Problem: find an index N where the sum of the integers to the left of N is equal to the sum of the integers to the right of N. If there is no index that would make this happen, return -1.

My solution

function findEvenIndex(arr) {
  var sum = i => i.reduce((a, b) => a + b),
    l = arr.length;

  for (let j = 0; j <= l; j++) {
    if (sum(arr.slice(0, j - 1)) === sum(arr.slice(j, l))) {
      return j
    } else {
      continue;
    }
  }
  return -1

}

I've set the for loop procedure as follows to see what's going on

  for(let j = 0; j <= arr.length; j++){
    var left = arr.slice(0, j-1), right = arr.slice(j)
    console.log(left, right)
  } 
/* returns 
[1] [3,4,3,2,1] 
[1,2] [4,3,2,1] 
[1,2,3] [3,2,1]
as expected
*/

However, when try to console.log the sum of these arrays:

function sum(i){ return i.reduce((a, b) => a+b)} 
    
    var l = arr.length;
  
  for(let j = 0; j <= l; j++){
    var left = arr.slice(0, j-1), right = arr.slice(j)
    console.log(sum(left), sum(right))
  }

Using the snippet above, findEvenIndex([1,2,3,4,3,2,1]) returns "15 16"?