const reducer = (sum, currentValue) => sum + currentValue;

function findEvenIndex(array) {
  if (array && array.length < 1) {
    return -1;
  }
  for (let i = 0; i < array.length; i++) {
    const left = [];
    const right = [];
    let leftSum = 0;
    let rightSum = 0;
    //make an array of all items on the left of i (excluding i)
    left.push(...array.slice(0, i));
    //make an array of all items on the right of i (excluding i)
    right.push(...array.slice(i + 1, array.length));
    if(left.length >= 1){
      leftSum  = left.reduce(reducer)
    }
    if(right.length >= 1){
      rightSum = right.reduce(reducer);
    }
    if(leftSum == rightSum){
      return i;
    }
  }
  return -1;
}