Persistent Bugger - kata

function persistence(num) {
   let digits = num.toString();
  if (digits.length === 1) {
    return 0;
  }
  let calculate =1;
  let counter = 0;
  while (digits.length > 1) {
    for (let i = 0; i < digits.length; i++) {
        calculate *= digits[i];
        }
        digits = String(calculate);
        calculate = 1;
    counter++;
    }


  return counter;
}

console.log(persistence(39));

/* Description:

Write a function, persistence, that takes in a positive parameter num and returns its multiplicative persistence, which is the number of times you must multiply the digits in num until you reach a single digit.

For example (Input --> Output):

39 --> 3 (because 3*9 = 27, 2*7 = 14, 1*4 = 4 and 4 has only one digit, there are 3 multiplications)
999 --> 4 (because 9*9*9 = 729, 7*2*9 = 126, 1*2*6 = 12, and finally 1*2 = 2, there are 4 multiplications)
4 --> 0 (because 4 is already a one-digit number, there is no multiplication)
*/