How do make a click on a particular element do a certain thing (e.g., open a menu) and once the menu is open, any click anywhere do a different thing (e.g., close the menu)?

to toggle items with onclick events, you would check the items status and then apply the opposite of it eg:

function toggleMenu(menuID) {
e = document.getElementById(menuID)
     if (e.style.display == 'block') {
        e.style.display = 'none';
     } else {
        e.style.display = 'block';
     }
}