Balance points

balancePoint.js

function sum(a, b) {
   return a + b;
}

var BalancePoint = function(input) {
  if (!Array.isArray(input)) {
    return -1;
  }

  var totalSum = input.reduce(sum, 0);
  var leftSum = 0;

  return input.reduce(function(point, current, i) {
    if (i > 0) {
      leftSum += input[i-1];
    }

    var rightSum = totalSum - leftSum - current;

    if (leftSum === rightSum) {
      point = i;
    }

    return point;
  }, -1);
}

BalancePoint([ 2, 7, 4, 5, -3, 8, 9, -1 ]); // 3