Simple snippet for creating a mobile navigation with pure vanilla JavaScript

navigation-trigger-handler.js

// Replace this with your burger icon element ID
var trigger = document.getElementById('mobile-navigation-trigger');

// Replace this with with your navigation element ID
var navigationList = document.getElementById('navigation-list');

// open or close the mobile flyout
function toggleNavigation() {
    if (navigationList.classList.contains('is-open')) {
        navigationList.classList.remove('is-open');
    } else {
        navigationList.classList.add('is-open');
    }
}

// attach the main event listener to the icon
function initMobileNavigation() {
    trigger.addEventListener('click', toggleNavigation.bind(this));
}

// initialise once the document is loaded
document.addEventListener('DOMContentLoaded', initMobileNavigation.bind(this));