Summary

Defines a new property directly on an object, or modifies an existing property on an object, and returns the object.

Method of Object
Implemented in	JavaScript 1.8.5
ECMAScript Edition	ECMAScript 5th Edition

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.

Description

This method allows 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), whose values may be changed, and which may be deleted. This method allows these extra details to be changed from their defaults.

Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors. A <dfn>data descriptor</dfn> is a property that has a value, which may or may not be writable. An <dfn>accessor descriptor</dfn> is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both. All descriptors regardless of flavor include the configurable and enumerable fields.

A property descriptor is an object with the following fields:

value

The value associated with the property. (data descriptors only). Defaults to undefined.

writable

True if and only if the value associated with the property may be changed. (data descriptors only). Defaults to false.

get

A function which serves as a getter for the property, or undefined if there is no getter. (accessor descriptors only). Defaults to undefined.

set

A function which serves as a setter for the property, or undefined if there is no setter. (accessor descriptors only). Defaults to undefined.

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. Defaults to false.

enumerable

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

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 imputed. All of the Boolean-valued fields default to false. The value, get, and set fields default to undefined.

Examples

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;
Object.defineProperty(o, "b", {get : function(){ return bValue; },
                               set : function(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: function() { 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 current configuration. If the old descriptor had its configurable attribute set to false (the property is said "non-configurable"), then no attribute besides writable can be changed. In that case, it is also not possible to switch back and forth from data/accessor properties type (a property which would have been defined without get/set/value/writable is called "generic" and is "typed" as a data descriptor).

If a property is non-configurable, its writable attribute can only be changed to false.

A TypeError is thrown when non-configurable property attributes are changed unless it's the writable attribute or if the current and new values are equal.

Writable attribute

When the writable property attribute is set to false, the property is said to be "non-writable". It cannot be assigned.

Example

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

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

console.log(o.a); // logs 37
o.a = 25;
console.log(o.a); // logs 37. The assignment didn't work. No error thrown (it would throw in strict mode)

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 or not.

Example

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' then 'd'

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 writable) can be changed.

Example

var o = {};
Object.defineProperty(o, "a", { get : function(){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 : function(){}}); // throws a TypeError (set was undefined previously)
Object.defineProperty(o, "a", {get : function(){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 o.a configurable attribute was true, none of the error would be thrown and the property would be deleted at the end.

Adding properties and default values

It's important to consider the way default values of attributes are applied. It's different 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});

Cross-browser concerns

Redefining the length property of an Array object

It is possible to redefine the length property of arrays, but not all browsers permit this redefinition.  Firefox 4 will throw a TypeError on any attempt 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, and changing writability seems to silently not work in some circumstances.  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 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.

Internet Explorer 8 specific case

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

• Trying to use Object.defineProperty() on native objects throws an error.
• Property attributes must be set to some values. true, true, 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 to delete the property first. If the property isn't deleted, it stays as it was before the reconfiguration attempt.

Browser compatibility

Based on Kangax's compat tables.

See also

• Object.defineProperties()
• object.watch()
• object.unwatch()
• get
• set
• Object.create()
• Additional Object.defineProperty examples