Every property has hidden attributes — writable, enumerable, configurable — that control whether it can change, show up in loops, or be deleted.

Inspecting descriptors

JavaScript

const obj = { visible: 1 };
console.log(Object.getOwnPropertyDescriptor(obj, "visible"));
Object.defineProperty(obj, "secret", { value: 42 });   // attributes default to false!
console.log(Object.getOwnPropertyDescriptor(obj, "secret"));
console.log(obj, Object.keys(obj), obj.secret);

Output

{ value: 1, writable: true, enumerable: true, configurable: true }
{ value: 42, writable: false, enumerable: false, configurable: false }
{ visible: 1 } [ 'visible' ] 42

Attribute	When `false`
`writable`	The value cannot be reassigned (throws in strict mode)
`enumerable`	Hidden from `for…in`, `Object.keys`, spread and `JSON.stringify`
`configurable`	Cannot be deleted, and its attributes cannot be changed (except writable → false)
`get` / `set`	Accessor property instead of a data property (no `value`/`writable`)

JavaScript

const account = {};
Object.defineProperty(account, "id", { value: "AC-1", enumerable: true });
try {
  account.id = "hacked";
} catch (e) {
  console.log(e.message);
}
try {
  delete account.id;
} catch (e) {
  console.log(e.message);
}
console.log(account.id);

Output

Cannot assign to read only property 'id' of object '#<Object>'
Cannot delete property 'id' of #<Object>
AC-1

preventExtensions, seal, freeze

	Add props	Delete props	Change values	Check
`Object.preventExtensions`	No	Yes	Yes	`Object.isExtensible`
`Object.seal`	No	No	Yes	`Object.isSealed`
`Object.freeze`	No	No	No	`Object.isFrozen`

JavaScript

const sealed = Object.seal({ a: 1 });
sealed.a = 2;
try { sealed.b = 3; } catch (e) { console.log("seal:", e.message); }

const frozen = Object.freeze({ a: 1, nested: { b: 1 } });
try { frozen.a = 2; } catch (e) { console.log("freeze:", e.message); }
frozen.nested.b = 99;                 // freeze is SHALLOW
console.log(sealed, frozen, Object.isFrozen(frozen.nested));

Output

seal: Cannot add property b, object is not extensible
freeze: Cannot assign to read only property 'a' of object '#<Object>'
{ a: 2 } { a: 1, nested: { b: 99 } } false

Deep freeze

JavaScript

function deepFreeze(obj) {
  for (const value of Object.values(obj)) {
    if (value && typeof value === "object") deepFreeze(value);
  }
  return Object.freeze(obj);
}
const config = deepFreeze({ db: { host: "localhost", ports: [5432] } });
try {
  config.db.ports.push(1);
} catch (e) {
  console.log(e.message);
}

Output

Cannot add property 1, object is not extensible

What is the difference between Object.freeze and const?

seal vs freeze?