code: function _stringPad(that, maxLength, fillString, left) {
  	  var S = String(_defined(that));
  	  var stringLength = S.length;
  	  var fillStr = fillString === undefined ? ' ' : String(fillString);
  	  var intMaxLength = _toLength(maxLength);
  	  if (intMaxLength <= stringLength || fillStr == '') return S;
  	  var fillLen = intMaxLength - stringLength;
  	  var stringFiller = _stringRepeat.call(fillStr, Math.ceil(fillLen / fillStr.length));
  	  if (stringFiller.length > fillLen) stringFiller = stringFiller.slice(0, fillLen);
  	  return left ? stringFiller + S : S + stringFiller;
  	}
docs: https://github.com/tc39/proposal-string-pad-start-end