https://codility.com/demo/take-sample-test/

codility.js

function solution(A) {
  function rightSum(A, N, i, res) {
    i = typeof i !== 'undefined' ? i : 0;
    res = typeof res!== 'undefined' ? res : 0;

    if (i>=N) {
      return res;
    } else {                
      return rightSum (A, N, i+1, res+A[i]);
    }
  }

  function leftSum(A, N, res) {
    res = typeof res!== 'undefined' ? res : 0;

    if (N<A.length) {
      return leftSum(A, N+1, res+A[N]);
    } else {
      return res;                
    }
  }

  function loop(A, N, res) {
    if (N>=A.length) {
      return res;
    } else if (rightSum(A, N)===leftSum(A, N+1)) {
      return N;
    } else {
      return loop(A, N+1, res);
    }
  }

  return loop(A, 0, -1);    
}