Does not work properly when a negative number is given

You could also do (0...number) instead of (1...number)

Interesting! I was not aware of the range syntax with 3 dots, where 1...n exludes the end of the range.

O my love. So beautiful

short and pithy，easy to read

Not computation efficient.
I think it should use math to solve this.

Good catch. Thanks.

This returns nil when number < 3.

It would probably be best to add || 0 onto the end of line 2.

I wouldn't really say this is a best practice. To me best practice is the code that runs with the highest efficiency, this one ran in 136ms when I checked it.
The code will work, but looping through n times is unneccessary.
You could make the code more efficient. Just think: you only care about the multiples of 3 and 5