const fact = n => n < 1
  ? 1
  : Array(n)
    .fill(0)
    .map((_, i) => i + 1)
    .reduce((a, v) => a * v, 1);

const countOf5 = n => Math.floor((n + 5) / 10);
const countOf10 = n => Math.floor(n / 10);

// step-by-step way
const countOfZeros = n => countOf5(n) + countOf10(n);
// the easiest way, count 5 and 10
// const countOfZeros2 = n => Math.floor(n / 5);

// prevent incorrect e-typed numbers
for (let i = 0; i < 21; i++) {
  const zeros = countOfZeros(i);
  console.log(`factorial(${i}) = ${fact(i)}   // ${zeros} zero${zeros % 10 === 1 ? '' : 's'}`);
}