How to get the last blank row of the entire active sheet for google app script

by .getMaxRows() :

function nbRows(){
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheets()[0];
Logger.log(sheet.getMaxRows());
}

Based from answer https://stackoverflow.com/a/9102463/1677912

function getLastSheetRowByColumnArray() {
  var column = sheet.getRange('A:A');
  var values = column.getValues(); // get all data in one call
  var ct = 0;
  while ( values[ct] && values[ct][0] == "" || values[ct] && values[ct][0] != "" ) {
    ct++;
  }
  return (ct);
}