Codility Equi Solution in Javascript - 100% score

Equi.js

/*
A zero-indexed array A consisting of N integers is given. An equilibrium index of this array is any integer P such that 0 ≤ P < N and the sum of elements of lower indices is equal to the sum of elements of higher indices, i.e. 

A[0] + A[1] + ... + A[P−1] = A[P+1] + ... + A[N−2] + A[N−1].

Sum of zero elements is assumed to be equal to 0. This can happen if P = 0 or if P = N−1.
*/

function solution(A) {
  var sum = A.reduce(function(pv, cv) { return pv + cv; });
  var sum_right;
  var sum_left = 0;
  
  for(i = 0; i < A.length; i++) {
    sum_right = sum - sum_left - A[i];
    if (sum_left === sum_right) return i;
    sum_left += A[i];
  }
  
  return -1;
}

Thanks for the codes, but it would fail when A is empty or a single number array, here is the correct answer:

function solution(A){
            var sum
            var sum_left = 0
            var sum_right

            if(!A || A.length == 0){
                return -1
            }else if(A.length == 1){
                sum = A[0]
            }else{
                sum = A.reduce(function(total, num){ return total + num })
            }

            for(var i = 0; i < A.length; i++){
                sum_right = sum - sum_left - A[i]
                if(sum_left == sum_right){ return i }
                sum_left += A[i]
            }

            return -1
        }