Thanks to you @benjaminzwhite I was able to solve this kata :)

Some tests in JavaScript pass in empty strings, and your solution fails on these. Empty strings should be treated as "0".
Yes, this input sucks. But still, even though you say you check agains it, it's not done correctly.

Javascript, also tried my solution in https://www.codewars.com/kata/525f4206b73515bffb000b21 as they say its duplicated and it works there

In what language?

Hi @yoshida_haru - the goal of this kata is not about specific data types; in fact it's not even about computers/programming languages really.

Imagine you had 2 huge numbers written on a piece of paper, or a school blackboard, etc. - how would you add them up (without even needing a computer)?