JavaScript의 배열에서 가능한 산술 시퀀스 계산

const arr = [1, 2, 3, 5, 7, 9];

const output = 5;

[1, 2, 3], [1, 3, 5], [1, 5, 9], [3, 5, 7] and [5, 7, 9]

const arr = [1, 2, 3, 5, 7, 9];
const countAP = (arr = []) => {
   let i, j, k;
   let { length: len } = arr;
   let count = 0;
   for (i = 0; i < len - 2; i++){
      for (k = i + 2; k < len; k++){
         let temp = arr[i] + arr[k];
         let div = temp / 2;
         if ((div * 2) == temp){
            for (j = i + 1; j < k; j++){
               if (arr[j] == div){
                  count += 1;
               }
            }
         }
      }
   }
   return count;
};
console.log(countAP(arr));

5