Javascript String genUrl()

/*//from www  . j  av  a  2 s  .c o m
* Helper mutator functions
* The MIT License
* Akhil Pandey
*/

// Mutator function for generating the link

String.prototype.genUrl = function () {
 let i = 0,
 str = "",
 num = Math.floor(Math.random()*(Math.pow(1000, 4))).toString()

 let store = {
  big: ['V', 'X', 'A', 'D', 'G', 'Y', 'D', 'U', 'O', 'Q'],
  lil: ['w', 'b', 'k', 'm', 'c', 'z', 'i', 's', 'f', 'p']
 }

 while(str.length < 20) {
  let count = Number(num[i])
  str += num[i] + store.big[count] + store.lil[count]
  i++
 }

 return (this + str)
}