document.getElementsByClassName

getElementsByClassName.js

// Just to test, Run it on https://www.hackerearth.com/practice/data-structures/hash-tables/basics-of-hash-tables/tutorial/

var root = document.getRootNode();

var count = 0;
var list = [];
function search(head) {
  if (head.classList && head.classList.contains("light")) {
    count++;
    list.push(head);
  }
  if (head.hasChildNodes()) {
    [].slice
      .call(head.childNodes)
      .filter(e => e.nodeType == Node.ELEMENT_NODE)
      .forEach(e => {
        return search(e);
      });
  }

  return;
}

search(root);
console.info(list);
document.getElementsByClassName("light");