Search/ filter function javascript does not work

Question

I would like to modify the code I saw on the W3school and the code is to generate a filter/ search function. the original code is

<script>
function myFunction() {
  // Declare variables
  var input, filter, ul, li, a, i, txtValue;
  input = document.getElementById('myInput');
  filter = input.value.toUpperCase();
  ul = document.getElementById("myUL");
  li = ul.getElementsByTagName('li');

  // Loop through all list items, and hide those who don't match the search query
  for (i = 0; i < li.length; i++) {
    a = li[i].getElementsByTagName("a")[0];
    txtValue = a.textContent || a.innerText;
    if (txtValue.toUpperCase().indexOf(filter) > -1) {
      li[i].style.display = "";
    } else {
      li[i].style.display = "none";
    }
  }
}
</script>

The code below is meant to compare the input with the data-caption attribute of the anchor element but it does not work. Can anyone kindly point out what is wrong? Thanks so much!

function search() {   
  var input, filter, ul, li, a, i, txtValue;
  input = document.getElementById('search-input');
  filter = input.value.toUpperCase();
  ul = document.getElementById('search-opt');
  li = ul.getElementsByTagName('li');
  for (i = 0; i < li.length; i++) {
    a = li[i].getElementsByTagName("a")[0].getAttribute('data-caption');
    txtValue = a.textContent || a.innerText;
    if (txtValue.toUpperCase().indexOf(filter) > -1) {
      li[i].style.display = "";
    } else {
      li[i].style.display = "none";
    }
  }
}

Answer 1

Try removing the .getAttribute('data-caption') , maybe it helps because W3Schools doesn't use it in their code.