constructor( handModel, controller, path, handedness, loader = null, onLoad = null ) {

		/**
		 * The WebXR controller.
		 *
		 * @type {Group}
		 */
		this.controller = controller;

		/**
		 * The hand model.
		 *
		 * @type {XRHandModel}
		 */
		this.handModel = handModel;

		/**
		 * An array of bones representing the bones
		 * of the hand skeleton.
		 *
		 * @type {Array<Bone>}
		 */
		this.bones = [];

		if ( loader === null ) {

			loader = new GLTFLoader();
			loader.setPath( path || DEFAULT_HAND_PROFILE_PATH );

		}

		loader.load( `${handedness}.glb`, gltf => {

			const object = gltf.scene.children[ 0 ];
			this.handModel.add( object );

			const mesh = object.getObjectByProperty( 'type', 'SkinnedMesh' );
			mesh.frustumCulled = false;
			mesh.castShadow = true;
			mesh.receiveShadow = true;

			const joints = [
				'wrist',
				'thumb-metacarpal',
				'thumb-phalanx-proximal',
				'thumb-phalanx-distal',
				'thumb-tip',
				'index-finger-metacarpal',
				'index-finger-phalanx-proximal',
				'index-finger-phalanx-intermediate',
				'index-finger-phalanx-distal',
				'index-finger-tip',
				'middle-finger-metacarpal',
				'middle-finger-phalanx-proximal',
				'middle-finger-phalanx-intermediate',
				'middle-finger-phalanx-distal',
				'middle-finger-tip',
				'ring-finger-metacarpal',
				'ring-finger-phalanx-proximal',
				'ring-finger-phalanx-intermediate',
				'ring-finger-phalanx-distal',
				'ring-finger-tip',
				'pinky-finger-metacarpal',
				'pinky-finger-phalanx-proximal',
				'pinky-finger-phalanx-intermediate',
				'pinky-finger-phalanx-distal',
				'pinky-finger-tip',
			];

			joints.forEach( jointName => {

				const bone = object.getObjectByName( jointName );

				if ( bone !== undefined ) {

					bone.jointName = jointName;

				} else {

					console.warn( `Couldn't find ${jointName} in ${handedness} hand mesh` );

				}

				this.bones.push( bone );

			} );

			if ( onLoad ) onLoad( object );

		} );

	}
Constructs a new XR hand mesh model.

@param {XRHandModel} handModel - The hand model.
@param {Group} controller - The WebXR controller.
@param {?string} path - The model path.
@param {XRHandedness} handedness - The handedness of the XR input source.
@param {?Loader} [loader=null] - The loader. If not provided, an instance of `GLTFLoader` will be used to load models.
@param {?Function} [onLoad=null] - A callback that is executed when a controller model has been loaded.