B8', 'B4'], min: 500, max: 3000, opacity: 1};
Map.addLayer(ee.Image(bestNDVICollection.get(0)), visuVeg,'test 2017');
Map.addLayer(ee.Image(bestNDVICollection.get(1)), visuVeg,'test 2018');
Map.addLayer(ee.Image(bestNDVICollection.get(2)), visuVeg,'test 2019');
Map.addLayer(ee.Image(bestNDVICollection.get(3)), visuVeg,'test 2020');

//-------------------------
function makeBestImage(studyMonth, refSetS2){
  /* 
   *   @notice Function used by map() to make a list of mosaic images using pixels with the best NDVI
   *  @param {ee.Number} studyMonth: month to generate ee.DateRange (1 to 12)
   *  @param {ee.imageCollection} refSetS2: full set of images, several years
   *  @dev {function} addNDVI: function to add NVI channel named 'NDVI' to an image
   *   @author Vu Tien Khang, licence MIT, Jan 2021
   */
  return function(year){
    var mosaicStartDate = ee.Date.fromYMD(year, studyMonth, 1); 
    var range2Mosaic = ee.DateRange(mosaicStartDate, mosaicStartDate.advance(1, 'month'));
    var filtered = refSetS2.filterDate (range2Mosaic);
    var with_NDVI = filtered.map (addNDVI("B8", "B4"));
    return with_NDVI.qualityMosaic ('NDVI')
        .set('system:time_start', with_NDVI.first().get('system:time_start'));
  };
}

//---------------------------
function addNDVI(NIR, RED){ 
  /* 
  *   @notice Function used by map() to add to "image" the band NDVI (normalized difference NIR and Red)
  *   @param {ee.String} NIR: name of band NIR in image, depends on the satellite
  *   @param {ee.String} RED: name of band RED in image, depends on the satellite
  *   @author Vu Tien Khang, licence MIT, Jan 2021
   */
    return function (image) {
      var ndvi = image.normalizedDifference ([NIR, RED]).rename ('NDVI');
      return image.addBands (ndvi);
      }}