main.js

function show_hidden_menu() {
    var ele = document.getElementById('hidden_menu');
    if (ele.className.indexOf('w3-show') == -1) {
        ele.className += ' w3-show w3-top';
    } else {
        ele.className = ele.className.replace(' w3-show w3-top', '');
    }
}

function set_element_display(elem_id, prop) {
    document.getElementById(elem_id).style.display = prop;
}

function includeHTML() {
    var z, i, elmnt, file, xhttp;
    /*loop through a collection of all HTML elements:*/
    z = document.getElementsByTagName("*");
    for (i = 0; i < z.length; i++) {
        elmnt = z[i];
        /*search for elements with a certain atrribute:*/
        file = elmnt.getAttribute("w3-include-html");
        if (file) {
            /*make an HTTP request using the attribute value as the file name:*/
            xhttp = new XMLHttpRequest();
            xhttp.onreadystatechange = function () {
                if (this.readyState == 4) {
                    if (this.status == 200) { elmnt.innerHTML = this.responseText; }
                    if (this.status == 404) { elmnt.innerHTML = "Page not found."; }
                    /*remove the attribute, and call this function once more:*/
                    elmnt.removeAttribute("w3-include-html");
                    includeHTML();
                }
            }
            xhttp.open("GET", file, true);
            xhttp.send();
            /*exit the function:*/
            return;
        }
    }
}