constructor( json, dracoLoader ) {

			if ( ! dracoLoader ) {

				throw new Error( 'THREE.GLTFLoader: No DRACOLoader instance provided.' );

			}

			this.name = EXTENSIONS.KHR_DRACO_MESH_COMPRESSION;
			this.json = json;
			this.dracoLoader = dracoLoader;
			this.dracoLoader.preload();

		}
DRACO THREE.Mesh Compression Extension

Specification: https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_draco_mesh_compression