Hell yeah! That's what I'm talking about

how's it work?

Ah thank you! I see my mistake now =D

If the actual object field was called name you could use either obj['name'] or obj.name. However, name is a variable that represents some other value in this case.

Can someone please explain why obj[name] works here, but obj.name does not?

I thought an object could use either syntax.

It seems that if the outer loop is over wishlist, then one doesn't need to check for uniqueness.

I notice that solution assumes the input will always contain one and only one of each of the numbers 0 through 9, except for the missing one.
Since I like to write code that can handle the unexpected, I set my JavaScript solution up to find the first non-negative integer not contained in the array, or at least that was my intention. I realized as I submitted it, that it will return the first missing or duplicated non-negative integer. So for [9, 1, 2, 3, 4, 7, 5, 6, 5, 0] it would return 5, while for [1, 0] it would return 2, and for [27, 0, 18, 19, 2, 3, 4, 5, 1, 15, -7, 16, 17, 6, 7, 12, 13, 14, 20, 21, 8, 9, 10, 11, 22, 24, 25, 26, 99, 27, 135] it would return 23.

Nevermind, it was me :)

Is it just me or this solution doesn't work anymore?

These days I've learned a lot about reduce method. Thanks for this clever solution !

Agree, though it is still slower in Firefox and IE11.
I am sure it will be someday the fastest solution.

I just tested it in Chrome 40, and map is now slightly faster than using a for loop.

Voted this one up for being more efficient than the indexOf solution.