when you use //, it basically rounds to the smaller number, if you have 1.3 it becomes 1, if you have 79.8, it becomes 79,
so it always round to the smaller, but what happens when the number is negative, -79.8, the smaller number is -80, and that's
the way to do ceil and floor without using any of the method, number // divisor to get the floor, and -(-number//divisor),
to have the ceil, pretty neat :D

thanks for that lesson! ;)

Wow, reading about why it works was educative :D

Rounding towards negative infinity. https://python-history.blogspot.com/2010/08/why-pythons-integer-division-floors.html

How does this even work?

Hi, Actually i had same thoughts about if else heirarchy(still i have same).
Before writing this comment i executed following code in C in Code Blocks 16.01
int x=1;
if(x==1)
printf("1");
else if(x>0)
printf("2");
else if(x<3);
printf("3");

and i got 13 as answer(in C) so based on this fact that if else is same for every programming language i commented on this.
but but i also tested this in differnet language after reading ur comment i got answer as 1 only.

That's not how else ifs work. Once one if is satisfied, it breaks out of the else if chain. By your logic, 15 would actually be added thrice: 15 % 3 == 0 and 15 % 5 == 0 so that will trigger the first one, then the second else if will trigger and so will the third.

The test case implies there is an array being passed, yet you say that number is not an array.

Edit: Also, you are simply console.log(abc) rather than return abc, might that be the issue?

If a number such as 15 has encountered in your case it will be added twice
1st in first if case and 2ndly in 3rd else if case so just put your 1st if statement having '||' instead of '&&' so it will take care of every case.