Hello I made 2 scripts for scrolling, one make navbar smaller second one show the button for going on start of page. When I added second one (for button) my first one for navbar didn't work. I deleted second one and it worked. What I need to change?

For navbar

        window.onscroll = function() {
  var el = document.getElementsByClassName('header')[0];
  var className = 'small';
  if (el.classList) {
    if (window.scrollY > 10)
      el.classList.add(className);
    else
      el.classList.remove(className);
  }
};

        window.smoothScroll = function(target) {
    var scrollContainer = target;
    do { //find scroll container
        scrollContainer = scrollContainer.parentNode;
        if (!scrollContainer) return;
        scrollContainer.scrollTop += 1;
    } while (scrollContainer.scrollTop == 0);

    var targetY = 0;
    do { //find the top of target relatively to the container
        if (target == scrollContainer) break;
        targetY += target.offsetTop;
    } while (target = target.offsetParent);

    scroll = function(c, a, b, i) {
        i++; if (i > 30) return;
        c.scrollTop = a + (b - a) / 30 * i;
        setTimeout(function(){ scroll(c, a, b, i); }, 20);
    }
    // start scrolling
    scroll(scrollContainer, scrollContainer.scrollTop, targetY, 0);
}

For button

//Get the button
var mybutton = document.getElementById("myBtn");

// When the user scrolls down 20px from the top of the document, show the button
window.onscroll1 = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}

HTML

     <button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>  
        
  <div class="header">
    <div class="container">
      <ul class="nav">
        <li><a href="#">Home</a></li>
      </ul>
    </div>
  </div>