What does Object.freeze() do?

JavaScriptJunior~1 min read

Object.freeze() is a built-in JavaScript method that "freezes" an object, making it fully immutable: properties cannot be changed, removed, or added.

In short

Syntax:

javascript

Object.freeze(obj)

Argument	Description
`obj`	The object to "freeze"

Returns the same object, but with frozen properties.

Example - freezing an object

javascript

const user = {
  name: 'Bohdan',
  age: 25
};

Object.freeze(user);

user.name = 'Oleh';   // will not change
user.city = 'Kyiv';   // cannot add a new one
delete user.age;      // cannot delete

console.log(user); // { name: 'Bohdan', age: 25 }

After Object.freeze() the object becomes immutable:

properties cannot be overwritten;
new ones cannot be deleted or added;
configurable or writable cannot be changed.

Checking whether an object is frozen

javascript

console.log(Object.isFrozen(user)); // true

Important: the freeze is shallow

If there are nested objects inside, they remain mutable:

javascript

const user = {
  name: 'Bohdan',
  address: {
    city: 'Kyiv'
  }
};

Object.freeze(user);

user.address.city = 'Lviv'; // works!
console.log(user.address.city); // "Lviv"

So Object.freeze() protects only the top level. For a "deep" freeze you need to freeze recursively.

Example of a deep freeze

javascript

function deepFreeze(obj) {
  Object.freeze(obj);

  for (const key in obj) {
    if (typeof obj[key] === 'object' && obj[key] !== null && !Object.isFrozen(obj[key])) {
      deepFreeze(obj[key]);
    }
  }

  return obj;
}

const user = {
  name: 'Bohdan',
  address: { city: 'Kyiv' }
};

deepFreeze(user);

user.address.city = 'Lviv'; // will not change
console.log(user.address.city); // "Kyiv"

Now the object and all its content are fully "frozen".

Difference from other methods

Method	What it does	Can change values	Can add/remove properties
`Object.freeze()`	Fully freezes	no	no
`Object.seal()`	Forbids adding/removing, but allows changing values	yes	no
`Object.preventExtensions()`	Forbids adding new properties	yes	yes (existing ones can be removed)

Example: `freeze` vs `seal`

javascript

const user = { name: 'Bohdan' };

Object.seal(user);
user.name = 'Oleh'; // allowed
delete user.name;   // not allowed

Object.freeze(user);
user.name = 'Max'; // not allowed

SUMMARY

Property	Behavior
`Object.freeze()`	makes the object fully immutable
not allowed	adding, removing, or changing properties
works shallowly	nested objects remain mutable
check	`Object.isFrozen(obj)`

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