A FizzBuzz implementation in JS/ES6. It could be a lot smaller and 'smarter' but it aims to be readable and understandable at the first glance. I also like the reusability.

someFizzBuzz.js

/**
 * FizzBuzz for a single value.
 * @param {number} number - The value to be tested.
 * @returns {number | string} - The same Number or 'Fizz', 'Buzz', 'FizzBuzz'.
 */
function fizzBuzz(number = 0) {
  if (number === 0) {
    return 0;
  }
  
  let word = '';
    
  if (number % 3 === 0) {
    word = 'Fizz';
  }
    
  if (number % 5 === 0) {
    word += 'Buzz';
  }
    
  return (word.length > 0) ? word : number;
}

/**
 * FizzBuzz over a numerical range.
 * @param {number} a - First range parameter.
 * @param {number} b - Optional second range parameter. Default = 0.
 * @returns {array} - An array of all 'FizzBuzzed' values in given range.
 */
function fizzBuzzFromRange(a, b = 0) {
  let min = Math.min(a, b);
  let max = Math.max(a, b);
  
  let it = [];
  
  for (let i = min; i <= max; i++) {
    it.push(fizzBuzz(i));
  }
  
  return it;
}

/**
 * FizzBuzz over all values of an array.
 * @param {array} arr - The input array. Default = [] (empty array).
 * @returns {array} - The 'FizzBuzzed' array.
 */
function fizzBuzzFromArray(arr = []) {
  return arr.map((number) => fizzBuzz(number));
}

// usage examples
console.log(fizzBuzz(15));

fizzBuzzFromRange(100).forEach((value) => console.log(value));
console.log(fizzBuzzFromRange(100).filter(value => value === 'FizzBuzz').length);

fizzBuzzFromRange(-50, 50).forEach((value) => console.log(value));

let numbers = [];
for (let i = 1; i <= 15; i++) {
  numbers.push(i);
}

fizzBuzzFromArray(numbers).forEach((value) => console.log(value));