Getting Image ID in Google Earth Engine?

Question

In the below code I'm going to get image id in a function but returns error. Could anyone guide me please? How can I solve this?

code link: https://code.earthengine.google.com/154cea3df7a29cdf0a42b13f3a71f42c

Map.centerObject(table);
Map.addLayer(table);

var uhi = ee.ImageCollection("YALE/YCEO/UHI/Summer_UHI_yearly_pixel/v4")
.filterBounds(table)
.mean().clip(table)

print(uhi)


// normalization process

var normal = function(img){
  
  var id = ee.String(img.get('system:id'))
  
  var min = ee.Number(img.reduceRegion(ee.Reducer.min(), table, 1000).get(id));
  var max = ee.Number(img.reduceRegion(ee.Reducer.max(), table, 1000).get(id));
  
  var normalization = (img.subtract(min)).divide(max.subtract(min));
  
  return normalization;
};


var uhiNormal = normal(uhi.select('Daytime'));

print(uhiNormal);

Answer 1

As @HMSP pointed out, after using mean() at the beginning of the code, you lost the ID because you are converting the collection into a mean composite without any id. Following code produces normalized images (by using your normalization formula) with its respective id for each image in Image Collection.

Map.centerObject(table, 11);
Map.addLayer(table);

var imageVisParam = {"opacity":1,
                     "bands":["Daytime"],
                     "min":0.07318689642113053,
                     "max":0.9887331944707479,
                     "palette":["1016ff","14fff5","ffb62d","fdff8f"]};

var uhi = ee.ImageCollection("YALE/YCEO/UHI/Summer_UHI_yearly_pixel/v4")
  .filterBounds(table);

print("first image", uhi.first());

var id = ee.String(uhi.first().get('system:id'));

print("id first image", id);

// normalization process

var listOfImages = uhi.toList(uhi.size());

var normal = listOfImages.map(function (img){
  
  var id = ee.String(ee.Image(img).get('system:id'));
  
  var min = ee.Image(img)
    .reduceRegion(ee.Reducer.min(), table)
    .get('Daytime');
    
  var max = ee.Image(img)
    .reduceRegion(ee.Reducer.max(), table)
    .get('Daytime');
  
  var normalization = (ee.Image(img)
    .subtract(ee.Number(min)))
    .divide(ee.Number(max).subtract(ee.Number(min)));
  
  return normalization.set('system:id', id)
                      .select('Daytime')
                      .clip(table);

});

print("List of images in Image Collection", normal);
Map.addLayer(ee.Image(normal.get(0)),imageVisParam, 'normalized');

First image of normalized collection is added to Map Canvas for verification purpose. It can be observed in following picture.