Smooth scrolling without external libraries

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
    <script src="script.js"></script>
   <style>
      .largeBlock{ min-width: 300px; height:300px; border: 1px solid red;}      
    </style>  
  </head>
  <body>
	
    <h1>Smooth Scroll without jQuery!</h1>   
	<div id="anchor-toc">
	<h2>Table of content (toc)</h2>
	<ul>
      <li><a href="#anchor-2" onclick="smoothScroll('anchor-2');">smooth scroll to Anchor 2<a/></li>
	  <li><a href="#chapter-4" onclick="smoothScroll('chapter-4');">smooth scroll to Chapter 4<a/></li>
    </ul>
	</div>
	
    <h2 id="target-1">Chapter 1</h2>
    <div class="largeBlock"> some text</div>
    
    
    <h2 id="anchor-2">Anchor 2</h2>
	<a href="#anchor-toc" onclick="smoothScroll('anchor-toc');">scroll smooth to toc<a/>
    <div class="largeBlock">This is the textblock below anchor 2</div>
    
    
    <h2>Chapter 3</h2>
    <div class="largeBlock"> some content </div>
    
    
    <h2 id="chapter-4">Chapter 4</h2>
	<a href="#anchor-toc" onclick="smoothScroll('anchor-toc');">scroll smooth to toc<a/>
    <div class="largeBlock"> This is the text of chapter 4</div>
    
    <h2>Chapter 5</h2>
    <div class="largeBlock"> some content </div>
	<h2>Chapter 6</h2>
    <div class="largeBlock"> some content </div>
	<a href="#anchor-toc" onclick="smoothScroll('anchor-toc');">scroll smooth to toc<a/>
  </body>

</html>

// Code goes here

// based on
// http://www.itnewb.com/tutorial/Creating-the-Smooth-Scroll-Effect-with-JavaScript
// use href="#anchorID" in your hyperlinks 
// with smoothScroll('destinationAnchorID');return false; as the onclick event.
// <a href="#anchor-1" onclick="smoothScroll('anchor-1-id');">smooth scroll to Anchor 1<a/>


function currentYPosition() {
    // Firefox, Chrome, Opera, Safari
    if (self.pageYOffset) return self.pageYOffset;
    // Internet Explorer 6 - standards mode
    if (document.documentElement && document.documentElement.scrollTop)
        return document.documentElement.scrollTop;
    // Internet Explorer 6, 7 and 8
    if (document.body.scrollTop) return document.body.scrollTop;
    return 0;
}


function elmYPosition(eID) {
    var elm = document.getElementById(eID);
    var y = elm.offsetTop;
    var node = elm;
    while (node.offsetParent && node.offsetParent != document.body) {
        node = node.offsetParent;
        y += node.offsetTop;
    } return y;
}


function smoothScroll(eID) {
    var startY = currentYPosition();
    var stopY = elmYPosition(eID);
    var distance = stopY > startY ? stopY - startY : startY - stopY;
    if (distance < 100) {
        scrollTo(0, stopY); return;
    }
    var speed = Math.round(distance / 100);
    if (speed >= 20) speed = 20;
    var step = Math.round(distance / 25);
    var leapY = stopY > startY ? startY + step : startY - step;
    var timer = 0;
    if (stopY > startY) {
        for ( var i=startY; i<stopY; i+=step ) {
            setTimeout("window.scrollTo(0, "+leapY+")", timer * speed);
            leapY += step; if (leapY > stopY) leapY = stopY; timer++;
        } return;
    }
    for ( var i=startY; i>stopY; i-=step ) {
        setTimeout("window.scrollTo(0, "+leapY+")", timer * speed);
        leapY -= step; if (leapY < stopY) leapY = stopY; timer++;
    }
  return false;
}

/* Styles go here */