Dangerous JavaScript Code

function kopiraj() {
  var copyText = document.getElementById("toCopy");
  copyText.select();
  document.execCommand("Copy");
  document.getElementById("telefon").innerHTML = 'Copied';
}
<button type="button" onclick="kopiraj()">Copy</button>
<input type="hidden" id="toCopy" value="123456789">
<p id="telefon"></p>