check.js

var paragraph = "the quick brown fox jumps over the lazy dog";

check_excluded_letters(paragraph);

function check_excluded_letters(paragraph)
{
    var alphabet = 'abcdefghijklmnopqrstuvwxyz';
    var alphabet_dictionary = alphabet.split("");    
    var check = paragraph.replace(/\s/g, '').split("");
    var result=[];
    alphabet_dictionary.forEach( element => {      
      if(!check.includes(element)) {
        result.push(element);
      }
    });
    return result;
}