Object.defineProperty

The Object.defineProperty() method defines a new property directly on an object, or modifies an existing property on an object, and returns the object.

Syntax

Object.defineProperty(obj, prop, descriptor)

Parameters

obj

The object on which to define the property.

prop

The name of the property to be defined or modified.

descriptor

The descriptor for the property being defined or modified.

Return value

The object that was passed to the function.

Description

This method allows a precise addition to or modification of a property on an object. Normal property addition through assignment creates properties which show up during property enumeration (for...in loop or Object.keys method), whose values may be changed, and which may be deleted. This method allows these extra details to be changed from their defaults. By default, values added using Object.defineProperty() are immutable.

Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors. A data descriptor is a property that has a value, which may or may not be writable. An accessor descriptor is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.

Both data and accessor descriptors are objects. They share the following required keys:

configurable

true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.
false

enumerable

true if and only if this property shows up during enumeration of the properties on the corresponding object.
false

A data descriptor also has the following optional keys:

value

The value associated with the property. Can be any valid JavaScript value (number, object, function, etc).
Defaults to undefined.

writable

true if and only if the value associated with the property may be changed with an assignment operator.
false

An accessor descriptor also has the following optional keys:

get

A function which serves as a getter for the property, or undefined if there is no getter. When the property is accessed, this function is called without arguments and with this set to the object through which the property is accessed (this may not be the object on which the property is defined due to inheritance). The return value will be used as the value of the property.
Defaults to undefined.

set

A function which serves as a setter for the property, or undefined if there is no setter. When the property is assigned to, this function is called with one argument (the value being assigned to the property) and with this set to the object through which the property is assigned.
Defaults to undefined.

If a descriptor has neither of value, writable, get and set keys, it is treated as a data descriptor. If a descriptor has both value or writable and get or set keys, an exception is thrown.

Bear in mind that these attributes are not necessarily the descriptor's own properties. Inherited properties will be considered as well. In order to ensure these defaults are preserved, you might freeze the Object.prototype upfront, specify all options explicitly, or point to null with Object.create(null).

// using __proto__
var obj = {};
var descriptor = Object.create(null); // no inherited properties
// not enumerable, not configurable, not writable as defaults
descriptor.value = 'static';
Object.defineProperty(obj, 'key', descriptor);

// being explicit
Object.defineProperty(obj, 'key', {
  enumerable: false,
  configurable: false,
  writable: false,
  value: 'static'
});

// recycling same object
function withValue(value) {
  var d = withValue.d || (
    withValue.d = {
      enumerable: false,
      writable: false,
      configurable: false,
      value: null
    }
  );
  d.value = value;
  return d;
}
// ... and ...
Object.defineProperty(obj, 'key', withValue('static'));

// if freeze is available, prevents adding or
// removing the object prototype properties
// (value, get, set, enumerable, writable, configurable)  
(Object.freeze || Object)(Object.prototype);

Examples

If you want to see how to use the Object.defineProperty method with a binary-flags-like syntax, see additional examples.

Creating a property

When the property specified doesn't exist in the object, Object.defineProperty() creates a new property as described. Fields may be omitted from the descriptor, and default values for those fields are inputted.

var o = {}; // Creates a new object

// Example of an object property added
// with defineProperty with a data property descriptor
Object.defineProperty(o, 'a', {
  value: 37,
  writable: true,
  enumerable: true,
  configurable: true
});
// 'a' property exists in the o object and its value is 37

// Example of an object property added
// with defineProperty with an accessor property descriptor
var bValue = 38;
Object.defineProperty(o, 'b', {
  // Using shorthand method names (ES2015 feature).
  // This is equivalent to:
  // get: function() { return bValue; },
  // set: function(newValue) { bValue = newValue; },
  get() { return bValue; },
  set(newValue) { bValue = newValue; },
  enumerable: true,
  configurable: true
});
o.b; // 38
// 'b' property exists in the o object and its value is 38
// The value of o.b is now always identical to bValue,
// unless o.b is redefined

// You cannot try to mix both:
Object.defineProperty(o, 'conflict', {
  value: 0x9f91102,
  get() { return 0xdeadbeef; }
});
// throws a TypeError: value appears
// only in data descriptors,
// get appears only in accessor descriptors

Modifying a property

When the property already exists, Object.defineProperty() attempts to modify the property according to the values in the descriptor and the object's current configuration. If the old descriptor had its configurable attribute set to false the property is said to be “non-configurable”. It is not possible to change any attribute of a non-configurable accessor property. For data properties, it is possible to modify the value if the property is writable, and it is possible to change writable attribute from true to false. It is not possible to switch between data and accessor property types when the property is non-configurable.

A TypeError is thrown when attempts are made to change non-configurable property attributes (except value and writable, if permitted) unless the current and new values are the same.

Writable attribute

When the writable property attribute is set to false, the property is said to be “non-writable”. It cannot be reassigned.

var o = {}; // Creates a new object

Object.defineProperty(o, 'a', {
  value: 37,
  writable: false
});

console.log(o.a); // logs 37
o.a = 25; // No error thrown
// (it would throw in strict mode,
// even if the value had been the same)
console.log(o.a); // logs 37. The assignment didn't work.

// strict mode
(function() {
  'use strict';
  var o = {};
  Object.defineProperty(o, 'b', {
    value: 2,
    writable: false
  });
  o.b = 3; // throws TypeError: "b" is read-only
  return o.b; // returns 2 without the line above
}());

As seen in the example, trying to write into the non-writable property doesn't change it but doesn't throw an error either.

Enumerable attribute

The enumerable property attribute defines whether the property shows up in a for...in loop and Object.keys() or not.

var o = {};
Object.defineProperty(o, 'a', {
  value: 1,
  enumerable: true
});
Object.defineProperty(o, 'b', {
  value: 2,
  enumerable: false
});
Object.defineProperty(o, 'c', {
  value: 3
}); // enumerable defaults to false
o.d = 4; // enumerable defaults to true
         // when creating a property by setting it

for (var i in o) {
  console.log(i);
}
// logs 'a' and 'd' (in undefined order)

Object.keys(o); // ['a', 'd']

o.propertyIsEnumerable('a'); // true
o.propertyIsEnumerable('b'); // false
o.propertyIsEnumerable('c'); // false

Configurable attribute

The configurable attribute controls at the same time whether the property can be deleted from the object and whether its attributes (other than value and writable) can be changed.

var o = {};
Object.defineProperty(o, 'a', {
  get() { return 1; },
  configurable: false
});

Object.defineProperty(o, 'a', {
  configurable: true
}); // throws a TypeError
Object.defineProperty(o, 'a', {
  enumerable: true
}); // throws a TypeError
Object.defineProperty(o, 'a', {
  set() {}
}); // throws a TypeError (set was undefined previously)
Object.defineProperty(o, 'a', {
  get() { return 1; }
}); // throws a TypeError
// (even though the new get does exactly the same thing)
Object.defineProperty(o, 'a', {
  value: 12
}); // throws a TypeError

console.log(o.a); // logs 1
delete o.a; // Nothing happens
console.log(o.a); // logs 1

If the configurable attribute of o.a had been true, none of the errors would be thrown and the property would be deleted at the end.

Adding properties and default values

It is important to consider the way default values of attributes are applied. There is often a difference between simply using dot notation to assign a value and using Object.defineProperty(), as shown in the example below.

var o = {};

o.a = 1;
// is equivalent to:
Object.defineProperty(o, 'a', {
  value: 1,
  writable: true,
  configurable: true,
  enumerable: true
});

// On the other hand,
Object.defineProperty(o, 'a', { value: 1 });
// is equivalent to:
Object.defineProperty(o, 'a', {
  value: 1,
  writable: false,
  configurable: false,
  enumerable: false
});

Custom Setters and Getters

The example below shows how to implement a self-archiving object. When temperature property is set, the archive array gets a log entry.

function Archiver() {
  var temperature = null;
  var archive = [];

  Object.defineProperty(this, 'temperature', {
    get() {
      console.log('get!');
      return temperature;
    },
    set(value) {
      temperature = value;
      archive.push({ val: temperature });
    }
  });

  this.getArchive = function() { return archive; };
}

var arc = new Archiver();
arc.temperature; // 'get!'
arc.temperature = 11;
arc.temperature = 13;
arc.getArchive(); // [{ val: 11 }, { val: 13 }]

In this example, a getter always returns the same value.

var pattern = {
    get() {
        return 'I always return this string, ' +
               'whatever you have assigned';
    },
    set() {
        this.myname = 'this is my name string';
    }
};

function TestDefineSetAndGet() {
    Object.defineProperty(this, 'myproperty', pattern);
}

var instance = new TestDefineSetAndGet();
instance.myproperty = 'test';
console.log(instance.myproperty);
// I always return this string, whatever you have assigned

console.log(instance.myname); // this is my name string

Inheritance of properties

If an accessor property is inherited, its get and set methods will be called when the property is accessed and modified on descendant objects. If these methods use a variable to store the value, this value will be shared by all objects.

function myclass() {
}

var value;
Object.defineProperty(myclass.prototype, "x", {
  get() {
    return value;
  },
  set(x) {
    value = x;
  }
});

var a = new myclass();
var b = new myclass();
a.x = 1;
console.log(b.x); // 1

This can be fixed by storing the value in another property. In get and set methods, this points to the object which is used to access or modify the property.

function myclass() {
}

Object.defineProperty(myclass.prototype, "x", {
  get() {
    return this.stored_x;
  },
  set(x) {
    this.stored_x = x;
  }
});

var a = new myclass();
var b = new myclass();
a.x = 1;
console.log(b.x); // undefined

Unlike accessor properties, value properties are always set on the object itself, not on a prototype. However, if a non-writable value property is inherited, it still prevents from modifying the property on the object.

function myclass() {
}

myclass.prototype.x = 1;
Object.defineProperty(myclass.prototype, "y", {
  writable: false,
  value: 1
});

var a = new myclass();
a.x = 2;
console.log(a.x); // 2
console.log(myclass.prototype.x); // 1
a.y = 2; // Ignored, throws in strict mode
console.log(a.y); // 1
console.log(myclass.prototype.y); // 1

Specifications

Browser compatibility

Compatibility notes

Redefining the length property of an Array object

It is possible to redefine the length property of arrays, subject to the usual redefinition restrictions. (The length property is initially non-configurable, non-enumerable, and writable. Thus on an unaltered array, it's possible to change the length property's value or to make it non-writable. It is not allowed to change its enumerability or configurability, or if it is non-writable to change its value or writability.) However, not all browsers permit this redefinition.

Firefox 4 through 22 will throw a TypeError on any attempt whatsoever (whether permitted or not) to redefine the length property of an array.

Versions of Chrome which implement Object.defineProperty() in some circumstances ignore a length value different from the array's current length property. In some circumstances changing writability seems to silently not work (and not throw an exception). Also, relatedly, some array-mutating methods like Array.prototype.push don't respect a non-writable length.

Versions of Safari which implement Object.defineProperty() ignore a length value different from the array's current length property, and attempts to change writability execute without error but do not actually change the property's writability.

Only Internet Explorer 9 and later, and Firefox 23 and later, appear to fully and correctly implement redefinition of the length property of arrays. For now, don't rely on redefining the length property of an array to either work, or to work in a particular manner. And even when you can rely on it, there's really no good reason to do so.

Internet Explorer 8 specific notes

Internet Explorer 8 implemented a Object.defineProperty() method that could only be used on DOM objects. A few things need to be noted:

• Trying to use Object.defineProperty() on native objects throws an error.
• Property attributes must be set to some values. The configurable, enumerable and writable attributes should all be set to true for data descriptor and true for configurable, false for enumerable for accessor descriptor.(?) Any attempt to provide other value(?) will result in an error being thrown.
• Reconfiguring a property requires first deleting the property. If the property isn't deleted, it stays as it was before the reconfiguration attempt.

See also

• Enumerability and ownership of properties
• Object.defineProperties()
• Object.propertyIsEnumerable()
• Object.getOwnPropertyDescriptor()
• Object.prototype.watch()
• Object.prototype.unwatch()
• get
• set
• Object.create()
• Additional Object.defineProperty examples
• Reflect.defineProperty()