Hidden properties with Symbol

Learn that Symbol properties are hidden and do not appear in for...in.

Symbol keys

A symbol can be used as a property key of an object. Unlike string keys, symbol keys are hidden from for...in and Object.keys(); they also disappear during serialization.

This makes them ideal for storing internal metadata that should not appear in normal iteration. To access symbol keys, use Object.getOwnPropertySymbols(), because Object.keys returns only string keys.

Object.getOwnPropertySymbols

To reach symbol-keyed properties, use Object.getOwnPropertySymbols(obj). This method returns an array of all symbol keys on the object.

String and symbol keys live in separate slots: Object.keys() returns strings only, and Object.getOwnPropertySymbols() returns symbols only. Together they give the full picture of an object.

Exercise Hide and reveal a symbol property

Create a symbol named id. Add a string key name: 'Alex' and a symbol key [id]: 101 to the user object. Use for...in to walk user and log the keys (only name should appear). Then log the symbol keys using Object.getOwnPropertySymbols(user).

Show example solution

const id = Symbol('id');
const user = {
  name: 'Alex',
  [id]: 101
};
for (let key in user) {
  console.log(key);
}
console.log(Object.getOwnPropertySymbols(user));

Test yourself

If an object has a symbol-keyed property, what does a for...in loop do?
- It iterates over the symbol keys too
- It skips the symbol keys
- It throws an error
- It iterates over only the symbol keys
Show answer

It skips the symbol keys — Correct. for...in only iterates over string and number keys; symbol keys are skipped.

Use a symbol as a key

const secret = Symbol('id'); const user = { name: 'Alex', [____]: 42 };

Show answer

const secret = Symbol('id'); const user = { name: 'Alex', [secret]: 42 };

Symbol key properties

Symbol key

Hidden from for...in
Object.keys()

String keys only
Object.getOwnPropertySymbols()

Returns symbol keys
Symbol property

Lives in a separate slot

Next lesson: Symbol use cases

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