factorial and sum of fibonacci(n) using recursion in JavaScript

file.js

const factorial = (n, acc=1) => {
  if (n < 1) { return undefined; }  
  
  if (n === 1) { return acc; }
  
  return factorial(n-1, n*acc);
};

const sumFibonacci = (n, prev=0, next=1, acc=0) => {
  if (n === 0) { return acc; }
  
  return sumFibonacci(n-1, next, next+prev, acc+prev+next);
};

factorial(5); // 120
sumFibonacci(6); // 19