Mini JavaScript loader

script-loader.js

<script id="in-head">
/*jslint
    browser:true
*/

/**
 * Load a script in an asynchronous manner.
 * @param {string} uri
 * @param {callback} loadCallback
 * @param {callback} errorCallback
 */
function _loadScriptAsync(uri, loadCallback, errorCallback) {
    "use strict";
    var element;
    var firstScript;
    var hash;

    hash = uri.split("").reduce(function (a, b) {
        a = ((a<<5) - a) + b.charCodeAt(0);

        return a&a;
    }, 0);
    firstScript = document.getElementsByTagName("script")[0];
    element = document.createElement("script");
    element.src = uri;
    element.async = true;
    element.id = "script_" + String(hash).replace("-", "m");
    if (typeof loadCallback === "function") {
        element.onload = loadCallback;
    }
    if (typeof errorCallback === "function") {
        element.onerror = errorCallback;
    }
    firstScript.parentNode.insertBefore(element, firstScript);

    return element.id
}

function prefixLoadContactForm(forms) {
    _loadScriptAsync("https://code.jquery.com/jquery-3.3.1.slim.min.js", function () {
        // Set up contact forms
        jQuery(forms).each(function (index, form) {
            jQuery(form).css("backgroundColor", "green");
        });
    }, function () {
        // Report error
        // https://docs.bugsnag.com/platforms/javascript/reporting-handled-errors/
        if (typeof bugsnagClient !== "object" ) {
            return;
        }
        bugsnagClient.notify(new Error("Script load error"));
    });
}
</script>

<script id="in-body-footer">
    (function () {
        var contactForms;
        contactForms = document.querySelectorAll('#main');
        if (contactForms.length === 0) {
            return;
        }
        prefixLoadContactForm(contactForms);
    }());
</script>

TODO Separate attrs:

function _loadFile(name, attributes, callback) {
    var element = document.createElement(name);

    for (att in attributes) {
        element[att] = attributes[att];
    }
    if (element.readyState) {
        // IE compatibility
        element.onreadystatechange = function () {
            if (element.readyState === "loaded" || element.readyState === "complete") {
                element.onreadystatechange = null;
                if (typeof callback === "function") {
                    callback();
                }
            }
        }
    } else {
        element.onload = function () {
            if (typeof callback === "function") {
                callback();
            }
        }
    }
    document.getElementsByTagName("head")[0].appendChild(element);
}

_loadFile("script", {
    "type": "text/javascript",
    "src": "theme.js",
    // https://www.growingwiththeweb.com/2014/02/async-vs-defer-attributes.html
    "defer": true
});