Object.freeze()

Summary

The Object.freeze() method freezes an object: that is, prevents new properties from being added to it; prevents existing properties from being removed; and prevents existing properties, or their enumerability, configurability, or writability, from being changed. In essence the object is made effectively immutable. The method returns the object being frozen.

Syntax

Object.freeze(obj)

Parameters

obj

The object to freeze.

Description

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).

Values cannot be changed for data properties. Accessor properties (getters and setters) work the same (and still give the illusion that you are changing the value). Note that values that are objects can still be modified, unless they are also frozen.

Examples

var obj = {
  prop: function (){},
  foo: "bar"
};

// New properties may be added, existing properties may be changed or removed
obj.foo = "baz";
obj.lumpy = "woof";
delete obj.prop;

var o = Object.freeze(obj);

assert(Object.isFrozen(obj) === true);

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

// ...and in strict mode such attempts will throw TypeErrors
function fail(){
  "use strict";
  obj.foo = "sparky"; // throws a TypeError
  delete obj.quaxxor; // throws a TypeError
  obj.sparky = "arf"; // throws a TypeError
}

fail();

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

The following example shows that object values in a frozen object can be mutated (freeze is shallow).

obj = {
  internal : {}
};

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

obj.internal.a // "aValue"

// To make obj fully immutable, freeze each object in obj.
// To do so, we use this function.

function deepFreeze (o) {
  var prop, propKey;
  Object.freeze(o); // First freeze the object.
  for (propKey in o) {
    prop = o[propKey];
    if (!o.hasOwnProperty(propKey) || !(typeof prop === "object") || Object.isFrozen(prop)) {
      // If the object is on the prototype, not an object, or is already frozen, 
      // skip it. Note that this might leave an unfrozen reference somewhere in the
      // object if there is an already frozen object containing an unfrozen object.
      continue;
    }

    deepFreeze(prop); // Recursively call deepFreeze.
  }
}

obj2 = {
  internal : {}
};

deepFreeze(obj2);
obj2.internal.a = "anotherValue";
obj2.internal.a; // undefined

Specifications

Browser compatibility

Based on Kangax's compat table.

See also

• Object.isFrozen
• Object.preventExtensions
• Object.isExtensible
• Object.seal
• Object.isSealed