Shifts letters to next letter in alphabet (c -> d, z -> a) and ensures vowels in new string are CAPS in Javascript ES6. Includes tests!

alphabetShift.js

/*  --  Problem  --
 *  Have the function alphabetShift(str) take the str parameter
 *  being passed and modify it using the following algorithm.
 *  Replace every letter in the string with the letter following it
 *  in the alphabet (ie. c becomes d, z becomes a). Then capitalize
 *  every vowel in this new string (a, e, i, o, u) and finally return
 *  this modified string.
 */

/*  --  Examples  --
 *
 *  Input:"hello*3"
 *  Output:"Ifmmp*3"
 *
 *  Input:"fun times!"
 *  Output:"gvO Ujnft!"
 *
 */

const alphabetShift = (str) => {

  let ary = str.split('');
  const length = ary.length;
  let new_ary = [];

  for (let i = 0; i < length; i++ ) {
    if( /^[a-yA-Y]+$/.test(ary[i]) ) {
      let next_letter = String.fromCharCode( (ary[i]).charCodeAt(0) + 1 );
      new_ary.push( "eiou".includes(next_letter) ? next_letter.toUpperCase() : next_letter );
    } else if ( /^[zZ]+$/.test(ary[i]) ){
      new_ary.push('A');
    } else {
      new_ary.push(ary[i]);
    }
  }

  return new_ary.join('');

}

// -- Test "suite"

// tallies
let pass = 0;
let fail = 0;

// basic test function for newString
const test = (name, str, expected) => {
  console.log(' testing ' + name);

  let newString = alphabetShift(str);

  if (newString === expected ){
    pass++;
    console.log('  - pass! 🐳\n');
  } else {
    fail++;
    console.log('  - fail! 🐞  got ' + newString + ' expected ' + expected + '\n');
  }

};

// test runner

console.log('\nStarting Test Suite for alphabetShift.js \n');

test('next letter', 'jklm', 'klmn');
test('next letter from caps', 'JKLM', 'KLMN');
test('z and Z to A', 'zZ', 'AA');
test('vowels to caps', 'zdhnt', 'AEIOU');
test('special chars', '/ ! @ # $ % ^ & * ; : " " ( ) [ ] { } /', '/ ! @ # $ % ^ & * ; : " " ( ) [ ] { } /');

console.log('\nResults are in:');
console.log(' ' + pass + ' Wins, ' + fail + ' Loses'); // print tallies