[рони]

vurdalak21,

const sumOdd = function(nums) {
    return !nums.reduce((balance, num, i) => balance +=  i % 2 ? num : -num, 0)
}

const removeIndex = function(nums, ind) {
    nums = nums.slice(0);
    nums.splice(ind, 1);
    return nums
}

const waysToMakeFair = function(nums) {
    return Object.keys(nums).filter(ind => sumOdd(removeIndex(nums, ind)))
};
alert(waysToMakeFair( [2,1,6,4]));