Its a Custom Web Component. Do not use the constructor directly with the new keyword. Instead, use one of the 3 following methods:

Create the Web Component and call its init method:

const myMenu = document.createElement("a-menu").init(config)

Or use the shorthand for it:

const myMenu = createMenu({
 items: [
     "This is a title", // Simple text is considered a title
     {
         icon: "fas fa-check",
         text: "Do this",
         action: () => {...}
     },
     "-", // Menu separator
     {
         hidden: !canSeeThisEntry, // It's possible to hide a menu entry using the hidden property
         icon: "fas fa-cube",
         text: "Do that",
         action: () => {...}
     },
     "Parameters:", // Text entries are processed as section titles inside the menu
     {
         icon: "fas fa-circle",
         iconSize: "32px", // It's possible to alter the default icon size
         iconColor: "#00aaee", // It's possible to alter the default icon color
         text: "Do that",
         action: () => {...}
     }
 ]
})

myMenu.render().showAt(100, 100) // Display the menu at position 100, 100