findEvenIndex.js

// You are going to be given an array of integers. Your job is to take that array and 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.

// For example:

// Let's say you are given the array {1,2,3,4,3,2,1}:
// Your function will return the index 3, because at the 3rd position of the array, the sum of left side of the index ({1,2,3}) and the sum of the right side of the index ({3,2,1}) both equal 6.

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