Pete Pete - 1 year ago 56
Javascript Question

What is the best JavaScript code to create an img element

I want to create a simple bit of JS code that creates an image element in the background and doesn't display anything. The image element will call a tracking URL (such as Omniture) and needs to be simple and robust and work in IE 6 =< only. Here is the code I have:

var oImg=document.createElement("img");
oImg.setAttribute('src', '');
oImg.setAttribute('alt', 'na');
oImg.setAttribute('height', '1px');
oImg.setAttribute('width', '1px');

Is this the simplest but most robust (error free) way to do it?

I cannot use a framework like jQuery. It needs to be in plain JavaScript.

Answer Source
oImg.setAttribute('width', '1px');

px is for CSS only. Use either:

oImg.width= '1';

to set a width through HTML, or: '1px';

to set it through CSS.

Note that old versions of IE don't create a proper image with document.createElement(), and old versions of KHTML don't create a proper DOM Node with new Image(), so if you want to be fully backwards compatible use something like:

// IEWIN boolean previously sniffed through eg. conditional comments

function img_create(src, alt, title) {
    var img= IEWIN? new Image() : document.createElement('img');
    img.src= src;
    if (alt!=null) img.alt= alt;
    if (title!=null) img.title= title;
    return img;

Also be slightly wary of document.body.appendChild if the script may execute as the page is in the middle of loading. You can end up with the image in an unexpected place, or a weird JavaScript error on IE. If you need to be able to add it at load-time (but after the <body> element has started), you could try inserting it at the start of the body using body.insertBefore(body.firstChild).

To do this invisibly but still have the image actually load in all browsers, you could insert an absolutely-positioned-off-the-page <div> as the body's first child and put any tracking/preload images you don't want to be visible in there.