How to reverse a string with recursion?

Question

JavaScript

How to reverse a string recursively using the length of the string and getting a substring?
I do not understand how to set a condition for recursion?

const reverse = (str) => {
  let res = '';
  const index = str.length-1;
  if (index === 0 || index === 1) return str;
  else {
    res += iter(str, index)
  }
};

const iter = (str, index) => {
  if (index !== 0) return iter(str, index-1);
}

Answer 1

const reverse = str => str.length < 2 ? str : reverse(str.slice(1)) + str[0];

or

const reverse = ([ c, ...str ]) => c ? reverse(str) + c : '';