Алгоритмы. fizzBuzz.

fizzBuzz.js

/**
 * Classical task.
 *
 * 3 - Fizz
 * 5 - Buzz
 * 15 - FizzBuzz
 */
function fizzBuzz() {
  let result;
  
  for (let i = 1; i < 31; i++) {
    const isFizz = i % 3 === 0;
    const isBuzz = i % 5 === 0;
    
    const result = (isFizz && isBuzz) ?
      'FizzBuzz' :
        isFizz ? 'Fizz' :
          isBuzz ? 'Buzz' : i;
    
    console.log(result);
  }
}

fizzBuzz();

/**
 * Write a function that takes an integer and returns an array [A, B, C],
 * where A is the number of multiples of 3 (but not 5) below the given integer,
 * B is the number of multiples of 5 (but not 3) below the given integer and C is the number
 * of multiples of 3 and 5 below the given integer.
 * For example, solution(20) should return [5, 2, 1]
 */
function solution(number) {
  let result = [0, 0, 0];
  
  for (let i = 1; i < number; i++) {
    const isFizz = i % 3 === 0;
    const isBuzz = i % 5 === 0;
    const incrementByIndex = index => (result[index] || 0) + 1;
    
    (isFizz && isBuzz) ?
      result[2] = incrementByIndex(2) :
        isFizz ? result[0] = incrementByIndex(0) :
          isBuzz ? result[1] = incrementByIndex(1) : null;
  }
  
  return result;
}

// Smaller variant:
function solution(number) {
  --number;
  const C = Math.floor(number / 15);
  const A = Math.floor(number / 3) - C;
  const B = Math.floor(number / 5) - C;
  return [c3, c5, c15];
}

console.log(solution(20)); // [5, 2, 1]