Reclassification of MODIS collection in Google Earth Engine

Question

I want to reclassify the MCD43A4_006_NDVI collection and change all negative values into NAs. I have the following code but I am not sure how to do this.

Any ideas?

 var geometry = /* color: #d63000 */ee.Geometry.Polygon(
            [[[107.4876749263151, 43.53145035042672],
              [107.2899210200651, 40.74603425026213],
              [114.4530069575651, 40.56266845308453],
              [118.6717569575651, 42.72968876300252]]]);

        var modisNDVI = ee.ImageCollection("MODIS/MCD43A4_006_NDVI")
        .filterBounds(geometry)
        .map(function(image){return image.clip(geometry)}) 
        .filter(ee.Filter.calendarRange(7,7,'month'));


 // Here I have no idea how to continue and set the "values less than 0"         
            var reclassified = modisNDVI.map(function(img){
          return img.gt(-1) // or gt()
        })

        var check = ee.Image(modisNDVI.first());
    Map.centerObject(check);    
    Map.addLayer (check, {min:0.0,max:1,palette:['FFFFFF','CC9966','CC9900','996600','33CC00','009900','006600','000000']},'NDVI');

    print(modisNDVI)


    var batch = require('users/fitoprincipe/geetools:batch') 
    batch.Download.ImageCollection.toDrive(modisNDVI, "Folder", {scale:500,region: geometry});

Answer 1

You can do it with masking.

// Masking pixels with values less than 0.  This will take a while.     
    var reclassified = modisNDVI.map(function(img){
      return img.updateMask(img.gt(0))// only pixels with values greater than 0
    })

var check = ee.Image(modisNDVI.first());
Map.centerObject(check);    
Map.addLayer (check, {min:0.0,max:1,palette:['FFFFFF','CC9966','CC9900','996600','33CC00','009900','006600','000000']},'NDVI');

Once masked, the pixels will no longer be used in any further calculation that you do.