JSFiddle - React, Tailwind, and code Playground

JavaScript

var isPrime = function(n) {
  if (n % 2 === 0 || n % 3 === 0) return false;
  for (var i = 5; i <= Math.sqrt(n); i += 6) {
    if (n % i === 0 || n % (i + 2) === 0) return false;
  }
  return true;
}

var getPrimes = function() {
  var p = [2, 3], i;
  for (i = 5; p.length < 26; i += 6) {
    isPrime(i) && p.push(i);
    isPrime(i+2) && p.push(i+2);
  }
  return p;
};

var indexInAlphabet = function(l) {
  return l.toLowerCase().charCodeAt(0) - 97;  
};

var your_x = 'z';
var your_prime = getPrimes()[indexInAlphabet(your_x)];
console.log(your_prime);