How can I use jQuery in Greasemonkey scripts in Google Chrome?

From "User Script Tip: Using jQuery - Erik Vold's Blog"

// ==UserScript==
// @name         jQuery For Chrome (A Cross Browser Example)
// @namespace    jQueryForChromeExample
// @include      *
// @author       Erik Vergobbi Vold & Tyler G. Hicks-Wright
// @description  This userscript is meant to be an example on how to use jQuery in a userscript on Google Chrome.
// ==/UserScript==

// a function that loads jQuery and calls a callback function when jQuery has finished loading
function addJQuery(callback) {
  var script = document.createElement("script");
  script.setAttribute("src", "//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js");
  script.addEventListener('load', function() {
    var script = document.createElement("script");
    script.textContent = "window.jQ=jQuery.noConflict(true);(" + callback.toString() + ")();";
    document.body.appendChild(script);
  }, false);
  document.body.appendChild(script);
}

// the guts of this userscript
function main() {
  // Note, jQ replaces $ to avoid conflicts.
  alert("There are " + jQ('a').length + " links on this page.");
}

// load jQuery and execute the main function
addJQuery(main);

I have written a few functions based on the Erik Vold's script to help run me run functions, code and other scripts in a document. You can use them to load jQuery into the page and then run code under the global window scope.

Example Usage

// ==UserScript==
// @name           Example from http://stackoverflow.com/q/6834930
// @version        1.3
// @namespace      http://stackoverflow.com/q/6834930
// @description    An example, adding a border to a post on Stack Overflow.
// @include        http://stackoverflow.com/questions/2246901/*
// ==/UserScript==

var load,execute,loadAndExecute;load=function(a,b,c){var d;d=document.createElement("script"),d.setAttribute("src",a),b!=null&&d.addEventListener("load",b),c!=null&&d.addEventListener("error",c),document.body.appendChild(d);return d},execute=function(a){var b,c;typeof a=="function"?b="("+a+")();":b=a,c=document.createElement("script"),c.textContent=b,document.body.appendChild(c);return c},loadAndExecute=function(a,b){return load(a,function(){return execute(b)})};

loadAndExecute("//ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js", function() {
    $("#answer-6834930").css("border", ".5em solid black");
});

You can click here to install it, if you trust that I'm not trying to trick you into installing something malicious and that nobody has edited my post to point to something else. Reload the page and you should see a border around my post.

Functions

load(url, onLoad, onError)

Loads the script at url into the document. Optionally, callbacks may be provided for onLoad and onError.

execute(functionOrCode)

Inserts a function or string of code into the document and executes it. The functions are converted to source code before being inserted, so they lose their current scope/closures and are run underneath the global window scope.

loadAndExecute(url, functionOrCode)

A shortcut; this loads a script from url, then inserts and executes functionOrCode if successful.

Code

function load(url, onLoad, onError) {
    e = document.createElement("script");
    e.setAttribute("src", url);

    if (onLoad != null) { e.addEventListener("load", onLoad); }
    if (onError != null) { e.addEventListener("error", onError); }

    document.body.appendChild(e);

    return e;
}

function execute(functionOrCode) {
    if (typeof functionOrCode === "function") {
        code = "(" + functionOrCode + ")();";
    } else {
        code = functionOrCode;
    }

    e = document.createElement("script");
    e.textContent = code;

    document.body.appendChild(e);

    return e;
}

function loadAndExecute(url, functionOrCode) {
    load(url, function() { execute(functionOrCode); });
}

Use jQuery without fear of conflicts, by calling jQuery.noConflict(true). Like so:

function GM_main ($) {
    alert ('jQuery is installed with no conflicts! The version is: ' + $.fn.jquery);
}

add_jQuery (GM_main, "1.7.2");

function add_jQuery (callbackFn, jqVersion) {
    jqVersion       = jqVersion || "1.7.2";
    var D           = document;
    var targ        = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;
    var scriptNode  = D.createElement ('script');
    scriptNode.src  = 'http://ajax.googleapis.com/ajax/libs/jquery/'
                    + jqVersion
                    + '/jquery.min.js'
                    ;
    scriptNode.addEventListener ("load", function () {
        var scriptNode          = D.createElement ("script");
        scriptNode.textContent  =
            'var gm_jQuery  = jQuery.noConflict (true);\n'
            + '(' + callbackFn.toString () + ')(gm_jQuery);'
        ;
        targ.appendChild (scriptNode);
    }, false);
    targ.appendChild (scriptNode);
}

But, For cross-browser scripts, why not take advantage of a nice, fast, local copy of jQuery, when you can?

The following works as a Chrome userscript and a Greasemonkey script, and it uses the nice local @require copy of jQuery, if the platform supports it.

// ==UserScript==
// @name     _Smart, cross-browser jquery-using script
// @include  http://YOUR_SERVER.COM/YOUR_PATH/*
// @require  http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js
// @grant    GM_info
// ==/UserScript==

function GM_main ($) {
    alert ('jQuery is installed with no conflicts! The version is: ' + $.fn.jquery);
}

if (typeof jQuery === "function") {
    console.log ("Running with local copy of jQuery!");
    GM_main (jQuery);
}
else {
    console.log ("fetching jQuery from some 3rd-party server.");
    add_jQuery (GM_main, "1.7.2");
}

function add_jQuery (callbackFn, jqVersion) {
    var jqVersion   = jqVersion || "1.7.2";
    var D           = document;
    var targ        = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;
    var scriptNode  = D.createElement ('script');
    scriptNode.src  = 'http://ajax.googleapis.com/ajax/libs/jquery/'
                    + jqVersion
                    + '/jquery.min.js'
                    ;
    scriptNode.addEventListener ("load", function () {
        var scriptNode          = D.createElement ("script");
        scriptNode.textContent  =
            'var gm_jQuery  = jQuery.noConflict (true);\n'
            + '(' + callbackFn.toString () + ')(gm_jQuery);'
        ;
        targ.appendChild (scriptNode);
    }, false);
    targ.appendChild (scriptNode);
}

If the page already has jQuery, then just follow this template:

// ==UserScript==
// @name          My Script
// @namespace     my-script
// @description   Blah
// @version       1.0
// @include       http://site.com/*
// @author        Me
// ==/UserScript==

var main = function () {

    // use $ or jQuery here, however the page is using it

};

// Inject our main script
var script = document.createElement('script');
script.type = "text/javascript";
script.textContent = '(' + main.toString() + ')();';
document.body.appendChild(script);

The simple way is using required keyword:

// @require     http://code.jquery.com/jquery-latest.js