/**
 * User type definition
 * @typedef {Object} User
 * @property {string} email
 * @property {string} [nickName]
 */

/** @type {User} */
const user = {email: ""}; // as expected email is required, but nickName is not
const nickName = user.nickName; // expected type string|undefined, but type is string

// expected error TS2532: Object is possibly 'undefined'.
console.log(nickName.length)