参考サイト - GAC なぜなにGAC->JavaScript

JavaScript部分

//ブラウザ判別
var IE = navigator.appName.indexOf("Microsoft Internet Explorer",0) != -1;

flag = false;

function getAnchorPos(elementID){
//ターゲットアンカーのx座標、y座標を求める
var objnew = new Object();
	if(document.getElementById){
		var obj = document.getElementById(elementID);
		objnew.x = obj.offsetLeft;
		objnew.y = obj.offsetTop;
		while((obj = obj.offsetParent) != null){
			objnew.x += obj.offsetLeft;
			objnew.y += obj.offsetTop;
		}
	}
	else if(document.all){
	var obj = document.all(elementID);
	objnew.x = obj.offsetLeft;
	objnew.y = obj.offsetTop;
	while((obj = obj.setParent) != null){
			objnew.x += obj.offsetLeft;
			objnew.y += obj.offsetTop;
		}
	}
	else if(domument.layers){
		objnew = document.anchors[elementID].x;
		objnew = document.anchors[elementID].y;
	}
	else{
		objnew .x = 0;
		objnew.y = 0;
	}
	return objnew;
}

function goToAnchor(elementID){
	if((getAnchorPos(elementID).x >=0 || getAnchorPos(elementID).y >=0)){
		pageScroll(0,getAnchorPos(elementID).y,10);
		}
	else{
		flag = true;
	}
}

function getInnerSize(){
//ウィンドウサイズを取得するスクリプト
var obj = new Object();
	if(document.all || (document.getElementById && IE)){
	obj.width = document.body.clientWidth;
	obj.height = document.body.clientHeight;
	}
	else if(document.layers || document.getElementById){
	obj.width = window.innerWidth;
	obj.height = window.innerHeight;
	}
	return obj;
}

function getScrollLeft(){
//現在の横位置を取得
	if(IE){
		return document.body.scrollLeft;
	}
	else if(window.pageXOffset){
	return window.pageXOffset;
	}
	else{
	return 0;
	}
}

function getScrollTop(){
//現在の縦位置を取得
	if(IE){
		return document.body.scrollTop;
	}
	else if(window.pageYOffset){
	return window.pageYOffset;
	}
	else{
	return 0;
	}
}

var timeID;
function pageScroll(endX,endY,frms,cuX,cuY){
//ページスクロール本体
	if(timeID){
	clearTimeout(timeID);
	}
	if(endX < 0){
		endX = 0;
	}
	if(endY < 0){
	endY = 0;
	}
	if(!cuX){
	cuX = getScrollLeft();
	}
	if(!cuY){
	cuY = getScrollTop();
	}
	if(endY > cuY && endY > (getAnchorPos('end').y) - getInnerSize().height) {
	endY = (getAnchorPos('end').y - getInnerSize().height)+1;
	}
	cuX += (endX - getScrollLeft())/frms;
	if(cuX < 0){cuX = 0;}
	cuY +=(endY - getScrollTop())/frms;
	if(cuY < 0){cuY = 0;}
	var posiX = Math.floor(cuX);
	var posiY = Math.floor(cuY);
	window.scrollTo(posiX,posiY);
	if(posiX != endX || posiY != endY){
	timeID = setTimeout("pageScroll("+endX+","+endY+","+frms+","+cuX+","+cuY+")",16);
	}
}

body部分

<div><a name="#top" id="top">ここがトップ</a></div>
~中略～
<a href="#top" onClick="javascript:goToAnchor('top');return flag;" id="end">ここをクリック</a>