Creating a Rule in NOAA/CDR/AVHRR/NDVI/V5 to only display NDVI for dense vegetation Google Earth Engine

Question

I'm trying to apply a rule to NOOA NDVI the already classified region to only display dense vegetation areas but it's saying NDVI is not a function.

Here is the code:

//collecting data using ndvi
var good = ee.ImageCollection('NOAA/CDR/AVHRR/NDVI/V5')
                  .filter(ee.Filter.date('2018-05-01', '2018-06-01'))
                  //.median()
                  //.good.lt(0.27).and(good.gt(0.18))
                  //.map(function(good){return good.lt(0.18).and(good.gt(0.27))})
                  .map(function(good){return good.clip(roi)});

Map.setCenter (37.577717495842506,0.3597340638009545,12);
var ndvi = good.select('NDVI')
var ndvi2 =ndvi
          .where(ndvi.gt(0.27).and(ndvi.lte(0.36)), 4)
          .where(ndvi.gt(0.36).and(ndvi.lte(0.74)), 5)
var ndvi2 = ndvi2.clip(roi);

The error shows this

Line 14: ndvi.gt is not a function

Answer 1

You are applying an image function (.gt()) to an image collection. So you have to select an image, or map the logic in a function over the entire imageCollection. Here is the solution to applying it to an image:

var ndviImage = ndvi.first() // now it is an image

var ndvi2 =ndviImage
          .where(ndviImage.gt(0.27).and(ndviImage.lte(0.36)), 4)
          .where(ndviImage.gt(0.36).and(ndviImage.lte(0.74)), 5)

https://code.earthengine.google.com/461a18758e1ee190a0b9449e0a6e3ffc