because null === null evaluates to true
if I'm understanding your question correctly

Read the posts below. Like this one: https://www.codewars.com/kata/reviews/55908ab5932c2fc59f000070/groups/55c8b47c16ffe086d7000056#6312e85473f6480018781578

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The problem is that I can't use Number?

You can use any function as long as you implement the correct validation logic. Usage of Number is not restricted for this kata.

Everything works fine in my VS Code.

Your code is definitely incorrect if it fails for basic cases like "1234". You must be incorrectly (locally) testing your function.