Object.freeze vs Object.seal in JavaScript

JavaScript provides built-in methods to control the mutability of objects, with Object.freeze() and Object.seal() being the two primary approaches for locking down object structures. While both methods prevent properties from being added or deleted, the core difference lies in how they handle property values: Object.seal() allows you to modify the values of existing properties, whereas Object.freeze() makes all existing properties completely immutable and read-only.

Understanding Object.seal()

The Object.seal() method takes an object and prevents new properties from being added to it, while also marking all existing properties as non-configurable.

When an object is sealed: * Cannot add properties: New properties cannot be added. * Cannot delete properties: Existing properties cannot be deleted using the delete operator. * Cannot reconfigure properties: Property descriptors cannot be changed (for example, converting a data property into an accessor property). * Can modify values: You can still change the values of existing writable properties.

const user = { name: "Alice", role: "Admin" };
Object.seal(user);

user.role = "User"; // Works: existing values can be updated
user.age = 30;      // Fails: new properties cannot be added
delete user.name;   // Fails: properties cannot be deleted

Understanding Object.freeze()

The Object.freeze() method provides the highest level of integrity for a JavaScript object. It performs everything that Object.seal() does, with the additional step of making all data properties non-writable.

When an object is frozen: * Cannot add properties: New properties cannot be added. * Cannot delete properties: Existing properties cannot be removed. * Cannot reconfigure properties: Descriptors cannot be modified. * Cannot modify values: Existing property values cannot be altered.

const config = { apiEndpoint: "https://api.example.com", timeout: 5000 };
Object.freeze(config);

config.timeout = 10000; // Fails: existing values cannot be changed
config.retries = 3;     // Fails: new properties cannot be added
delete config.timeout;  // Fails: properties cannot be deleted

Key Differences at a Glance

To check the state of an object, JavaScript provides corresponding checking methods: Object.isSealed(obj) and Object.isFrozen(obj).

Shallow Mutation Behavior

Both Object.freeze() and Object.seal() perform shallow operations. If a sealed or frozen object contains nested objects or arrays, those nested structures remain fully mutable unless they are also explicitly sealed or frozen recursively.