OP solved it, closing

What's the difference?

Do you pass the challenge?
How did you deal with bug arrays?

Why do you think those symbols were all the same? Or all different?

They were all represented the same, but that's not the same thing.

I was able to get the test to give me a pass. But my code was definitely not fully functional. This kata might need more test cases.

It's somewhat difficult to tell what to search for, because you do not need to use any specific function or language construct here. You need a good idea. You already noticed that parseInt will return unvalid result for large arrays. Now the question is, do you really need parseInt? Can you perform the operation of add 1 to a number expressed as array of digits without building a value out of the array? Actually you can! For example, you can use the same algorithm you do when adding long numbers on paper:

 [7, 6, 3, 0, 5, 9]
+                1
------------------
 [?, ?, ?, ?, ?, ?]

It's one possible way.

I had the same question. Still trying to figure it out.

It's likely just an older solution when const/let weren't formally adopted in javascript yet.