How do I access pages jQuery from user script?

Question

I want to write a small userscript to the page that already contains jQuery. I can access $ object from chrome developer console, but can not from user script -- it just says that jQuery || $ || window.jQuery is undefined.

PS: user script is installed as an extension.

Answer 1

Userscripts don't run in the same context as the page. In order to access the page's javascript environment, you need to run it like so:

var execute = function (body) {
    if(typeof body === "function") { body = "(" + body + ")();"; }
    var el = document.createElement("script");
    el.textContent = body;
    document.body.appendChild(el);
    return el;
};

execute(function() {
    $.noop();
}

Bonus: you can also load external scripts from a CDN.

var load = function (src, on_load, on_error) {
    var el = document.createElement("script");
    el.setAttribute("src", src);
    if (on_load != null) { el.addEventListener("load", on_load); }
    if (on_error != null) { el.addEventListener("error", on_error); }
    document.body.appendChild(el);
    return el;
};
load("//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js", function () {
    execute(function(){$.noop();});
});