Broken but not sure why

index.js

const mapping = (f) => (reducing) => (result, input, index) => reducing(result, f(input, index));
const filtering = (predicate) => (reducing) => (result, input, index) => predicate(input) ? reducing(result, input, index) : result;

const ALPHABET = 'abcdefghijklmnopqrstuvwxyz'.split('');
const NUMBERS = '0123456789'.split('');
const isAlphaNumeric = (letter) => ALPHABET.indexOf(letter) !== -1 || NUMBERS.indexOf(letter) !== -1;
const periodicallyPad = (current, index) => index !== 0 && index % 5 === 0 ? ' ' + current : current;
const concatenate = (xs, x, index) => xs.concat(x);

const result = 'I AM SO EXCITED ABOUT TRANSDUCERS'.toLowerCase().split('')
  .reduce(filtering(isAlphaNumeric)((xs, x) => xs.concat(x)), [])
  .reduce(mapping(periodicallyPad)((xs, x) => xs.concat(x)), [])
  .join('');
console.log(result);

const incorrectResult = 'I AM SO EXCITED ABOUT TRANSDUCERS'.toLowerCase().split('')
    .reduce(filtering(isAlphaNumeric)(mapping(periodicallyPad))((xs, x) => xs.concat(x)), [])
    .join('');
console.log(incorrectResult);