Error: "ee.ImageCollection(...).subtract is not a function" (Earth Engine)

Question

When I execute this expression:

var ETfraction = (Thot-LST)/(Thot-Tcold)  // **pseudo code**

I get an error:

ee.ImageCollection(...).subtract is not a function

Why does this not work? Here is my code:

//Find Reference ET (ETo) from Terraclimate

var dataset = ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE')                 
              .filter(ee.Filter.date('2015-01-01', '2017-12-30'))

.map(function(image)

{return image.clip(basin)});

var pet  = dataset.select('pet');

var pet = pet.map(function(img) {

 return img
    .multiply(0.1)
    //.subtract(273.15)

.copyProperties(img, ['system:time_start']);
});


var vis = {

min: 50,

max: 200.0,

 palette: [
    'ffffff', 'fcd163', '99b718', '66a000', '3e8601', '207401', '056201',
    '004c00', '011301'
  ],
};



Map.addLayer(pet,vis);

print(pet,'PET')






//Find LST from MODIS(MOD11A1.006 = Terar land surface temperature daily 1 km.)

var dataset = ee.ImageCollection('MODIS/006/MOD11A1')

.filter(ee.Filter.date('2015-01-01', '2017-12-30'))

.filterBounds(basin)

 .map(function(image){return image.clip(basin)});

var lst = dataset.select('LST_Day_1km');

var lst = lst.map(function(img) {

 return img

 .multiply(0.02)
    // .subtract(273.15)

.copyProperties(img, ['system:time_start']);
});


var landSurfaceTemperatureVis = {

 min: 14000.0,
  max: 16000.0,

 palette: [

  'blue', 'limegreen', 'yellow', 'darkorange', 'red'
  ],
};



//Ts=LST of each pixel

var monthly_lst = ee.List.sequence(0, 3*12)

.map(function(n) 
{ 

// .sequence: number of years from starting year to present

var start = ee.Date('2015-01-01').advance(n, 'month'); // Starting date

var end = start.advance(1, 'month'); // Step by each iteration

return lst.filterDate(start, end)

.mean()

 .set('system:time_start', start.millis());


});

var collection = ee.ImageCollection(monthly_lst);


//Thot = LST for the hot pixel

var Th_lst = ee.List.sequence(0, 3*12)
.map(function(n) { 

// .sequence: number of years from starting year to present

var start = ee.Date('2015-01-01').advance(n, 'month'); // Starting date

var end = start.advance(1, 'month'); // Step by each iteration


return lst.filterDate(start, end)

        .max()

        .set('system:time_start', start.millis());

});
print(Th_lst); 

var collection2 = ee.ImageCollection(Th_lst);

 print(collection2,'Thot');

 Map.addLayer(collection2,vis);


//Tcold = LST for the cold pixel
var Tc_lst = ee.List.sequence(0, 3*12)
.map(function(n) { 

// .sequence: number of years from starting year to present

var start = ee.Date('2015-01-01').advance(n, 'month'); // Starting date

var end = start.advance(1, 'month'); // Step by each iteration


return lst.filterDate(start, end)

        .min()

        .set('system:time_start', start.millis());

});

print(Tc_lst); 

var collection3 = ee.ImageCollection(Tc_lst);

 print(collection3,'Tcold');

 Map.addLayer(collection3,vis);


// // //ET fractional (ETf=(Thot-LST)/(Thot-Tcold))

var etf = ee.ImageCollection(Th_lst)

          .subtract(ee.Image(monthly_lst))

          .divide((ee.Image(Th_lst))

          .subtract(ee.Image(Tc_lst)))

         .rename('ETF').cast({'ETF': 'float'});

print(etf);

Map.addLayer(etf,vis);

Code Editor link

Answer 1

This is because ee.ImageCollection doesn't have a subtract() method. If you need to subtract something from every image in a collection, you need to map over the collection.

var collection = ee.ImageCollection('COPERNICUS/S2')
  .limit(10)

var subtracted = collection.map(function (image) {
  return image.subtract(42)
})