/**
 * Part 4
 *
 * write a function the returns a FizzBuzz string for some number N (counting up from 1).
 * - for every number that isn't a multiple of 3 or 5, return a period "."
 * - for every number that is a multiple of 3 (but not 5), return "fizz"
 * - for every number that is a multiple of 5 (but not 3), return "buzz"
 * - for every number that is a multiple of 3 and 5, return "fizzbuzz"
 */

function fizzbuzz(N){
    // YOUR CODE HERE
}

console.assert(fizzbuzz(1) === ".")
console.assert(fizzbuzz(2) === "..")
console.assert(fizzbuzz(3) === "..fizz")
console.assert(fizzbuzz(5) === "..fizz.buzz")
console.assert(fizzbuzz(10) === "..fizz.buzzfizz..fizzfizzbuzz")

Have a go at it first at least. We're help you if you get stuck. Although I will say all you need is a loop going up to N and IF statements that determine what to output based on modulo division of the current number.
Its that hard, although it might seem like it to a beginner.