Freeze & Seal

JavaScript · Reference cheat sheet

📋 Overview

Object.freeze, Object.seal, and Object.preventExtensions progressively lock objects against extension and mutation. They are shallow. For true immutability in apps, combine with conventions, structuredClone, or immutable libraries — and know what each level allows.

🔧 Core concepts

*existing configurable props can still be deleted if not sealed.

• Checks: Object.isFrozen, isSealed, isExtensible.
• Strict mode: failed mutations throw TypeError; sloppy mode fails silently.
• Shallow: nested objects remain mutable unless frozen too.

const cfg = Object.freeze({ theme: "dark", nested: { a: 1 } });
cfg.theme = "light"; // ignored / TypeError
cfg.nested.a = 2; // still works — shallow!

💡 Examples

"use strict";

const sealed = Object.seal({ x: 1 });
sealed.x = 2; // OK
// sealed.y = 3; // TypeError
// delete sealed.x; // TypeError

const frozen = Object.freeze({ x: 1 });
// frozen.x = 2; // TypeError

Object.preventExtensions(arrLike);
// arrLike.newProp = 1; // TypeError

// Deep freeze (simple)
function deepFreeze(obj) {
  Object.freeze(obj);
  for (const value of Object.values(obj)) {
    if (value && typeof value === "object" && !Object.isFrozen(value)) {
      deepFreeze(value);
    }
  }
  return obj;
}

// Constants pattern
const ROUTES = Object.freeze({
  HOME: "/",
  ABOUT: "/about",
});

// Arrays
const list = Object.freeze([1, 2, 3]);
// list.push(4); // TypeError

// Copy then freeze
const next = Object.freeze({ ...state, count: state.count + 1 });

⚠️ Pitfalls

• Shallow freeze is the #1 surprise — nested mutation still works.
• Freezing does not make values deeply immutable or protect against Proxy targets specially.
• const ≠ immutable — only rebinding is blocked; freeze the object too.
• Frozen arrays reject mutator methods; some libraries assume extensible objects.
• Cannot unfreeze — make a new object instead.

🔗 Related

• objects.md — property descriptors
• strict_mode.md — TypeError on mutate
• structured_clone.md — deep copy
• proxy_reflect.md — deeper control
• spread_rest.md — immutable updates