Load external JS script

gistfile1.js

/**
 * Import/include an additional script in the document head
 * @param {string} src the source file
 * @param {function} callback the callback function
 */
function importScript(src, callback) {

    var script = document.createElement('script');
    script.setAttribute('type', 'text/javascript');
    script.setAttribute('src', src);
        
    if (typeof callback === 'function') {
        script.onload = callback;
        script.onreadystatechange = function() {
            if (this.readyState == 'complete') {
                callback();
            }
        };
    }
    
    document.head.appendChild(script);
}