The largest palindrome comes from 15*125 = 1875 -> 8.

There is a smarter way to do this than using recursion :)

Is there a smaller pyramid that has the same longest slide.
Take a look at the small pyramid and think about it.

Or, if you're using javascript (which I guess based on your previous katas), you can just use console.log:

  function isMerge(s, part1, part2) {
    console.log('Params:', s, ':', part1, ':', part2);
    // ...your solution
  }

I added the colons between the parameters so it will be easier to see when one of them is an empty string.

For example:

def is_merged(s, part1, part2):
    print(s, part1, part2)
    # ... your solution

But better use repr() because you will have to deal with empty strings.

You are trying to call toUpperCase on an undefined variable, check it exisits and is not undefined when you call that method on it.

CW has some issue when I post some comments...

becasue testcase check for the function returned value, console.log() only used to print information to your screen