var nestedNumbers = [1,2,3,[1,2,3,[12]]];

var sumOfArray = function(arr, sum) {
  for (var i = 0; i < arr.length; i++) {
    if (typeof arr[i] === 'object' && arr[i].length) {
     sum = sumOfArray(arr[i], sum);
    } else {
      sum += arr[i];
    }
  }
  return sum;
};

console.log(sumOfArray(nestedNumbers, 0));