Object.freeze / Object.seal

TL;DR

Explanation

Object.freeze(obj): no new properties, no changes, no deletions. Silently fails in non-strict mode, throws TypeError in strict mode. Object.isFrozen() checks. Object.seal(obj): no new/deleted properties, existing writable properties can change. Readonly check with Object.isSealed(). Both are SHALLOW — nested objects remain mutable. Deep freeze requires recursion. Use cases: constants, config objects, action type maps in Redux, enums before JS had them. For deep immutability: recursively freeze, use Immer, or use TypeScript readonly.

Common Misconception

Why It Matters

Common Mistakes

• Assuming freeze is deep — nested objects still mutable.
• Not knowing freeze silently fails in non-strict mode.
• Using freeze on large objects in hot paths — slight performance cost.

Code Examples

const CONFIG = { db: { host: 'localhost', port: 5432 } };
Object.freeze(CONFIG);
CONFIG.db.port = 9999; // Works! db is not frozen

function deepFreeze(obj) {
    Object.getOwnPropertyNames(obj).forEach(name => {
        const val = obj[name];
        if (val && typeof val === 'object') deepFreeze(val);
    });
    return Object.freeze(obj);
}

const CONFIG = deepFreeze({ db: { host: 'localhost', port: 5432 } });
CONFIG.db.port = 9999; // TypeError in strict mode

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze

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