Javascript: overload method format of Date

hunttingteacher.user.es6

    let getPaddedComp = comp => parseInt(comp) < 10 ? '0' + comp : comp,
      o = {
        "[y|Y]{4}": date => date.getFullYear(), // year
        "[y|Y]{2}": date => date.getFullYear().toString().slice(2), // year
        "MM": date => getPaddedComp(date.getMonth() + 1), //month
        "M": date => date.getMonth() + 1, //month
        "[d|D]{2}": date => getPaddedComp(date.getDate()), //day
        "[d|D]{1}": date => date.getDate(), //day
        "h{2}": date => getPaddedComp(
          (date.getHours() > 12) ? date.getHours() % 12 : date.getHours()), //hour
        "h{1}": date => (date.getHours() > 12) ? date.getHours() % 12 : date.getHours(), //hour
        "H{2}": date => getPaddedComp(date.getHours()), //hour
        "H{1}": date => date.getHours(), //hour
        "m{2}": date => getPaddedComp(date.getMinutes()), //minute
        "m{1}": date => date.getMinutes(), //minute
        "s+": date => getPaddedComp(date.getSeconds()), //second
        "f+": date => getPaddedComp(date.getMilliseconds()), //millisecond,
        "b+": date => (date.getHours() >= 12) ? 'PM' : 'AM'
      };
    $.extend(Date.prototype, {
      toString: function(format) {
        let formattedDate = format;
        for(var k in o) {
          if(new RegExp("(" + k + ")").test(format)) {
            formattedDate = formattedDate.replace(RegExp.$1, o[k](this));
          }
        }
        return formattedDate;
      }
    });