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Make Internal Links Scroll Smoothly with JavaScript
Pages: 1, 2, 3, 4, 5, 6

The complete code looks like this:


/* Smooth scrolling
   Changes links that link to other parts of this page to scroll
   smoothly to those links rather than jump to them directly, which
   can be a little disorienting.
   
   sil, http://www.kryogenix.org/
   
   v1.0 2003-11-11
   v1.1 2005-06-16 wrap it up in an object
*/

var ss = {
  fixAllLinks: function() {
    // Get a list of all links in the page
    var allLinks = document.getElementsByTagName('a');
    // Walk through the list
    for (var i=0;i<allLinks.length;i++) {
      var lnk = allLinks[i];
      if ((lnk.href && lnk.href.indexOf('#') != -1) && 
          ( (lnk.pathname == location.pathname) ||
            ('/'+lnk.pathname == location.pathname) ) && 
          (lnk.search == location.search)) {
        // If the link is internal to the page (begins in #)
        // then attach the smoothScroll function as an onclick
        // event handler
        ss.addEvent(lnk,'click',ss.smoothScroll);
      }
    }
  },

  smoothScroll: function(e) {
    // This is an event handler; get the clicked on element,
    // in a cross-browser fashion
    if (window.event) {
      target = window.event.srcElement;
    } else if (e) {
      target = e.target;
    } else return;
  
    // Make sure that the target is an element, not a text node
    // within an element
    if (target.nodeType == 3) {
      target = target.parentNode;
    }
  
    // Paranoia; check this is an A tag
    if (target.nodeName.toLowerCase() != 'a') return;
  
    // Find the <a name> tag corresponding to this href
    // First strip off the hash (first character)
    anchor = target.hash.substr(1);
    // Now loop all A tags until we find one with that name
    var allLinks = document.getElementsByTagName('a');
    var destinationLink = null;
    for (var i=0;i<allLinks.length;i++) {
      var lnk = allLinks[i];
      if (lnk.name && (lnk.name == anchor)) {
        destinationLink = lnk;
        break;
      }
    }
  
    // If we didn't find a destination, give up and let the browser do
    // its thing
    if (!destinationLink) return true;
  
    // Find the destination's position
    var destx = destinationLink.offsetLeft; 
    var desty = destinationLink.offsetTop;
    var thisNode = destinationLink;
    while (thisNode.offsetParent && 
          (thisNode.offsetParent != document.body)) {
      thisNode = thisNode.offsetParent;
      destx += thisNode.offsetLeft;
      desty += thisNode.offsetTop;
    }
  
    // Stop any current scrolling
    clearInterval(ss.INTERVAL);
  
    cypos = ss.getCurrentYPos();
  
    ss_stepsize = parseInt((desty-cypos)/ss.STEPS);
    ss.INTERVAL =
setInterval('ss.scrollWindow('+ss_stepsize+','+desty+',"'+anchor+'")',10);
  
    // And stop the actual click happening
    if (window.event) {
      window.event.cancelBubble = true;
      window.event.returnValue = false;
    }
    if (e && e.preventDefault && e.stopPropagation) {
      e.preventDefault();
      e.stopPropagation();
    }
  },

  scrollWindow: function(scramount,dest,anchor) {
    wascypos = ss.getCurrentYPos();
    isAbove = (wascypos < dest);
    window.scrollTo(0,wascypos + scramount);
    iscypos = ss.getCurrentYPos();
    isAboveNow = (iscypos < dest);
    if ((isAbove != isAboveNow) || (wascypos == iscypos)) {
      // if we've just scrolled past the destination, or
      // we haven't moved from the last scroll (i.e., we're at the
      // bottom of the page) then scroll exactly to the link
      window.scrollTo(0,dest);
      // cancel the repeating timer
      clearInterval(ss.INTERVAL);
      // and jump to the link directly so the URL's right
      location.hash = anchor;
    }
  },

  getCurrentYPos: function() {
    if (document.body && document.body.scrollTop)
      return document.body.scrollTop;
    if (document.documentElement && document.documentElement.scrollTop)
      return document.documentElement.scrollTop;
    if (window.pageYOffset)
      return window.pageYOffset;
    return 0;
  },

  addEvent: function(elm, evType, fn, useCapture) {
    // addEvent and removeEvent
    // cross-browser event handling for IE5+,  NS6 and Mozilla
    // By Scott Andrew
    if (elm.addEventListener){
      elm.addEventListener(evType, fn, useCapture);
      return true;
    } else if (elm.attachEvent){
      var r = elm.attachEvent("on"+evType, fn);
      return r;
    } else {
      alert("Handler could not be removed");
    }
  } 
}

ss.STEPS = 25;

ss.addEvent(window,"load",ss.fixAllLinks);

Wrapping Up

Your document's internal links will scroll to their destination, allowing your users to retain an awareness of the browser's location within the document, and how far they are from their starting point.

Stuart Langridge is an information architect for a law firm in the UK. He writes about JavaScript and DHTML, as well as pretty much anything else that catches his attention, at www.kryogenix.org.

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