benhowdle89 benhowdle89 - 3 months ago 46
Javascript Question

Check if Javascript script exists on page

I have a bookmarklet that I've made and it loads a script from my server onto the users current page. However I have an if check in my script that if a condition is not met then no action is taken. However if the user then meets that condition then the code is run, but has caused there to be two sets of scripts inserted into their page. Can i prevent this?

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

Answer

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.


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.

Comments