Test Javascript function

Total newbie here with JS, I have a basic JS function that I need to just test to see some output from it.

function GetDateWithSeparator(separator) {
  var today = new Date();

  var dateTime = ''

  if (separator === '-') {
    today.setDate(today.getDate() - 1)
    dateTime = today.getFullYear() + separator + ('0' + (today.getMonth() + 1)).slice(-2) + separator + ('0' + today.getDate()).slice(-2)
  }
  else {
    dateTime = today.getFullYear() + separator + ('0' + (today.getMonth() + 1)).slice(-2) + separator + ('0' + today.getDate()).slice(-2)
  }

  return dateTime;      
}

I am trying to test this in an online Javascript code tester but it’s not displaying any output. I also tried using console.log(dateTime) at the bottom of the script, however that still does not result in anything.

It seems like I need to supply some input value for the seperator argument, however I’m not sure what the syntax is to do this within a code tester tool. Any help is appreciated!