importScript: (function (head) {

    function loadError(error) {
        throw new URIError("The script " +
                            error.target.src + " is not accessible.");}

    return function (url, callback) {
        var existingScript = document.querySelectorAll("script[src='" +
                             url + "']");
        var isNewScript = (existingScript.length == 0);
        var script;
        if (isNewScript) {
            script = document.createElement("script")
            script.type = "text/javascript";
        }
        else {
            script = existingScript[0];
        }
        script.onerror = loadError;
        if (script.readyState) { //IE
            script.onreadystatechange = function () {
                if (script.readyState == "loaded" ||
                    script.readyState == "complete") {
                    script.onreadystatechange = null;
                    if (callback) {
                        callback(); }
                }
            };
        } else { // others than IE
            script.onload = callback; }

        if (isNewScript) {
            script.src = url;
            head.appendChild(script); }
    }
})(document.head || document.getElementsByTagName("head")[0])

importScript("myScript1.js");
importScript("myScript2.js", /* onload function: */
            function () { alert("The script has been OK loaded."); });