This is because first two items are checked before the for loop

Why do you return the string instead of false?

That's because of JS's limitation on amount of arguments you can pass to a function though.

Also, there are no performance requirements here, so I'd go for the shorter (and cleaner) code ;-)

HI can you tell me how to use this website

You didn't understand how to sort it, or how the sort method works.
See the example again:

348597 => [7,9,5,8,4,3]

Your code returns [ 9, 8, 7, 5, 4, 3 ]
Reversing is not the same as sort in descending order.

Consider a string like ")))(((". It would return true in your function, but that is obviously an invalid input.