Check if Javascript script exists on page

You can check whether your script is loaded like this:

function isMyScriptLoaded(url) {
    if (!url) url = "http://xxx.co.uk/xxx/script.js";
    var scripts = document.getElementsByTagName('script');
    for (var i = scripts.length; i--;) {
        if (scripts[i].src == url) return true;
    }
    return false;
}

Alternatively, you could do something like this:

<a href="javascript:
    if (!jsCode) {
        var jsCode = document.createElement('script');
        jsCode.setAttribute('src', 'http://xxx.co.uk/xxx/script.js');
        document.body.appendChild(jsCode);
    }
 ">Bookmarklet</a>

This "pollutes" the global namespace with the jsCode variable, but that might be a necessary evil. You could rename it to something that is unlikely to appear in the document where the bookmarklet is run.

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Please note that while the javascript URI scheme is okay for bookmarklets as in this case, it's not considered to be a good practice for normal use.

Just check the selector length. Here's an example using jQuery:

if ($('script[src="http://xxx.co.uk/xxx/script.js"]').length > 0) {
    //script exists
}