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 Mesh Compression Extension

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