dateformat yymmdd to yy-mm-dd

function dateFormat(value, pattern) {

            var i = 0,

            date = value.toString();

      return pattern.replace(/#/g, _ => date[i++]);

      }

 

 

var tmpDate = dateFormat(date, '####-##-##');

                    var selDate = new Date(tmpDate);

              var day = selDate.getDay();