Accessing the DOM (Document Object Model) drains processor resources (CPU). Therefore, when you reference the same DOM element several times in JavaScript, store its reference within a variable to avoid re-parsing the DOM for the same element.
Instead of:
document.getElementById('menu').property1 = 'foo';
document.getElementById('menu').property2 = 'bar';
write:
$menu = document.getElementById('menu');
$menu.property1 = 'foo';
$menu.property2 = 'bar';