How to Freeze Object in JavaScript: Syntax and Examples

Use Object.freeze(object) to make an object immutable in JavaScript. This method prevents adding, deleting, or changing properties of the object.

Syntax

The syntax to freeze an object is simple:

• Object.freeze(object): Takes the object you want to freeze as an argument.
• Returns the same object but now immutable.

javascript

const frozenObject = Object.freeze(object);

Example

This example shows how freezing an object stops changes to its properties:

javascript

const user = { name: 'Alice', age: 25 };
Object.freeze(user);

user.age = 30; // This will not change the age
user.city = 'New York'; // This will not add a new property

console.log(user);

Output

{"name":"Alice","age":25}

Common Pitfalls

Freezing only works on the object itself, not on nested objects inside it. Also, Object.freeze() is shallow, so nested objects can still be changed.

Trying to modify a frozen object in strict mode throws an error, but in non-strict mode it fails silently.

javascript

const person = {
  name: 'Bob',
  address: { city: 'Paris' }
};
Object.freeze(person);

person.address.city = 'London'; // This change works because address is not frozen

console.log(person.address.city); // Outputs: London

Output

London

Quick Reference

• Object.freeze(obj): Makes obj immutable (shallow).
• Does not freeze nested objects.
• Returns the frozen object.
• Use Object.isFrozen(obj) to check if frozen.

Key Takeaways

Use Object.freeze() to make an object immutable and prevent changes.

Freezing is shallow; nested objects remain mutable unless frozen separately.

Modifying a frozen object fails silently in non-strict mode and throws in strict mode.

Check if an object is frozen using Object.isFrozen().

Freezing helps protect objects from accidental changes in your code.