Freezing and sealing objects

This chapter follows Classes in JavaScript.

JavaScript objects are mutable by default. Three built-ins shrink the set of allowed changes at the object level (shallowly).

preventExtensions, seal, freeze

API	Add keys	Delete keys	Change values	Change descriptors
`Object.preventExtensions(obj)`	No	Yes	Yes	Yes
`Object.seal(obj)`	No	No	Yes	No (except value write on data props)
`Object.freeze(obj)`	No	No	No	No

1
const config = { host: 'localhost', port: 3000 };
2
Object.freeze(config);
3
config.port = 8080; // silent fail in sloppy mode; TypeError in strict mode

Nested objects are not frozen unless you recurse. For deep trees, walk and freeze, or treat data as immutable with new copies (spread, structuredClone, Immer, etc.).

Checking state

1
Object.isExtensible(obj);
2
Object.isSealed(obj);
3
Object.isFrozen(obj);

Use freeze for constants exported from a module; use seal when the key set is fixed but values may still update.

Next: Proxy and Reflect.