Combine Range As CSV

Not sure if I understand your goal. So here is a guess:

function to_csv(range) { 
  return range.join(',');
}

Which is actually the same as the native formula: =TEXTJOIN(",",TRUE,A1:C1)

Or:

function the_weird_way_to_set_a_formula() { 
  var row = 10; 
  var formula = 'TEXTJOIN(",",TRUE,' +
    [...'PQRSTUVWXYZ'.split(''), ...'ABC'.split('').map(letter => 'A' + letter)]
    .map(letter => letter + row).join(',') + ')';

  SpreadsheetApp.getActiveSheet().getRange('a1').setFormula(formula);
}

I'm not sure I understand the question but I'll give it a shot

function combineRangeValuesIntoOneCell() {
  const ss = SpreadsheetApp.getActive();
  const sh = ss.getSheetByName("Sheet0");
  const vs = sh.getRange(1,16,sh.getLastRow(),11).getValues();
  let o = vs.reduce((a,c,i) => {
    c.forEach(e => {
      a += e;
    });
    return a;
  },0);
  Logger.log(o);
}

Execution log
8:26:42 AM  Notice  Execution started
8:26:43 AM  Info    1155.0
8:26:44 AM  Notice  Execution completed