// @ts-check /** @typedef {{ x: number, y: number }} Point */ /** @typedef {Point & { z: number }} Point3D */ /** @type {Point3D} */ const point3d = { x: 1, y: 2, z: 3 }; // Example (would error if enabled): missing z property for Point3D // const point2d = { x: 1, y: 2 }; console.log(point3d.z);