How do I toggle an element’s class in pure JavaScript?

2014 answer: classList.toggle() is the standard and supported by most browsers.

Older browsers can use use classlist.js for classList.toggle():

var menu = document.querySelector('.menu') // Using a class instead, see note below.
menu.classList.toggle('hidden-phone');

As an aside, you shouldn’t be using IDs (they leak globals into the JS window object).