please edit your pen and remove the alert function.
Beyond that, I’m not sure I follow your question. However, you say:
Find the sum of all the multiples of 3 or 5 below 1000.
but your code seems to find the sum of all natural numbers below 1000 multiplied by 3 where the product is below 1000, and multiplied by 5 where the product is below 1000. I don’t know if this is a problem with the way you’re writing the code, or the way you’re reading the question.
In either case, this sounds like a homework question…? We’re not here to do homework for you. If there’s a specific question you’re asking, I’d be happy to help.