原生js获取YYYY-MM-DD格式日期

gistfile1.js

function getDate(){
  var param = arguments,
    date = new Date();
  if(param.length==0){
    return changeDateType(date);
  }else if(param.length==1){
    if(param[0].toString().indexOf("+")!="-1"){
       var nowTime = date.getTime(),
        preDate = param[0].substr(1,param[0].length-1),
        preDateTime = Number(preDate)*24*60*60*1000;
        nowTime = Number(nowTime)+preDateTime;
       var newDate = new Date(nowTime);
       return changeDateType(newDate);
    }else if(param[0].toString().indexOf("-")!="-1"){
       var nowTime = date.getTime(),
        preDate = param[0].substr(1,param[0].length-1),
        preDateTime = Number(preDate)*24*60*60*1000;
        nowTime = Number(nowTime)-preDateTime;
      var newDate = new Date(nowTime);
      return changeDateType(newDate);
    }else{
      return null;
    }
  }
   function changeDateType(date){
    var year = date.getFullYear(),
      mon = date.getMonth() < 9 ? '0'+(date.getMonth()+ 1) : date.getMonth() + 1 ,
      day = date.getDate() < 10 ? '0'+date.getDate() : date.getDate(),
      nowDate = year+"-"+mon+"-"+day;
    return nowDate;
  }
     
}