Moving vowels and consonants using JavaScript

Problem

We are required to write a JavaScript function that takes in a string of English alphabets. Our function should construct a new string and every consonant should be pushed forward 9 places through the alphabet. If they pass 'z', start again at 'a'. And every vowel should be pushed by 5 places.

Example

Following is the code −

const str = 'sample string';
const moveWords = (str = '') => {
   str = str.toLowerCase();
   const legend = 'abcdefghijklmnopqrstuvwxyz';
   const isVowel = char => 'aeiou'.includes(char);
   const isAlpha = char => legend.includes(char);
   let res = '';
   for(let i = 0; i < str.length; i++){
      const el = str[i];
      if(!isAlpha(el)){
         res += el;
         continue;
      };
      let pos;
      const ind = legend.indexOf(el);
      if(isVowel(el)){
         pos = (21 + ind) % 26;
      }else{
         pos = (ind + 9) % 26;
      };
      res += legend[pos];
   };
   return res;
};
console.log(moveWords(str));

Output

bvvyuz bcadwp