path: root/files/de/web/javascript/reference/global_objects/object/defineproperty/index.html

---
title: Object.defineProperty()
slug: Web/JavaScript/Reference/Global_Objects/Object/defineProperty
tags:
  - Méthode
  - Objekt
translation_of: Web/JavaScript/Reference/Global_Objects/Object/defineProperty
---
<div>{{JSRef}}</div>

<p>Die Methode <code><strong>Object.defineProperty()</strong></code> definiert eine neue Eigenschaft direkt auf ein Objekt, oder modifiziert eine Eigenschaft. Schließlich gibt die Funktion das Objekt zurück.</p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox"><code>Object.defineProperty(<var>obj</var>, <var>prop</var>, <var>descriptor</var>)</code></pre>

<h3 id="Parameter">Parameter</h3>

<dl>
 <dt><code>obj</code></dt>
 <dd>Das Objekt, welchem die neue Eigenschaft zugewiesen werden soll.</dd>
 <dt><code>prop</code></dt>
 <dd>Der Name der Eigenschaft, welche hinzugefügt oder modifiziert werden soll.</dd>
 <dt><code>descriptor</code></dt>
 <dd>Die Beschreibung/ der Wert, welche die neue Eigenschaft annehmen soll.</dd>
</dl>

<h3 id="Rückgabewert">Rückgabewert</h3>

<p>Das Objekt, welches behandelt wurde.</p>

<h2 id="Beschreibung">Beschreibung</h2>

<p>Diese Methode erlaubt präzises Hinzufügen oder Modifizieren von Eigenschaften eines Objektes. Normal property addition through assignment creates properties which show up during property enumeration ({{jsxref("Statements/for...in", "for...in")}} loop or {{jsxref("Object.keys")}} method), whose values may be changed, and which may be {{jsxref("Operators/delete", "deleted", "", 1)}}. This method allows these extra details to be changed from their defaults. Standardmäßig sind Werte die mit <code>Object.defineProperty()</code> hinzugefügt wurden unveränderbar.</p>

<p>Attribut Deskriptoren unterscheiden sich in zwei Varianten: Daten Deskriptoren und Zugiffsdeskriptoren. Ein <em>Datendeskriptor</em> ist ein Attribut welches einen Wert hat das schreibbar oder nicht schreibbar sein kann. Ein <em>Zugriffsdeskriptor</em> ist ein Attribut das mit einem "getter/setter Paar" beschrieben wird. Ein Deskriptor muss von einer dieser beiden Arten sein, er kann nicht beides sein.</p>

<p>Beide, Daten- und Zugriffsdeskriptoren sind Objekte. Sie teilen die folgenden benötigten Objektschlüssel:</p>

<dl>
 <dt><code>configurable</code></dt>
 <dd><code>true</code> if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.<br>
 <strong>Defaults to <code>false</code>.</strong></dd>
 <dt><code>enumerable</code></dt>
 <dd><code>true</code> if and only if this property shows up during enumeration of the properties on the corresponding object.<br>
 <strong>Defaults to <code>false</code>.</strong></dd>
</dl>

<p>Ein Datendeskriptor hat außerdem die folgenden, optionalen Schlüssel:</p>

<dl>
 <dt><code>value</code></dt>
 <dd>The value associated with the property. Can be any valid JavaScript value (number, object, function, etc).<br>
 <strong>Defaults to {{jsxref("undefined")}}.</strong></dd>
 <dt><code>writable</code></dt>
 <dd><code>true</code> if and only if the value associated with the property may be changed with an {{jsxref("Operators/Assignment_Operators", "assignment operator", "", 1)}}.<br>
 <strong>Defaults to <code>false</code>.</strong></dd>
</dl>

<p>Ein Zugriffsdeskriptor hat außerdem die folgenden, optionalen Schlüssel:</p>

<dl>
 <dt><code>get</code></dt>
 <dd>A function which serves as a getter for the property, or {{jsxref("undefined")}} if there is no getter. The function return will be used as the value of property.<br>
 <strong>Defaults to {{jsxref("undefined")}}.</strong></dd>
 <dt><code>set</code></dt>
 <dd>A function which serves as a setter for the property, or {{jsxref("undefined")}} if there is no setter. The function will receive as only argument the new value being assigned to the property.<br>
 <strong>Defaults to {{jsxref("undefined")}}.</strong></dd>
</dl>

<p>Bear in mind that these options are not necessarily the descriptor's own properties, and properties inherited from the prototype chain will be considered too. In order to ensure these defaults are preserved you might freeze the {{jsxref("Object.prototype")}} upfront or specify all options explicitly.</p>

<pre class="brush: js">// 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);
</pre>

<h2 id="Examples">Examples</h2>

<p>If you want to see how to use the <code>Object.defineProperty</code> method with a <em>binary-flags-like</em> syntax, see <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty/Additional_examples">additional examples</a>.</p>

<h3 id="Creating_a_property">Creating a property</h3>

<p>When the property specified doesn't exist in the object, <code>Object.defineProperty()</code> 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 <code>false</code>. The <code>value</code>, <code>get</code>, and <code>set</code> fields default to {{jsxref("undefined")}}. A property which is defined without <code>get</code>/<code>set</code>/<code>value</code>/<code>writable</code> is called “generic” and is “typed” as a data descriptor.</p>

<pre class="brush: js">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', {
  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
</pre>

<h3 id="Modifying_a_property">Modifying a property</h3>

<p>When the property already exists, <code>Object.defineProperty()</code> attempts to modify the property according to the values in the descriptor and the object's current configuration. If the old descriptor had its <code>configurable</code> attribute set to <code>false</code> (the property is said to be “non-configurable”), then no attribute besides <code>writable</code> can be changed. In that case, it is also not possible to switch back and forth between the data and accessor property types.</p>

<p>If a property is non-configurable, its <code>writable</code> attribute can only be changed to <code>false</code>.</p>

<p>A {{jsxref("TypeError")}} is thrown when attempts are made to change non-configurable property attributes (besides the <code>writable</code> attribute) unless the current and new values are the same.</p>

<h4 id="Writable_attribute">Writable attribute</h4>

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

<pre class="brush: js">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.
</pre>

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

<h4 id="Enumerable_attribute">Enumerable attribute</h4>

<p>The <code>enumerable</code> property attribute defines whether the property shows up in a {{jsxref("Statements/for...in", "for...in")}} loop and {{jsxref("Object.keys()")}} or not.</p>

<pre class="brush: js">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
</pre>

<h4 id="Configurable_attribute">Configurable attribute</h4>

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

<pre class="brush: js">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
</pre>

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

<h3 id="Adding_properties_and_default_values">Adding properties and default values</h3>

<p>It's 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 <code>Object.defineProperty()</code>, as shown in the example below.</p>

<pre class="brush: js">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
});
</pre>

<h3 id="Custom_Setters_and_Getters">Custom Setters and Getters</h3>

<p>Example below shows how to implement a self-archiving object. When <code>temperature</code> property is set, the <code>archive</code> array gets a log entry.</p>

<pre class="brush: js">function Archiver() {
  var temperature = null;
  var archive = [];

  Object.defineProperty(this, 'temperature', {
    get: function() {
      console.log('get!');
      return temperature;
    },
    set: function(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 }]
</pre>

<p>or</p>

<pre class="brush: js">var pattern = {
    get: function () {
        return 'I always return this string, whatever you have assigned';
    },
    set: function () {
        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

</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.2.3.6', 'Object.defineProperty')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td>Initial definition. Implemented in JavaScript 1.8.5.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-object.defineproperty', 'Object.defineProperty')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td></td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-object.defineproperty', 'Object.defineProperty')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td></td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<div>{{CompatibilityTable}}</div>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Firefox (Gecko)</th>
   <th>Chrome</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatGeckoDesktop("2")}}</td>
   <td>{{CompatChrome("5")}}</td>
   <td>{{CompatIE("9")}} [1]</td>
   <td>{{CompatOpera("11.60")}}</td>
   <td>{{CompatSafari("5.1")}} [2]</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>Android</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatGeckoMobile("2")}}</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatIE("9")}}</td>
   <td>{{CompatOperaMobile("11.5")}}</td>
   <td>{{CompatVersionUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<p>[1] In Internet Explorer 8 only on DOM objects and with some non-standard behaviors.</p>

<p>[2] Also supported in Safari 5, but not on DOM objects.</p>

<h2 id="Compatibility_notes">Compatibility notes</h2>

<h3 id="Redefining_the_length_property_of_an_Array_object">Redefining the <code>length</code> property of an <code>Array</code> object</h3>

<p>It is possible to redefine the {{jsxref("Array.length", "length")}} property of arrays, subject to the usual redefinition restrictions. (The {{jsxref("Array.length", "length")}} property is initially non-configurable, non-enumerable, and writable. Thus on an unaltered array it is possible to change the {{jsxref("Array.length", "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.</p>

<p>Firefox 4 through 22 will throw a {{jsxref("TypeError")}} on any attempt whatsoever (whether permitted or not) to redefine the {{jsxref("Array.length", "length")}} property of an array.</p>

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

<p>Versions of Safari which implement <code>Object.defineProperty()</code> ignore a <code>length</code> value different from the array's current {{jsxref("Array.length", "length")}} property, and attempts to change writability execute without error but do not actually change the property's writability.</p>

<p>Only Internet Explorer 9 and later, and Firefox 23 and later, appear to fully and correctly implement redefinition of the {{jsxref("Array.length", "length")}} property of arrays. For now, don't rely on redefining the {{jsxref("Array.length", "length")}} property of an array to either work, or to work in a particular manner. And even when you <em>can</em> rely on it, <a href="http://whereswalden.com/2013/08/05/new-in-firefox-23-the-length-property-of-an-array-can-be-made-non-writable-but-you-shouldnt-do-it/">there's really no good reason to do so</a>.</p>

<h3 id="Internet_Explorer_8_specific_notes">Internet Explorer 8 specific notes</h3>

<p>Internet Explorer 8 implemented a <code>Object.defineProperty()</code> method that could <a class="external" href="https://msdn.microsoft.com/en-us/library/dd229916%28VS.85%29.aspx">only be used on DOM objects</a>. A few things need to be noted:</p>

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

<h2 id="See_also">See also</h2>

<ul>
 <li><a href="/en-US/docs/Enumerability_and_ownership_of_properties">Enumerability and ownership of properties</a></li>
 <li>{{jsxref("Object.defineProperties()")}}</li>
 <li>{{jsxref("Object.propertyIsEnumerable()")}}</li>
 <li>{{jsxref("Object.getOwnPropertyDescriptor()")}}</li>
 <li>{{jsxref("Object.prototype.watch()")}}</li>
 <li>{{jsxref("Object.prototype.unwatch()")}}</li>
 <li>{{jsxref("Operators/get", "get")}}</li>
 <li>{{jsxref("Operators/set", "set")}}</li>
 <li>{{jsxref("Object.create()")}}</li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty/Additional_examples">Additional <code>Object.defineProperty</code> examples</a></li>
 <li>{{jsxref("Reflect.defineProperty()")}}</li>
</ul>