FizzBuzz.js

function getNum(input) {
	let conditions = [{num: 3, word: 'Fizz'}, {num: 5, word: 'Buzz'}]
	return conditions.map(({num, word}) => {
		return input % num === 0 ? word : '#'
	}).join('').replace(/#+/, input).replace(/(\d+)?([A-z]+)(\d+)?/g, '$2')
}

FuzzBuzzOriginal.js

function getNum(num) {
  return num % 3 === 0 ? (num % 5 === 0 ? 'FizzBuzz' : 'Buzz') : (num % 5 === 0 ? 'Fizz' : num.toString())
}

Adding more condtions is easy, just add another object to the conditions variable.
For example:

let conditions = [
  { num: 3, word: 'Fizz' },
  { num: 5, word: 'Buzz' },
  { num: 7, word: 'Boop' }
];