Finds a number's multiplicative persistence.

Persistence.js

function persistence(num) {
   let stringnum = num.toString()
   let count = 0;
   while ((stringnum).length > 1) {
     count++;
     let nums = stringnum.split("");
     let m = 1;
     for (let i = 0; i < nums.length; i++) {
       m = m*nums[i];
     }
     stringnum = m.toString();
   }
 return count;
}