I think push/pop are correct in a Stack (LIFO). Unshift/pop would be FIFO which (Queue)

Well, push and pop are faster and much more efficient in comparison to unshift and shift I believe.

I don't quite understand what you mean, you mean if obj[el] is non-existent, delete obj[el]?
But the question is we need to find out the odd one, not only the only one.

If you call variable as a stack you should use .unshift instead of push. Because stack involves the principle of first came, the latter came out.

not enought good because two loops instead of one. One loop is enought (use delete instead of = 1 and then get the only object field).