// Function that returns the index of element in array where the sum of all of the other elements to its left
// are equal to the sum of elements to its right.
// e.g. in [1, 3, 5, 9, 7, 2] the answer would be index of 3 (ie the number 9)
// since the elements to its left sum up to 9 (1+3+5=9)
// and also the elements to its right (7+2=9)
// if not matching element found -1 is returned.

const middle = (numArray) => (
  numArray.findIndex((el, idx) => {
    const data = numArray.reduce((obj, el, i) => {
      // sum of elements to the left
      if(i < idx) {
	obj.sumLeft += el;
	return obj;
      }
      // sum of elements to the right
      if(i > idx) {
	obj.sumRight += el;
        return obj;
      }
      return obj;
    }, { sumLeft: 0, sumRight: 0 });

    return data.sumLeft === data.sumRight
  })
);

// eg
const numbers = [1, 2, 3, 4, 10, 7, 3];
middle(numbers); // => 4