These are tested in the Classic Designer on Version 11.0.2212.30907.

Here are some formulas you can put in your Single Line field that will populate the current date.

Formatted as M/D/YYYY:

=MONTH(NOW())&"/"&DAY(NOW())&"/"&YEAR(NOW())

Formatted as YYYY-MM-DD: 

=YEAR(NOW())&"-"&RIGHT("00"&MONTH(NOW()),2)&"-"&RIGHT("00"&DAY(NOW()),2)

Formatted as DD/MM/YYYY: 

=RIGHT("00"&DAY(NOW()),2)&"/"&RIGHT("00"&MONTH(NOW()),2)&"/"&YEAR(NOW())

Then since you mentioned Javascript, here's some Javascript to do it (again, this is for the Classic Designer).  This is based on CSS Class Names dateValue1, dateValue2, and dateValue3 - and are set to update when the form is loaded if the fields are blank to begin with.  

$(document).ready(function () {
  
  var myDate = new Date();
  var m = myDate.getMonth()+1;    //one digit month (add 1 because JS defaults months 0-11)
  var m2 = ("00" + m).slice(-2);  //two digit month
  var d = myDate.getDate();       //one digit day of the month
  var d2 = ("00" + d).slice(-2);  //two digit day of the month
  var y = myDate.getFullYear();   //4-digit year
  if($('.dateValue1 input').val() == '') {
    $('.dateValue1 input').val(m + '/' + d + '/' + y); 
  }
  if($('.dateValue2 input').val() == '') {
    $('.dateValue2 input').val(y + '-' + m2 + '-' + d2); 
  }
  if($('.dateValue3 input').val() == '') {
    $('.dateValue3 input').val(d2 + '/' + m2 + '/' + y); 
  }
  
});