Random quote, how to replace AJAX with JS only? [on hold]

I am learning JS and found nice projekt, simple random quote by button press. Unfortunately some AJAX was used. I tried to rebuild it in JS only and failed.

Are there some useful websites which help in "translating" languages (AJAX->JS)? Could you look over my code and give me some feedback? Original:
https://codepen.io/miomate/pen/wRzPaQ
My changes:

function randomize(){
      var range = data.notes.length;
      var random_index = Math.floor(Math.random() * range);
      var item = data.notes[random_index];
      document.getElementByClassName("quote-content").innerHTML = item.quote;
      document.getElementByClassName("quote-book").innerHTML = item.title;
      document.getElementByClassName("quote-author").innerHTML = item.author;
    }

    document.getElementsByClassName("next").onlick = getQuote();
    function getQuote(){
      randomize();
    };

    document.addEventListener("DOMContentLoaded", function(event) { 
      randomize();
      getQuote();
   });

Thank you very much.