运用switch的穿透实现当前日期是这一年的第几天

switch的穿透作用计算当前日期是这一年的第几天

    var y =2020;
    var m = 04;
    var d =22;
    var sum = 0
    switch(m){
     case 12:
   		sum += 30;
     case 11:
     		sum += 31;
     case 10:
     		sum += 30;
     case 9:
    		sum += 31;
     case 8:
    		sum += 31;
     case 7:
     		sum += 30;
     case 6:
    		sum += 31;
     case 5:
     		sum += 30;
     case 4:
      		sum += 31;
     case 3:
    		if ((y % 4 == 0 && y % 100 != 0) || (y % 400 == 0)){
   				 sum += 29;
   	            }else{
    				 sum += 28;
     		}
     case 2:
   		 sum += 31;
     case 1：
    		 sum += d; break
    default:
             console.log("你输入的日期不符合规范")
       }
     console.log("今天是2020年的第"+sum)