scroll smooth css

                                    html {
  scroll-behavior: smooth;
}

                                    // Scroll to specific values
// scrollTo is the same
window.scroll({
  top: 2500, 
  left: 0, 
  behavior: 'smooth'
});

// Scroll certain amounts from current position 
window.scrollBy({ 
  top: 100, // could be negative value
  left: 0, 
  behavior: 'smooth' 
});

// Scroll to a certain element
document.querySelector('.hello').scrollIntoView({ 
  behavior: 'smooth' 
});

                                    // handle links with @href started with '#' only
$(document).on('click', 'a[href^="#"]', function(e) {
    // target element id
    var id = $(this).attr('href');

    // target element
    var $id = $(id);
    if ($id.length === 0) {
        return;
    }

    // prevent standard hash navigation (avoid blinking in IE)
    e.preventDefault();

    // top position relative to the document
    var pos = $id.offset().top;

    // animated top scrolling
    $('body, html').animate({scrollTop: pos});
});

                                    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
  </script>
<script>
$(document).ready(function(){
  // Add smooth scrolling to all links
  $("a").on('click', function(event) {

    // Make sure this.hash has a value before overriding default behavior
    if (this.hash !== "") {
      // Prevent default anchor click behavior
      event.preventDefault();

      // Store hash
      var hash = this.hash;

      // Using jQuery's animate() method to add smooth page scroll
      // The optional number (800) specifies the number of milliseconds it takes to scroll to the specified area
      $('html, body').animate({
        scrollTop: $(hash).offset().top
      }, 800, function(){

        // Add hash (#) to URL when done scrolling (default click behavior)
        window.location.hash = hash;
      });
    } // End if
  });
});
</script>

                                    /* ==========================================================================
   Smooth scroll
   ========================================================================== */
$('a[href*=#]:not([href=#])').on('click', function() {
    if (location.pathname.replace(/^\//, '') === this.pathname.replace(/^\//, '') && location.hostname === this.hostname) {
        var target = $(this.hash);
        target = target.length ? target : $('[name=' + this.hash.slice(1) + ']');
        if (target.length) {
            $('html,body').animate({

                scrollTop: (target.offset().top - 40)
            }, 1000);
            return false;
        }
    }
});