app.js

(() => {
  // https://en.wikipedia.org/wiki/Caesar_cipher
  class CaesarCipher {
    static RE = /[a-z]/;

    constructor(shift) {
      this.shift = shift;
    }

    encode(plaintext) {
      let ciphertext = "";

      for (let i = 0; i < plaintext.length; i++) {
        if (this.constructor.RE.test(plaintext.charAt(i))) {
          ciphertext += String.fromCharCode(
            ((plaintext.charCodeAt(i) - 97 + this.shift) % 26) + 97,
          );
        } else {
          ciphertext += plaintext.charAt(i);
        }
      }

      return ciphertext;
    }

    decode(ciphertext) {
      let plaintext = "";

      for (let i = 0; i < ciphertext.length; i++) {
        if (this.constructor.RE.test(ciphertext.charAt(i))) {
          plaintext += String.fromCharCode(
            ((ciphertext.charCodeAt(i) - 97 + 26 - this.shift) % 26) + 97,
          );
        } else {
          plaintext += ciphertext.charAt(i);
        }
      }

      return plaintext;
    }
  }

  const encoded = "13x.0ns60@fvzcyrybtva.pbz";
  const cipher = new CaesarCipher(13);
  const decoded = cipher.decode(encoded);

  document.getElementById("email").href = `mailto:${decoded}`;
})();