gee_reduce_n_reproject.js

// Make a suitable image for `reduceConnectedComponents()`
// by adding a label band to the `img_pop3` image.
img_pop3 = img_pop3.addBands(patchid.select('labels'));

// Calculate the total population in demand area
// defined by the previously added "labels" band
// and reproject to original scale
var patchPop = img_pop3.reduceConnectedComponents({
  reducer: ee.Reducer.sum(),
  labelBand: 'labels',
}).rename(['pop_total']).reproject(img_pop3.projection())