Q.

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:

 persistence(39) === 3 // because 3*9 = 27, 2*7 = 14, 1*4=4
                       // and 4 has only one digit
                 
 persistence(999) === 4 // because 9*9*9 = 729, 7*2*9 = 126,
                        // 1*2*6 = 12, and finally 1*2 = 2
                  
 persistence(4) === 0 // because 4 is already a one-digit number

A)

function persistence(num) {
  let numArr = num.toString().split('');
  let count = 0;
   while (numArr.length > 1) {
     numArr = numArr.reduce((ret, cur) => ret * cur).toString().split('');
     count++;
   }
  return count;
}