Google Earth Engine: error "Invalid type. Expected type: Number" trying to compute a Number while performing histogram matching on satellite images

Question

I am working on satellite imagery on Google Earth Engine (JavaScript), and I'm experiencing some issues while I try to perform histogram matching on two images from two different time periods. In particular, when I retrieve the histograms for the two images (different histograms for different spectral bands) I see that the histograms of one image are shorter than the histograms of the other image. So, I tried to compute the minimum length between the histograms from the two images to slice the longest histogram and match the length of the shortest one. I admit I exploited chatGPT to get help while developing the script (I am a beginner in JavaScript), as the only examples provided by Google Earth Engine tutorials on this subject are in Python and I needed to translate in JavaScript (https://developers.google.com/earth-engine/tutorials/community/histogram-matching).

This is the script that I'm using:

// Function for histograms lookup table 
function lookup(source_hist, target_hist) {   // in our case: source_hist is histYear2, target_hist is histYear1 (match year2 with year1)

  
  var source_values = source_hist.slice(1, 0, 1).project([0]); // 1D array of the unique pixel values from the source image histogram
  var source_counts = source_hist.slice(1, 1, 2).project([0]); // 1D array of the normalized counts corresponding to source_values

  var target_values = target_hist.slice(1, 0, 1).project([0]);
  var target_counts = target_hist.slice(1, 1, 2).project([0]);
  
  // Ensure the counts are equal in length
  var lengthSource = ee.Number(source_counts.length());
  var lengthTarget = ee.Number(target_counts.length());
  print("source length object type: ", ee.Algorithms.ObjectType(lengthSource));
  print("source length object type: ", ee.Algorithms.ObjectType(lengthTarget))
  
  // Compute the minimum length and convert it to an Earth Engine integer
  var minLength = ee.Algorithms.If(
    lengthSource.lt(lengthTarget), 
    lengthSource.toInt(), 
    lengthTarget.toInt()
  );
  print(minLength);
  
  // Slice to equal length
  source_values = source_values.slice(0, 0, minLength);
  source_counts = source_counts.slice(0, 0, minLength);
  target_values = target_values.slice(0, 0, minLength);
  target_counts = target_counts.slice(0, 0, minLength);

  // Normalize the counts: this normalization converts the counts into probabilities or proportions.
  source_counts = source_counts.divide(source_counts.reduce(ee.Reducer.sum(), [0]));
  target_counts = target_counts.divide(target_counts.reduce(ee.Reducer.sum(), [0]));

  var lookUp = source_counts.toList().map(function(n) { // for each n (a normalized count from the source histogram), we find a corresponding value from the target histogram
    return target_values.get(target_counts.gte(n).argmax());  
  });                                                         

  return {x: source_values.toList(), y: lookUp};
}
  
// Function to match histogram for a single band
function matchHistogram(bandName, year1, year2, scale) { 

  // Compute histograms for the selected band
  var histYear1 = ee.Array(year1.select(bandName).reduceRegion({
    reducer: ee.Reducer.autoHistogram({maxBuckets: 256}), 
    geometry: year1.geometry(),                           
    scale: scale,                                         
    maxPixels: 65536 * 4 - 1
  }).get(bandName));
  print('Histogram Year 1 for', bandName, histYear1);
  
  var histYear2 = ee.Array(year2.select(bandName).reduceRegion({
    reducer: ee.Reducer.autoHistogram({maxBuckets: 256}),
    geometry: year2.geometry(),
    scale: scale,   
    maxPixels: 65536 * 4 - 1
  }).get(bandName));
    print('Histogram Year 2 for', bandName, histYear2);
  
  
  // Generate the lookup table
  var lookupTable = lookup(histYear1, histYear2); // Note the order, matching hist2 to hist1 (so year2 to year1)
  // Print the lookup table for inspection
  // Note: This is for one band, modify as needed to inspect other bands
  print('Lookup Table for', bandName, lookupTable);

  // Apply the lookup table to match year2 histogram to year1 histogram and perform the interpolation.
  var matchedBand = year2.select(bandName).interpolate({
    x: lookupTable.x,
    y: lookupTable.y
  }).rename(bandName);
  print('Matched Band for', bandName, matchedBand);

  return matchedBand;
}


// List of PlanetScope band names
// b1: coastal blue, b2: blue, b3: green I, b4: green, b5: yellow, b6: red, b7: rededge, b8: Near-IR
var bands = ['b1', 'b2', 'b3', 'b4', 'b5', 'b6', 'b7', 'b8'];
// spatial resolution (meters)
var scale = 3; 


// Match histograms for all bands
var matchedBands = bands.map(function(bandName) {
  return matchHistogram(bandName, year1, year2, scale);
});

// Combine matched bands into a single image
var matchedYear2 = ee.Image.cat(matchedBands).rename(bands);
print("matched", matchedYear2);

Apparently the problem is related to the calculation of the minimum length (minLength) object, to use as the "end" argument in the .slice() function. When running the script, this is what appears in the Console:

Histogram Year 1 for b1 List (179 elements)

Histogram Year 2 for b1 List (183 elements)

source length object type:
Array

source length object type:
Array

ComputedObject (Error) Number.lt, argument 'left': Invalid type. Expected type: Number. Actual type: Long<0, 4294967295, dimensions=1>. Actual value: [179]

One thing I do not understand is why the ee.Algorithms.ObjectType() functions return "Array" when I used ee.Number to retrieve the length of the arrays as a number. Moreover, the Error refers to this snippet:

// Compute the minimum length and convert it to an Earth Engine integer
  var minLength = ee.Algorithms.If(
  lengthSource.lt(lengthTarget), 
  lengthSource.toInt(), 
  lengthTarget.toInt()
  );

Where I used also .toInt() to ensure the number was transformed into an integer.

I thank in advance whoever could help me in understanding my mistake and improve my code (I'm starting to think that there is a much simpler way to do this).