Weighted Average of Band Based on another Band in Image Collection [Google Earth Engine]

Question

Using monthly VIIRS data, I'm trying to estimate a weighted average of radiance where I want to weight by the number of cloud free days. Each image in the image collection has two bands: "avg_rad" and "cf_cvg." The below code calculates the average radiance in 2015; however, I'm wondering if there's a convenient way to calculate a weighted average, where "cf_cvg" is used as the weight.

var viirs = ee.ImageCollection('NOAA/VIIRS/DNB/MONTHLY_V1/VCMCFG');
var viirs_2015 = ee.ImageCollection(
    viirs.filterDate("2015-01-01","2015-12-31")
);

var viirs_2015_mean = viirs_2015.reduce(ee.Reducer.mean()); 
var viirs_2015_mean_rad = viirs_2015_mean.select("avg_rad_mean")

Answer 1

Something like this maybe (reference):

var viirs = ee.ImageCollection('NOAA/VIIRS/DNB/MONTHLY_V1/VCMCFG');
var viirs_2015 = ee.ImageCollection(
    viirs.filterDate("2015-01-01","2015-12-31")
);

var viirs_2015_mean = viirs_2015.reduce(ee.Reducer.sum().splitWeights())
    .divide(viirs_2015.select('cf_cvg').reduce(ee.Reducer.sum())); 

Map.addLayer(viirs_2015_mean, {}, 'weighted')
Map.addLayer(viirs_2015.select('avg_rad').reduce('mean'), {}, 'unweighted')