// Compute the trend of night-time lights.

// Adds a band containing image date as years since 1991.
function createTimeBand(img) {
  var year = ee.Date(img.get('system:time_start')).get('year').subtract(1991);
  return ee.Image(year).byte().addBands(img);
}

// Map the time band creation helper over the night-time lights collection.
// https://developers.google.com/earth-engine/datasets/catalog/NOAA_DMSP-OLS_NIGHTTIME_LIGHTS
var collection = ee.ImageCollection('NOAA/DMSP-OLS/NIGHTTIME_LIGHTS')
    .select('stable_lights')
    .map(createTimeBand);

// Compute a linear fit over the series of values at each pixel, visualizing
// the y-intercept in green, and positive/negative slopes as red/blue.
Map.addLayer(
    collection.reduce(ee.Reducer.linearFit()),
    {min: 0, max: [0.18, 20, -0.18], bands: ['scale', 'offset', 'scale']},
    'stable lights trend');