You were modifying your input weren't you?

Your isprime function is not correct. Google is your friend.

Let's say you create a "three", its value should stay at 3, then you add "seven" and get "ten", then you have the same "three" and add "eight" to it, you'd expect you'd get "eleven", when you mutated the value, you'll get "eighteen" instead. I don't know if this helps (that's what the sample tests do, create a single number and add different values to it).

You're mutating number property in add method, you shouldn't.

Because let arry=arr; doesn' create a copy, that's another reference to the same array.

Assuming you're using JavaScript, you don't need to use arrays, you can directly solve the kata using string functions like slice and replace.