How to include jquery.js in another js file?

Appending a script tag inside the document head programmatically does not necessarily mean that the script will be available immediately. You should wait for the browser to download that file, parse and execute it. Some browsers fire an onload event for scripts in which you can hookup your logic. But this is not a cross-browser solution. I would rather “poll” for a specific symbol to become available, like this:

var theNewScript = document.createElement("script");
theNewScript.type = "text/javascript";
theNewScript.src = "http://example.com/jquery.js";
document.getElementsByTagName("head")[0].appendChild(theNewScript);
// jQuery MAY OR MAY NOT be loaded at this stage
var waitForLoad = function () {
    if (typeof jQuery != "undefined") {
        $.get("myfile.php");
    } else {
        window.setTimeout(waitForLoad, 1000);
    }
};
window.setTimeout(waitForLoad, 1000);