Hint
Symbol keys are non-enumerable by default — hidden from Object.keys, for...in, and JSON.stringify.
const id = Symbol('id');
const user = { name: 'Alice', [id]: 42 };
console.log(user.name);
console.log(user[id]);
console.log(Object.keys(user).length);Alice 42 1
Explanation: Symbol keys must be accessed directly with [id]. Object.keys only returns string keys — length is 1.
Key Insight: Symbol keys are non-enumerable by default — hidden from Object.keys, for...in, and JSON.stringify.