Q.

Given an array of integers, find the one that appears an odd number of times.

There will always be only one integer that appears an odd number of times.

Examples
[7] should return 7, because it occurs 1 time (which is odd).
[0] should return 0, because it occurs 1 time (which is odd).
[1,1,2] should return 2, because it occurs 1 time (which is odd).
[0,1,0,1,0] should return 0, because it occurs 3 times (which is odd).
[1,2,2,3,3,3,4,3,3,3,2,2,1] should return 4, because it appears 1 time (which is odd).

A)

function findOdd(A) {
  let ret = {};
  for (el in A) A[el] in ret ? ret[A[el]]++ : ret[A[el]] = 1;
  for (el in ret) if (ret[el] % 2 === 1) return parseInt(el)
}