Google Apps Script custom function - 2nd parameter array is empty

Confirmed the logic of the function worked as expected by using arrays within the function.

However, once I passed arrays as parameters issue with the 2nd parameter reared up it's head.

Basically, the function loops through a 2D array that's a calendar, find the last (most recent) activity and then return the next value from the alt_activity range. If the previous activity is the last value from the alt_activity range, start from the start of the alt_activity range once more.

Sample data

/**
* Looks for the alternate workout in the LAST matching value within the calendar.
* @param {array} input The calendar array to be searched.
* @return The first match starting from the last row (WEEK) and column (DAY) of the @param.
* @customfunction
*/
function calender_search(calendar_range, alt_activities_range) 
{

  /**var calendar_range = [[ , ,          ,             ,            ,     'Chess'],
                        [ , ,          ,             ,   'Pilates',   'Elevate'],
                        [ , ,          ,  'Kundalini',	'Scrabble',	'Gardening'],
                        [ , , 'Darning',	'Crossword',	   'Chess',	  'Pilates']];        
*/                                       


 //var alt_activities_range = [ 'Chess', 'Pilates', 'Elevate', 'Kundalini', 'Scrabble', 'Gardening', 'Darning', 'Crossword'];

 
 
  for (let week = calendar_range.length - 1; week >= 0; week--) 
  {
    for (let day = calendar_range[week].length - 1; day >= 0; day--) 
    {
      //return calendar_range[week][day]; // test where the functionality gets to.
      return alt_activities_range[day]; // array is empty when passed in as a parameter
      

      if (alt_activities_range.indexOf(calendar_range[week][day]) > -1) 
      {
        //NOT entering this if statement!!! return (JSON.stringify(calendar_range)); 
/** 
        if (calendar_range[week][day] === alt_activities_range[alt_activities_range.length - 1]) 
        {
          return alt_activities_range[0];
        }
        // Go to index of match PLUS 1 for NEXT value to be returned
        return alt_activities_range[(alt_activities_range.indexOf(calendar_range[week][day]) + 1) % alt_activities_range.length];
      }
  */  
    }
  }
}
}

Table 1

Table 2

I figured out a (crude) work around for the issue. I copied the alt_wrkout_range parameter to a local variable (alt_wrkout_ARRAY). The first mention of the al_wrkout_range is within an IF clause so I'm guessing that this is where things stop working. An explicity variable seems to do the trick.

/**
* Looks for the alternate workout in the LAST matching value within the calendar.
* @param {array} input The calendar array to be searched.
* @return The first match starting from the last row (WEEK) and column (DAY) of the @param.
* @customfunction
*/
function calender_search(calendar_range, alt_activities_range) 
{
  var alt_activities_array=alt_activities_range[0].slice();

  for (let week = calendar_range.length - 1; week >= 0; week--) 
  {
    for (let day = calendar_range[week].length - 1; day >= 0; day--) 
    {

      if (alt_activities_array.indexOf(calendar_range[week][day]) > -1) 
      {
        if (calendar_range[week][day] === alt_activities_array[alt_activities_array.length - 1]) 
        {
          return alt_activities_array[0];
        }
        // Go to index of match PLUS 1 for NEXT value to be returned
        return alt_activities_array[(alt_activities_array.indexOf(calendar_range[week][day]) + 1) % alt_activities_array.length];
      }
    }
  }
}

Perhaps someone will explain why the IF statement was causing the issue