Object.freeze()
===============

The **`Object.freeze()`** static method *freezes* an object. Freezing an object prevents extensions and makes existing properties non-writable and non-configurable. A frozen object can no longer be changed: new properties cannot be added, existing properties cannot be removed, their enumerability, configurability, writability, or value cannot be changed, and the object's prototype cannot be re-assigned. `freeze()` returns the same object that was passed in.

Freezing an object is the highest integrity level that JavaScript provides.

Try it
------

Syntax
------


```js
Object.freeze(obj)
```

### Parameters

`obj`

The object to freeze.

### Return value

The object that was passed to the function.

Description
-----------

Freezing an object is equivalent to preventing extensions and then changing all existing properties' descriptors' `configurable` to `false` — and for data properties, `writable` to `false` as well. Nothing can be added to or removed from the properties set of a frozen object. Any attempt to do so will fail, either silently or by throwing a `TypeError` exception (most commonly, but not exclusively, when in strict mode).

 For data properties of a frozen object, their values cannot be changed since the `writable` and
 `configurable` attributes are set to `false`. Accessor properties (getters and setters) work the same — the property value returned by the getter may still change, and the setter can still be called without throwing errors when setting the property. Note that values
 that are objects can still be modified, unless they are also frozen. As an object, an
 array can be frozen; after doing so, its elements cannot be altered and no elements can
 be added to or removed from the array.

Private properties do not have the concept of property descriptors. Freezing an object with private properties does not prevent the values of these private properties from being changed. (Freezing objects is usually meant as a security measure against external code, but external code cannot access private properties anyway.) Private properties cannot be added or removed from the object, whether the object is frozen or not.

`freeze()` returns the same object that was passed into the function. It
 *does not* create a frozen copy.

 A `TypedArray` or a `DataView` with elements will cause a `TypeError`,
 as they are views over memory and will definitely cause other possible issues:


```js
Object.freeze(new Uint8Array(0)); // No elements
// Uint8Array []

Object.freeze(new Uint8Array(1)); // Has elements
// TypeError: Cannot freeze array buffer views with elements

Object.freeze(new DataView(new ArrayBuffer(32))); // No elements
// DataView {}

Object.freeze(new Float64Array(new ArrayBuffer(64), 63, 0)); // No elements
// Float64Array []

Object.freeze(new Float64Array(new ArrayBuffer(64), 32, 2)); // Has elements
// TypeError: Cannot freeze array buffer views with elements
```


 Note that as the standard three properties (`buf.byteLength`,
 `buf.byteOffset` and `buf.buffer`) are read-only (as are those of
 an `ArrayBuffer` or `SharedArrayBuffer`), there is no reason for
 attempting to freeze these properties.

Unlike `Object.seal()`, existing properties in objects frozen with `Object.freeze()` are made immutable and data properties cannot be re-assigned.

Examples
--------

### Freezing objects


```js
const obj = {
  prop() {},
  foo: "bar",
};

// Before freezing: new properties may be added,
// and existing properties may be changed or removed
obj.foo = "baz";
obj.lumpy = "woof";
delete obj.prop;

// Freeze.
const o = Object.freeze(obj);

// The return value is just the same object we passed in.
o === obj; // true

// The object has become frozen.
Object.isFrozen(obj); // === true

// Now any changes will fail
obj.foo = "quux"; // silently does nothing
// silently doesn't add the property
obj.quaxxor = "the friendly duck";

// In strict mode such attempts will throw TypeErrors
function fail() {
  "use strict";
  obj.foo = "sparky"; // throws a TypeError
  delete obj.foo; // throws a TypeError
  delete obj.quaxxor; // returns true since attribute 'quaxxor' was never added
  obj.sparky = "arf"; // throws a TypeError
}

fail();

// Attempted changes through Object.defineProperty;
// both statements below throw a TypeError.
Object.defineProperty(obj, "ohai", { value: 17 });
Object.defineProperty(obj, "foo", { value: "eit" });

// It's also impossible to change the prototype
// both statements below will throw a TypeError.
Object.setPrototypeOf(obj, { x: 20 });
obj.__proto__ = { x: 20 };
```

### Freezing arrays


```js
const a = [0];
Object.freeze(a); // The array cannot be modified now.

a[0] = 1; // fails silently

// In strict mode such attempt will throw a TypeError
function fail() {
  "use strict";
  a[0] = 1;
}

fail();

// Attempted to push
a.push(2); // throws a TypeError
```


 The object being frozen is *immutable*. However, it is not necessarily
 *constant*. The following example shows that a frozen object is not constant
 (freeze is shallow).


```js
const obj1 = {
  internal: {},
};

Object.freeze(obj1);
obj1.internal.a = "aValue";

obj1.internal.a; // 'aValue'
```


 To be a constant object, the entire reference graph (direct and indirect references to
 other objects) must reference only immutable frozen objects. The object being frozen is
 said to be immutable because the entire object *state* (values and references to
 other objects) within the whole object is fixed. Note that strings, numbers, and
 booleans are always immutable and that Functions and Arrays are objects.

#### What is "shallow freeze"?

 The result of calling `Object.freeze(object)` only applies to the
 immediate properties of `object` itself and will prevent future property
 addition, removal or value re-assignment operations *only* on
 `object`. If the value of those properties are objects themselves, those
 objects are not frozen and may be the target of property addition, removal or value
 re-assignment operations.


```js
const employee = {
  name: "Mayank",
  designation: "Developer",
  address: {
    street: "Rohini",
    city: "Delhi",
  },
};

Object.freeze(employee);

employee.name = "Dummy"; // fails silently in non-strict mode
employee.address.city = "Noida"; // attributes of child object can be modified

console.log(employee.address.city); // "Noida"
```


 To make an object immutable, recursively freeze each non-primitive property
 (deep freeze). Use the pattern on a case-by-case basis based on your design when you
 know the object contains no cycles in the reference
 graph, otherwise an endless loop will be triggered. An enhancement to
 `deepFreeze()` would be to have an internal function that receives a path
 (e.g. an Array) argument so you can suppress calling `deepFreeze()`
 recursively when an object is in the process of being made immutable. You still run a
 risk of freezing an object that shouldn't be frozen, such as `window`.


```js
function deepFreeze(object) {
  // Retrieve the property names defined on object
  const propNames = Reflect.ownKeys(object);

  // Freeze properties before freezing self
  for (const name of propNames) {
    const value = object[name];

    if ((value && typeof value === "object") || typeof value === "function") {
      deepFreeze(value);
    }
  }

  return Object.freeze(object);
}

const obj2 = {
  internal: {
    a: null,
  },
};

deepFreeze(obj2);

obj2.internal.a = "anotherValue"; // fails silently in non-strict mode
obj2.internal.a; // null
```

Specifications
--------------

| Specification |
| --- |
| ECMAScript Language Specification # sec-object.freeze |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

* `Object.isFrozen()`
* `Object.preventExtensions()`
* `Object.isExtensible()`
* `Object.seal()`
* `Object.isSealed()`