Polydivisible.js

// https://www.codewars.com/kata/556206664efbe6376700005c/solutions/javascript/me/best_practice

const isPolydivisible = (n, b) => n.toString().split('').every(
  (d, i, a) => (a.join('').slice(0, i + 1))
  .split('').reverse()
  .map(num => Number(num) || CHARS.indexOf(num))
  .reduce((a, c, i) =>
    (c * (Math.pow(b, i))) + Number(a)
  )
  % (i + 1) === 0
);


const getPolydivisible = (n, b) => {
  const convertToBaseFromTen = (num, base, concat = '') => {
    if (Math.floor(num / base) >= 1) {
      return convertToBaseFromTen(Math.floor(num / base), base, concat + (num % base > 9 ? CHARS[num % base] : num % base))
    } else {
      return (concat + Math.floor(num % base)).split('').reverse().join('')
    }
  };
  const deducer = n >= b ? b : 0;
  let inc = deducer;
  let dec = n;
  let highestPoly;
  
  while (dec > deducer) {
    const numInBase = convertToBaseFromTen(inc, b);
    
    if (isPolydivisible(numInBase, b)) {
      highestPoly = numInBase;
      dec--;
    }
    
    inc++;
  }
  
  return n <= b ? CHARS[n - 1] : highestPoly;
}