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Using PrototypeJS with Greasemonkey

For those of you not already hooked on it, Greasemonkey is an amazing add-on for Firefox that basically lets you inject your own Javascript into a page on the fly.

Don't like the way a page looks? Want to change things around? Is it a page that doesn't belong to you? Well, thanks to Greasemonkey, you can make that page your bitch. The problem is that writing Javascript without using PrototypeJS^*^ sucks. It's tedious and you end up having to write way to much code to do the simplest things.

*OK, jQuery is fine too.

There are two obstacles we have to overcome to use PrototypeJS in a Greasemonkey script

  1. Prototype has to be included on the page the script is running on, not within the Greasemonkey script. Otherwise Prototype won't actually be able to access any of the page elements
  2. You have to handle the case where a page is already including Prototype otherwise you end up including it twice and the whole thing dies.
  3. That's not quite it either.

The Greasemonkey Javascript we're going to write is going to do the following things

  1. Check the page to see if it's already including Prototype (we'll also check for Scriptaculous while we're at it).
  2. If it's not included, add a new <script> element to the <head> of the page and set the src to Prototype
  3. Set up a handler for the window load event
  4. When the window loads, get handles to the Prototype functions we're interested in

Here's a complete script using the above strategy. This particular example only works for It does an Effect.Shake and puts a red border on each <img> tag on the page.

// ==UserScript==// @name           Prototype Test// @namespace @description    This is a test script// @include*// ==/UserScript==var scriptTags = document.getElementsByTagName('script');var addPrototype = true;var addScriptaculous = true;//Loop over all script tags in the page header//and check to see if Prototype or Scriptaculous are already being includedfor(i in scriptTags) {    if( scriptTags[i].src.match(/prototype.*?\.js/) ) {        addPrototype = false;    }    if( scriptTags[i].src.match(/scriptaculous.*?\.js/) ) {        addScriptaculous = false;    }}var scripts = [];var idx = 0;if(addPrototype) {    scripts[idx] = '';    idx++}if(addScriptaculous) {    scripts[idx] = '';    idx++}//Add any missing script tags to the page headerfor (i in scripts) {    var script = document.createElement('script');    script.src = scripts[i];    document.getElementsByTagName('head')[0].appendChild(script);}//Handler for the window load eventwindow.addEventListener('load', function(event) {    //Get handles to the Prototype and Scriptaculous functions we're going to use    $ = unsafeWindow['window'].$;    $$ = unsafeWindow['window'].$$;    Effect = unsafeWindow['window'].Effect;        $$('img').each( function(elem) {        Effect.Shake(elem);        elem.setStyle( {border:  '3px dotted red'});    });}, 'false');

The important part of the above is this

$ = unsafeWindow['window'].$;

The problem is that Greasemonkey doesn't know about the PrototypeJS functions because they only exist inside the actual page.

The unsafeWindow lets us get a handle to the page copy of the the various PrototypeJS functions.