文件拓展名、周几的写法优化

const extname = (filename) => {
   let arr=filename.split(".");
   let len=arr.length;
   let extname=''
   if(len>1){
     extname = arr[len-1];
     extname =arr[len-2]==''?'': `.${extname}`;
     return extname
   }
   return extname
}

const extname2 = (filename) => {
   let index=filename.lastIndexOf (".");
  let len = filename.length
  if(index<=0||index==len-1){
    return ''
  }else{
   return filename.substring(index,len)

  }
}

let now = new Date() 
let week = now.getDay()
//写法一 
switch(week){
  case 1:console.log('周一')
    break;
     case 2:console.log('周二')
    break;
     case 3:console.log('周三')
    break;
     case 4:console.log('周四')
    break;
     case 5:console.log('周五')
    break;
     case 6:console.log('周六')
    break;
     case 0:console.log('周日')
    break;
}

//写法二 
let weekArr=['日',"一","二","三","四","五","六"]
console.log(`周${weekArr[week]}`)

//写法三 
let weekStr ='日一二三四五六'
console.log(`周${weekStr.charAt(week)}`)