Registering the hit test call back. Stores it in a variable to allow removing the event listener later, and using an arrow function to preserver this.

AnchorPositioning.js

// Listeners and Callbacks
// -------------------------------------------------------------------
// First, create a variable that stores an arrow function to the
// performHitTest method of this class. This method takes care of
// starting the hit test in the native AR system.
// We need this as an arrow function to have access to "this" in
// the class scope. Creating this function in a variable allows
// to remove the event listener once the scene closes.
var performHitTestCallback = (evt) => this.performHitTest(evt);