Buscar el índice en el que la suma de un array es igual por ambos lados

readme.js

/*
Tengo que decir, que desconocía la función "reduce" de los arrays de JavaScript, que hubiese simplificado un poco el código, sobre todo después de ver cómo lo han usado algunos en sus respuestas en el [Kata - Equal Sides Of An Array](https://www.codewars.com/kata/5679aa472b8f57fb8c000047/javascript)

Aquí te pongo un par de códigos que usan reduce.<br>
*/

//de zoid:

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;
}


//de Rahul:

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));


//de d-sheep:

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;
}

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.

Let's look at another one.
You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of the array,
the sum of left side of the index ({1}) and the sum of the right side of the index ({50,-51,1,1}) both equal 1.

Last one:
You are given the array {20,10,-80,10,10,15,35}
At index 0 the left side is {}
The right side is {10,-80,10,10,15,35}
They both are equal to 0 when added. (Empty arrays are equal to 0 in this problem)
Index 0 is the place where the left side and right side are equal.

Note: Please remember that in most programming/scripting languages the index of an array starts at 0.

Input:
An integer array of length 0 < arr < 1000. The numbers in the array can be any integer positive or negative.

Output:
The lowest index N where the side to the left of N is equal to the side to the right of N. If you do not find an index that fits these rules, then you will return -1.

Note:
If you are given an array with multiple answers, return the lowest correct index.
*/
function findEvenIndex(arr)
{
  //Code goes here!

  // Crear subarrays con los elementos de cada grupo y sumar los elementos.
  const cuantos = arr.length;
  const todos = [cuantos];

  if (cuantos == 1) return 0;

  let todos0 = 0;

  for (let index = 0; index < cuantos; index++) {
    let total = 0;
    let total2 = 0;
    for (let j = 0; j < index; j++) {
      total += arr[j];
    }
    for (let j = index; j < cuantos; j++) {
      total2 += arr[j];
    }

    let nums = [2];
    nums[0] = total;
    nums[1] = total2;
    todos[index] = nums;

    todos0 += (total + total2)
  }
  // Si todos los valores suman cero, devolver cero.
  if (todos0 == 0 ) return 0;

  // Para probar y mostrar los valores.
  console.log(arr);
  for (const iterator of todos) {
    //console.log(iterator);
    console.log(iterator[0] + " / " + iterator[1]);
  }

  // Si todos son iguales, devolver 0

  let iguales = 0;
  let menor = -1;
  // Buscar la posición que tenga todos[0] = todos[1] y todos[1] = todos[0]
  for (let i = 0; i < cuantos; i++) {
    for (let j = i + 1; j < cuantos; j++) {
      if (todos[i][0] == todos[j][1] && todos[i][1] == todos[j][0]) {
        if (i == j - 1) {
          iguales++;
          menor = i;
          break;
        }
      }
      // Esto funciona en este caso: [10,-80,10,10,15,35,20]
      // Es decir, cuando la suma anterior al último número es cero.
      else if (todos[i][0] == todos[j][0] && todos[i][1] == todos[j][1]) {
        if (menor == -1 && j == cuantos - 1) {
          menor = j;
          break;
        }
      }

    }
  }

  if (iguales == cuantos) {
    return 0;
  }

  return menor;
}

function pruebas(arr, res) {
    var index = findEvenIndex(arr);
    console.log("[" + arr.toString() + "] = " + res + " ?= (" + index + ")");
    if (res != index) {
      console.log("\tEl valor devuelto es " + index + " y debe ser " + res);
    }
}

//pruebas([10,-80,10,10,15,35,20], 6);
pruebas ([0,0,0,0,0], 0);
//pruebas([-1086,6717,-7114,-2318,1260,338,5654,5436,8309,7432,-35,-9036,4482,-4368,6400,9507,4156,-8254,-5228,900,4527,2571,579,-4240,-5867,9621,9414,-2139,-7346,-8227,5732,1594,-8122,2275,-3584,-7218,-6316,5825,-3246,-4306,8887,4481,7203,-6716,-9166,-752,-9365,-9961,-5238,6057,1353,4233,-3232,-7636,-1999,-5653,4325,-7752,-6451,-5026,-5753,3468,-2999,-8692,1662,2327,948,1734,6856,7246,7401,4603,6053,-9438,-8635,-12,-3666,1118,9824,-9068,7499,2890,3811,-4159,5428,-3228,-7955,-3939,-684,7103,-7794,3654,-1128,9041,8760,4240,-2579,6744,-4407,-5400,1397,-8805,-1556,-781,4067,3228,3042,5402,6850,-3362,-9483,2982,-7400,5877,8936,-1718,-9161,5601,2908,3870,-2906,-2992,-9019,-3364,1620,-4524,8869,-3888,-7930,-554,-4065,2914,-4199,3481,-7766,7222,8508,-6965,-4961,9112,8524,-5137,8998,-8230,-7633,-13,-3869,9705,-219,-3364,569,4549,-9147,-2116,6958,852,9467,-3427,-2142,892,7649,-3205,-3704,-7603,788,345,-26,6332,-7418,-5325,-8730,3331,9918,-1305,4430,-1627,-2804,4481,-6327,2602,-7784,7588,-7138,-9737,-289,85,-3401,608,-5671,6279,6645,-4843,5535,-7464,9164,-903,8884,649,5927,-1081,-6353,564,-5206,3895,8702,-2980,-6492,-510,-7315,852,8920,-6584,7072,2776,2399,3700,2383,-4656,582,-3981,-7015,6700,-2857,9826,9406,-7307,6194,3155,4115,-7697,3166,1659,-3276,-1548,5184,4142,-6273,-2615,-9411,3205,9259,-536,3781,7526,-8542,-1012,4696,-98,2406,2789,5536,-3345,-2718,6432,402,4870,9074,1083,5940,8332,7937,20,9627,1707,6783,7435,1433,-3781,-6481,7356,-903,-6883,520,-916,5563,3839,-8267,-5225,5567,-7653,170,-2680,4730,3149,2876,-7089,3830,7162,-3408,-7513,990,-9858,-1627,2624,5066,2669,-5286,-7800,4377,6202,8888,-8255,1720,-8404,-9526,-6653,-7665,-4535,2395,-1895,1850,-9579,-2329,267,9975,-6340,1601,-576,8098,262,-6305,-6653,-242,-7051,-6658,2864,6731,9064,6150,-1986,-3649,6685,3640,9388,-1923,9685,-1363,8082,9521,-2075,3890,-1945,1826,-4312,3206,4572,922,7832,5487,-2876,-6512], -1);

//pruebas([8], 0);

// pruebas([1,2,3,4,3,2,1], 3);
// pruebas([1,100,50,-51,1,1],1);
// pruebas([1,2,3,4,5,6],-1);
// pruebas([20,10,30,10,10,15,35],3);

/*
const Test = require('@codewars/test-compat');

describe("FindEvenIndex", function() {
  it("Tests", function() {
    Test.assertEquals(findEvenIndex([1,2,3,4,3,2,1]),3, "The array was: [1,2,3,4,3,2,1] \n");
    Test.assertEquals(findEvenIndex([1,100,50,-51,1,1]),1, "The array was: [1,100,50,-51,1,1] \n");
    Test.assertEquals(findEvenIndex([1,2,3,4,5,6]),-1, "The array was: [1,2,3,4,5,6] \n");
    Test.assertEquals(findEvenIndex([20,10,30,10,10,15,35]),3, "The array was: [20,10,30,10,10,15,35] \n");
  });
});
*/