To add to this, you can see what this looks like by doing a console.log() on the object after you have added keys to them. You'll see that the key's are simply a string of numbers, not actual numbers.

you can't store actual numbers as keys in object. so javascript automatically converts them to string when assigned to an object as a key

There is no reason to store the non odd occuring numbers. Doing so requires you to loop through again and find the odds. It's much slower than deleting a key if it's already in the object

This solution has a run time of O(n). Since the run time of a sorting algorithm is usually O(nlogn), and you'd still have to iterate through your array once more to count the number of times the integer appears, this solution would be faster.

prop is a string

Yes, similiar as mine, but much much shorter and more readable :) kudos

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.

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).

clean and functional.

spend time looking at the other solutions and use the Array and Enumerable methods that are available.

many people find until and unless to be unintuitive, and therefore use while and if instead.