hot to mock script requests

fake-script.js

document.createElement = function (
    createElement, // the native one
    createResponse // the function "in charge"
) {
    return function (nodeName) {
        var result, src;
        // if we are creating a script
        if (/^script$/i.test(nodeName)) {
            // result will be a place holder
            result = createElement.call(
                document,
                "meta"
            );
            // we need to monitor the src property
            Object.defineProperty(result, "src", {
                get: function () {
                    return src;
                },
                // when set ...
                set: function ($src) {
                    // we can check periodically ...
                    function T() {
                        // if the placeholder is in DOM
                        if (result.parentNode) {
                            // in this case we can put a real script
                            result = result.parentNode.insertBefore(
                                createElement.call(
                                    document,
                                    "script"
                                ),
                                result
                            );
                            // and set the encoded src
                            result.src = "data:text/javascript;base64," +
                                btoa(createResponse.call(result, src))
                            ;
                        } else {
                            // no DOM, no loading ... try later
                            setTimeout(T, 100);
                        }
                    }
                    // store the src
                    src = $src;
                    // and start checking 
                    T();
                }
            });
        } else {
            // just return the element
            result = createElement.call(
                document,
                nodeName
            );
        }
        return result;
    };
}(
    document.createElement,

    // must return a string
    function (src) {

        // this points to the current script
        // src is the address
        // if we know the callback ...

        if (/callback=([^&]+)/.test(src)) {
            return RegExp.$1 + '(' +
                JSON.stringify({dummy:"data"})
            + ')';
        }
    }
);


// example
function test(data) {
    alert(data.dummy);
}

document.documentElement.insertBefore(
    document.createElement("script"),
    document.documentElement.lastChild
).src = "page.php?callback=test";