JavaScript函数练习

1. 判断一个数是否是素数

         function isSushu (n) {
             n = n || 0;
             var isSu = true;
             for (var i = 2; i <= Math.sqrt(n); i++) {
                 if (n % i === 0) {
                     isSu = false;
                 }
             }
             return isSu;
         }
             console.log(isSushu(83));

2. 求阶乘

         function getJS (n) {
             var JS = 1;
             for (var i = 2; i <= n; i++) {
                 JS *= i;
             }
             return JS;
         }
         console.log(getJS(2));

3. 求1！ + 2！ + 3！ + ... +n!

         function getSum(n) {
             var sum = 0;
             for (var i = 1; i<= n; i++) {
                 var JS = 1;
                 for(var j = 1; j <= i; j++){
                     JS *= j;
                 }
             //    sum += getJS(i);
                 sum += JS;
             }
             return sum;
         }
         console.log(getSum(3));

4. 求斐波那契数列fibonacci中的第n个数是多少？？

// 1  1  2  3  5   8  13  21

         function fibonacci (n) {
             var arr = [];
             arr[0] = 1;
             arr[1] = 1;
             for(var i = 2; i < n; i++){
                 arr[i] = arr[i-1] + arr[i-2];
             }
             return arr[n-1];
         }

         console.log(fibonacci(3));