Hidden features of Greasemonkey

Question

What are some of the lesser-known but useful features and techniques that people are using in their Greasemonkey scripts?

(Please, just one feature per answer.)

Similar threads:

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Answer 1

Data can be persisted across page loads by storing it as a mozilla preference value via GM_setValue(keyname, value).

Here is a simple example that tallys the number of times your script has been executed - by a given browser:

var od = GM_getValue("odometer", 0);
od++;
GM_setValue("odometer", od);
GM_log("odometer=" + od);

GM values are analogous to cookies in that cookie values can only be accessed by the originated domain, GM values can only be accessed by the script that created them.

Answer 2

Anonymous statistics

Assuming you have a basic hosting service that provides access logging, you can easily track basic usage statistics for your script.

1. Place a gif file (eg, a logo image) on your own website.
2. In your script, attach an img element to the page that references the gif:

var img = document.createElement("img");
img.src = "http://mysite.com/logo.gif";
document.body.appendChild(img);

Now, each time a user executes your script, your hosting service will register a hit on that gif file.

To track more than one script, use a different gif file for each. Or add some kind of differentiating parameter to the URL, (eg: http://mysite.com/logo.gif?zippyver=1.0).

Answer 3

Greasemonkey scripts often need to search for content on a page. Instead of digging through the DOM, try using XPath to locate nodes of interest. The document.evaluate() method lets you provide an XPath expression and will return a collection of matching nodes. Here's a nice tutorial to get you started. As an example, here's a script I wrote that causes links in phpBB3 posts to open in a new tab (in the default skin):

// ==UserScript==
// @name           New Tab in phpBB3
// @namespace      http://robert.walkertribe.com/
// @description    Makes links in posts in phpBB3 boards open new tabs.
// ==/UserScript==

var newWin = function(ev) {
    var win = window.open(ev.target.href);
    if (win) ev.preventDefault();
};

var links = document.evaluate(
        "//div[@class='content']//a[not(@onclick) and not(@href='#')]",
        document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);

for (var i = 0; i < links.snapshotLength; i++) {
    var link = links.snapshotItem(i);
    link.addEventListener("click", newWin, true);
}

The XPath expression used in the code identifies all a elements that 1) do not have an onclick attribute, 2) whose href attribute is not set to "#", and 3) are found inside divs whose class attribute is set to "content".

Answer 4

Script header values, (@name, @description, @version, etc), can be made retrievable. This is preferable to maintaining the same constant values in multiple places in your script.

See Accessing Greasemonkey metadata from within your script?

Answer 5

Your script can add graphics into a page, even if you don't have any place to host files, via data URIs.

For example, here is a little button graphic:

var button = document.createElement("img");
button.src = "data:image/gif;base64,"
    + "R0lGODlhEAAQAKEDAAAA/wAAAMzMzP///yH5BAEAAAMALAAAAAAQABAAAAIhnI+pywOtwINHTmpvy3rx"
    + "nnABlAUCKZkYoGItJZzUTCMFACH+H09wdGltaXplZCBieSBVbGVhZCBTbWFydFNhdmVyIQAAOw=="
somenode.appendChild(button);

Here is an online image encoder.

And a wikipedia article about the Data URI standard.

Answer 6

A useful XPath technique is to specify your match relative to a node that you have already found. As a contrived example for stackoverflow:

// first we got the username link at the top of the page
var hdrdiv = document.evaluate(
    "//div[@id='headerlinks']/a[1]", document, null,
    XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;

// now we can retrieve text that follows it, (user's reputation score)
// (note that hdrdiv is now the contextNode argument, rather than document)
var reptext = document.evaluate(
    "following-sibling::span", hdrdiv, null,
    XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;

alert("Reputation Score: " + reptext.textContent);

You can match in any direction relative to the contextNode, ancestors, descendants, previous, following. Here is a helpful XPath reference.

Answer 7

GreaseMonkey scripts run when the DOM is ready, so you don't need to add onload events, you just start manipulating the DOM straight away in your GreaseMonkey script.

Answer 8

==UserScript==
...
@require http://ajax.googleapis.com/ajax/framework-of-your/choice.js
==/UserScript==

Answer 9

GM_setValue normally only stores 32-bit integers, strings, and booleans, but you can take advantage of the uneval() method (and a later eval() on retrieval) to store any object.

var foo={people:['Bob','George','Smith','Grognak the Destroyer'],pie:true};
GM_setValue('myVeryOwnFoo',uneval(foo));
var fooReborn=eval(GM_getValue('myVeryOwnFoo','new Object()'));
GM_log('People: '+fooReborn.people+'   Pie:'+fooReborn.pie);

I tend to use "new Object()" as my default in this case, but you could also use "({})". Just remember that "{}" evaluates as a string, not an object. As usual, eval() with care.