Code puzzle 3

const f = a => {
  const g = x => x[x .length - 1]; const b = {'+':1, '-':1, '*':2, '/':2}; let c = [];
  return a.split('').reduce((a, d) => {
  if (parseFloat(d)) a.push(d);
  if (d in b) { while (g(c) in b && b[d] <= b[g(c)]) a.push(c.pop()); c.push(d); }
  if (d == '(') c.push(d);
  if (d == ')') { while (g(c) != '(') a.push(c.pop()); c.pop();}
  return a; }, []).concat(c.reverse()).join('');
};