Setup
There are 2 ways to initialise the component.
Automatic
Add data-ecl-auto-init attribute to component's markup
<nav class="ecl-mega-menu" data-ecl-menu data-ecl-auto-init="MegaMenu">...</nav>
Use the ECL library's autoInit() (ECL.autoInit()) when your page is ready or other custom event you want to hook onto.
Manual
Get target element, create an instance and invoke init().
Given you have 1 element with an attribute data-ecl-mega-menu on the page:
var elt = document.querySelector('[data-ecl-mega-menu]');
var menu = new ECL.megaMenu(elt);
menu.init();