How to define a function that uses absolute value on GEE?

Question

I want to define a function for an index MMRI using the formula given below

I'm relatively new to JS and I'm having trouble defining it correctly.

First, I defined NDVI and MNDWI. This works fine.

var addIndicesL5 = function(img) {
      var ndvi = img.normalizedDifference(['B4','B3']).rename('NDVI');
      var mndwi = img.normalizedDifference(['B2','B5']).rename('MNDWI');
return img
    .addBands(ndvi)
    .addBands(mndwi)
};

Then, I defined a separate function to get their absolute values using

var ndvi_mndwi_abs = function(img){
  var ndvi_abs = (img.select('NDVI')).abs.rename('NDVI_ABS');
  var mndwi_abs = (img.select('MNDWI')).abs.rename('MNDWI_ABS');
  
            return img
    .addBands(ndvi_abs)
    .addBands(mndwi_abs);
};

(Following this, I plan to take the normalizedDifference of 'MNDWI_ABS' and 'NDVI_ABS' bands to get MMRI)

However, when I map ndvi_mndwi_abs function to the Image Collection, I get an error saying " img.select(...).abs.rename is not a function".

How can I rewrite this function?

Answer 1

You forgot the function call parentheses:

  //                                   vv
  var ndvi_abs = img.select('NDVI').abs().rename('NDVI_ABS');
  var mndwi_abs = img.select('MNDWI').abs().rename('MNDWI_ABS');
  //                                     ^^

Every operation on an Earth Engine Image is a method, which must be called (.abs()), not just named (.abs).

For future troubleshooting: The error you then saw was because the effect of the incorrect code was to try to call a method named rename on the method abs, rather than the result of calling abs(). No such method exists, so execution stopped with that error.