js实现阶乘

//while循环实现
function calNum(n) {
    var product = 1;
    while(n > 1){//1*5*4*3*2，1*n*(n-1)*(n-2)*...*2
        product *= n;
        n--;
    }
    return product;
}
console.log(calNum(5))

//js实现阶乘
	function calNum(n){
		var a = 1, str = '1*';
		for (var i = 2; i <= n; i++) {
			str += i + '*';
			a *= i;
		}
		str = str.substr(0,str.length-1);
		return str + '=' +a;
	}
	console.log(calNum(5));
       //打印结果为：1*2*3*4*5=120