What is not supported by Internet Explorer?

Question

JavaScript

There is a script:

const container = document.querySelector('.baloons-app__inner');
  
  function smoothScrollLoop(root, speed = 5) {
      if (!root || !root.firstElementChild) return;

  function elementScrollHeight(el) {
    const style = window.getComputedStyle(el);
    return el.clientHeight + parseFloat(style.marginTop);
  }

  let firstChild = root.getElementsByClassName('baloons-app__item')[0];
  let secondChild = root.getElementsByClassName('baloons-app__item')[1];
  let firstChildHeight = elementScrollHeight(firstChild);
  let secondChildHeight = elementScrollHeight(secondChild);
  let oldTimestamp = performance.now();
  let scrollTop;

  function step(newTimestamp) {
    const timePassed = newTimestamp - oldTimestamp;
    scrollTop = timePassed / 1000 * speed;
    root.style.transform = 'translateY(-' + scrollTop + 'px)';

    if(container.classList.contains('baloons-app__story')) {
      if (scrollTop > 0) {
        firstChild.classList.add('animate');
        if (scrollTop > secondChildHeight || scrollTop > firstChildHeight) {
          secondChild.classList.add('animate');
        }
      }
    }

    if (scrollTop >= firstChildHeight+secondChildHeight) {
      root.style.transform = 'translateY(-' + scrollTop - (firstChildHeight+secondChildHeight) + 'px)';
      firstChild.classList.remove('animate');
      secondChild.classList.remove('animate');
      root.appendChild(firstChild);
      root.appendChild(secondChild);
      firstChild = root.getElementsByClassName('baloons-app__item')[0];
      secondChild = root.getElementsByClassName('baloons-app__item')[1];
      firstChildHeight = elementScrollHeight(firstChild);
      secondChildHeight = elementScrollHeight(secondChild);
      oldTimestamp = newTimestamp;
    }

    window.requestAnimationFrame(step);
  }

  window.requestAnimationFrame(step);
}

smoothScrollLoop(container, 25);

In all browsers it works adequately, except for IE (who would doubt it).
Please tell me what may not work in this script in IE, and how it can be fixed.

Answer 1

Default function parameters are not supported.
Since speed is unlikely to be == 0, you can write instead

function smoothScrollLoop(root, speed) {
speed = speed || 5;