This is a very clever way of adding the multiples of 3 and 5 without repeating the common multiple. However I can't mark it as a best practice because it forces a traverse through all of the natural number (minus 1). If we're talking about a small input number it's negligeble, but if we're talking about a very high input number then it starts to take a toll.

i had no idea what i was doing and no way could have figured this out. thanks for reminding me im far from mastering 7 kyu

nice!

don't use var declaration in forloop or functions please, use let instead

This is straight forward and insightful.

Что если введут 100000, код будет перебирать каждое число ? Максимально не эффективно

O(n) is good, but you can go in steps of 15 until you get close enough for O((n/15)+6.5) (on average) :)

return sum = 0 by default then

So simple, yet so nice.

I think for code redability point of view should place if condition to return 0 for case of negative number.

filter returns a new array and doesn't mutate the argument.

Learn to match! The code works in O(n), this is a naive solution. If the input data value is greater than 10 ^10, the code will work very slowly (or even not at all). Using the sum of the arithmetic progression gives a constant time complexity. I don't understand why so many people marked such a decision as "the best".

This solution is at least 10 years old.