You are not supposed to convert it to number but manually implement multiplication algorithm on arbitray sized integers stored as string.
https://en.wikipedia.org/wiki/Multiplication_algorithm

watch the name of the kata: you're not supposed to use numbers.

Just an ignored variable https://stackoverflow.com/questions/27637013/what-is-the-meaning-of-an-underscore-in-javascript-function-parameter

what "_" mean ?

i written my code in the console and it do fine but here every time give one error!!!!!!

Ruby:

STDERR
main.rb:22: warning: constant ::Fixnum is deprecated

random tests => duplicate issue, (but only in js) see below, thanks

test added to all languages

No random tests and the solution below may pass but not fit the problem description:

https://www.codewars.com/kata/reviews/5e0e15bd4225f300010c2a9f/groups/5e0eb5644ebc200001b5d8fc

done

console.log the input

no its not wrong i test my solution many times!!

Because your solution is wrong. Read the error message.