_markDefs() {

		const nodeDefs = this.json.nodes || [];
		const skinDefs = this.json.skins || [];
		const meshDefs = this.json.meshes || [];

		// Nothing in the node definition indicates whether it is a Bone or an
		// Object3D. Use the skins' joint references to mark bones.
		for ( let skinIndex = 0, skinLength = skinDefs.length; skinIndex < skinLength; skinIndex ++ ) {

			const joints = skinDefs[ skinIndex ].joints;

			for ( let i = 0, il = joints.length; i < il; i ++ ) {

				nodeDefs[ joints[ i ] ].isBone = true;

			}

		}

		// Iterate over all nodes, marking references to shared resources,
		// as well as skeleton joints.
		for ( let nodeIndex = 0, nodeLength = nodeDefs.length; nodeIndex < nodeLength; nodeIndex ++ ) {

			const nodeDef = nodeDefs[ nodeIndex ];

			if ( nodeDef.mesh !== undefined ) {

				this._addNodeRef( this.meshCache, nodeDef.mesh );

				// Nothing in the mesh definition indicates whether it is
				// a SkinnedMesh or Mesh. Use the node's mesh reference
				// to mark SkinnedMesh if node has skin.
				if ( nodeDef.skin !== undefined ) {

					meshDefs[ nodeDef.mesh ].isSkinnedMesh = true;

				}

			}

			if ( nodeDef.camera !== undefined ) {

				this._addNodeRef( this.cameraCache, nodeDef.camera );

			}

		}

	}
Marks the special nodes/meshes in json for efficient parse.

@private