Basic encrypt and decrypt characters.

caeser_encrypt.js

const encrypt = (text, k = 2) => {
  const alphabet = [
    'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
    'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
  ];
  let newText = '';

  for (let char of text.toUpperCase()) {
    // ASCII => A = 65 (https://www.w3schools.com/charsets/ref_html_ascii.asp);
    const index = char.charCodeAt() - 65;
    const newIndex = index + k;
    const newChar = alphabet[newIndex];

    newText += newChar;
   }

  return newText;
};

const encrypt2 = (text, k = 2) => {
  const alphabet = [
    'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
  ];
  let newText = '';

  for (let char of text.toUpperCase()) {
    let newIndex = k;

    for (let i = 0; i < alphabet.length; i++) {
      if (alphabet[i] === char) {
        newIndex += i;
        break;
      }
    }

    const newChar = alphabet[newIndex];
    newText += newChar;
   }

  return newText;
};

const decrypt = (text, k = 2) => {
  const alphabet = [
    'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
    'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
  ];
  let newText = '';

  for (let char of text.toUpperCase()) {
    const index = char.charCodeAt() - 65;
    const newIndex = index - k;
    const newChar = alphabet[newIndex];

    newText += newChar;
   }

  return newText;
};