Toggling menus based on an ID
First time doing a review, so bear with me.

I've been thinking about how to make any changes to your code, but had a difficult time finding things that could be optimized. Overall, I think you did a great job.

Selectors - this may be a style preference, but I tend to lump everything under one var statement (delimited by commas), rather than having to write var each time.

Methods - I chose to pass the elementId into document.getElementById rather than having to write additional if statements to check specifically for either id. You're already checking to see what event.target.id is returning, might as well use it again.

Methods - closePanel, I created a function called checkClassListAndAddClass very long, but at least you know what it's doing. This function takes the selector as an argument, checks to see if the selector contains a class, and then returns the add class. I added a variable to that function to help remove some repetitive code.

Again, this is my first stab at a code review, so I'm happy to hear your feedback as well as any other feedback from the community. 

/**
* Require modules
*/
var dullBackground = require('./dullBackground');

module.exports = function () {

    'use strict';

    /**
    * Selectors
    */
    var menu = document.getElementById('nav-main'),
        overlay = document.getElementById('overlay-panel'),
        close = document.getElementById('overlay-close'),
        productsPanel = document.getElementById('page-products'),
        systemsPanel = document.getElementById('page-systems'),
        activeClass = 'active',
        hideClass = 'hide';

    /**
    * Methods
    */
    function togglePanel(event) {
        var elementId = event.target.id;

        event.preventDefault();

        if (elementId === ('page-products' || elementId === 'page-systems')) {
            dullBackground();
            overlay.classList.add(activeClass);
            document.getElementById(elementId).classList.remove(hideClass);
        }
    }

    function closePanel(event) {
        event.preventDefault();
        overlay.classList.remove(activeClass);

        checkClassListAndAddClass(productsPanel);
        checkClassListAndAddClass(systemsPanel);
    }

    function checkClassListAndAddClass(selector) {
        var selClassList = selector.classList;

        if (!selClassList.contains(hideClass)) {
            return selClassList.add(hideClass);
        }
    }

    /**
    * Events/APIs/init
    */
    menu.addEventListener('click', togglePanel, false);
    close.addEventListener('click', closePanel, false);

};