equalsidesarray.ts

function findEvenIndex(arr)
{
  for(var i=1; i<arr.length-1; i++) {
    if(arr.slice(0, i).reduce((a, b) =>  a+b) === arr.slice(i+1).reduce((a, b) =>  a+b)) {
      return i;
    }
  }
  return -1;
}

function findEvenIndex(arr)
{
  var left = 0, right = arr.reduce(function(pv, cv) { return pv + cv; }, 0);
  for(var i = 0; i < arr.length; i++) {
      if(i > 0) left += arr[i-1];
      right -= arr[i];
      
      if(left == right) return i;
  }
    return -1;
}

const sum = (a, from, to) => a.slice(from, to).reduce((a, b) => a + b, 0)
const findEvenIndex = a => a.findIndex((el, i) => sum(a, 0, i) === sum(a, i + 1));

A solution from other warriors that i like better than my solution...