charcodeplus3.js

// get string as argument
function switchCharCode(str) {
  // split string
  str = str.split('');
  
  str = str.map(function(char) {
    let char_code;
    if (char !== ' ') {
      // convert each char to Unicode value and add three
      char_code = char.charCodeAt(0);
      char_code = char_code + 3;
      if ((char_code >= 91 && char_code <= 96) || (char_code >= 123 && char_code <= 126)) {
        // wrap around back to start of alphabet if char goes past 'Z' or 'z'
        // Unicode values 91 to 96 / 123 to 126 represent non-alphabetic symbols
        char_code = char_code - 26;
      } else if (char_code >= 48 && char_code <= 64) {
        // undo previous action if char is numeric (0-9) or belonging to a set of ASCII symbols
        char_code = char_code - 3;
      }
    } else if (char == ' ') {
      // char is whitespace, return whitespace Unicode value
      char_code = 32;
    }
    // re-convert back to chars
    char_code = char_code.toString();
    char_code = String.fromCharCode(char_code);
    return char_code;
  })
  
  // return new string
  return str.join('');
}

console.log(switchCharCode('Hello World')); // "Khoor Zruog"