JavaScript 判断某年某月有几多天

function  getYearMonthDayNum(year,month){ 
	    var   dayNum   =   [31,28,31,30,31,30,31,31,30,31,30,31]; 
	    if(new   Date(year,1,29).getDate()==29){ 
	        dayNum[1]   =   29; 
	    } 
	    return   dayNum[month-1]; 
	}

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