The two functions are not the same; the second code nests the y loop inside the x loop, causing different behavior, while the first runs the loops separately.

langs do not work like that, at least 4 this level
try another algorithm

They are totally not identical, if you open your eyes you will see a difference between them :D

Leading 0's are not perrmited, as no "real" numbers have them (except 0 itself).

Your solution does have leading 0's, "03" != "3"

This kata is great !

This kata is great ! i scratched my head so hard trying to code a simple addition xD

You should treat each string as a representation of a base-10 integer. So, "0" -> 0 and "5" -> 5, so the expected answer is 5 and not the addition of 2 strings which yield NaN.

I think if leading zeros are allowed, it makes sense to say so in the condition or show it in the example for the task.

JavaScript fixes

• Updated to chai
• Added anti-cheat (taken from here)

Resoved issues (at least for JS): 1, 2, 3, 4, 5

I didn't know the python limtation. My code passes with simple casting with int(). Although I wanted to know maximum length of blocks by this algorithm ,is 4300 digits

Sum digit by digit: 0.454667329788208 seconds
Sum in blocks of 100: 0.02377772331237793 seconds
Sum in blocks of 1000: 0.030898571014404297 seconds
Sum in blocks of 4300: 0.09897232055664062 seconds

ValueError: Exceeds the limit (4300 digits) for integer string conversion: value has 4301 digits; use sys.set_int_max_str_digits() to increase the limit

Maybe your code is too short?

Anyway, "my code is too slow to pass" is not a kata issue. You need to figure out how to make your solution sufficiently fast.