fizz-buzz.js

const assert = require('assert');

const deepEqual = assert.deepEqual;

const isFizz = x => x % 3 === 0;
const isBuzz = x => x % 5 === 0;
const isFizzBuzz = x => isFizz(x) && isBuzz(x);

const match = x => {
  if (isFizzBuzz(x)) return 'FizzBuzz';
  if (isFizz(x)) return 'Fizz';
  if (isBuzz(x)) return 'Buzz';
  return x;
};

assert(match(1) === 1);
assert(match(3) === 'Fizz');
assert(match(5) === 'Buzz');
assert(match(15) === 'FizzBuzz');

const fizzBuzz = (x, max, buf = []) => {
  if (x > max) return buf;
  return fizzBuzz(x + 1, max, buf.concat([match(x)]));
};

deepEqual(fizzBuzz(1, 20), [1, 2, 'Fizz', 4, 'Buzz', 'Fizz', 7, 8, 'Fizz', 'Buzz', 11, 'Fizz', 13, 14, 'FizzBuzz', 16, 17, 'Fizz', 19, 'Buzz']);

fizzBuzz(1, 100).forEach(x => console.log(x));