Scrolling HTML Basics, Part III: Positioning A Scroll Box - www.docjavascript.com

Positioning A Scroll Box

There are many ways to position a scroll box. You can position it in absolute coordinates, but the desired location may change because of an ad put at the top of the page, for example. In this column we use the GIF method. We place a dummy GIF wherever we want in our page:

<IMG SRC="blank.gif"
        NAME="holdspace" ID="holdspace"
        WIDTH="350" HEIGHT="90"
        STYLE="visibility:hidden; position:relative;">

We then compute the coordinates of this GIF in our scrolling box code. We use the IMG's ID to find its object:

var holdingImage = document.images["holdspace"];
var canvasLeft = (NS4) ? holdingImage.x : holdingImage.getRealLeft();
var canvasTop = (NS4) ? holdingImage.y : holdingImage.getRealTop();
var canvasWidth = holdingImage.width;
var canvasHeight = holdingImage.height;

where getRealLeft() and getRealTop() are defined as in Column 13, Scrolling JavaScript Banners:

function getRealLeft() {
	xPos = this.offsetLeft;
	tempEl = this.offsetParent;
  	while (tempEl != null) {
  		xPos += tempEl.offsetLeft;
  		tempEl = tempEl.offsetParent;
  	}
	return xPos;
}
function getRealTop() {
	yPos = this.offsetTop;
	tempEl = this.offsetParent;
	while (tempEl != null) {
  		yPos += tempEl.offsetTop;
  		tempEl = tempEl.offsetParent;
  	}
	return yPos;
}

These functions are needed for Internet Explorer in cases where the IMG tag is positioned inside another container such as a table. See Column 13 for more details on the motivation.

Produced by Yehuda Shiran and Tomer Shiran

Created: December 21, 1998
Revised: December 21, 1998

URL: https://www.webreference.com/js/column32/position.html