Revision 784735 of WeakSet

<div>{{JSRef("Global_Objects", "WeakSet")}} {{ harmony() }}</div>

<h2 id="Summary">Summary</h2>

<p>The <strong><code>WeakSet</code></strong> object lets you store weakly held <em>objects</em> in a collection.</p>

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

<pre class="syntaxbox">
 new WeakSet([iterable]);</pre>

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt>iterable</dt>
 <dd>If an <a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of">iterable object </a>is passed, all of its elements will be added to the new <code>WeakSet</code>. null is treated as undefined.</dd>
</dl>

<h2 id="Description">Description</h2>

<p><code>WeakSet</code> objects are collections of objects. An object in the <code>WeakSet</code> may only occur once; it is unique in the <code>WeakSet</code>'s collection.</p>

<p>The main differences to the {{jsxref("Set")}} object are:</p>

<ul>
 <li>In contrast to <code>Sets</code>, <code>WeakSets</code> are <strong>collections of objects only</strong> and not of arbitrary values of any type.</li>
 <li>The <code>WeakSet</code> is <em>weak</em>: References to objects in the collection are held weakly. If there is no other reference to an object stored in the <code>WeakSet</code>, they can be garbage collected. That also means that there is no list of current objects stored in the collection. <code>WeakSets</code> are not enumerable.</li>
</ul>

<h2 id="Properties" name="Properties">Properties</h2>

<dl>
 <dt><code>WeakSet.length</code></dt>
 <dd>The value of the <code>length</code> property is 0.</dd>
 <dt>{{jsxref("WeakSet.prototype")}}</dt>
 <dd>Represents the prototype for the <code>Set</code> constructor. Allows the addition of properties to all <code>WeakSet</code> objects.</dd>
</dl>

<h2 id="Boolean_instances" name="Boolean_instances"><code>WeakSet</code> instances</h2>

<p>All <code>WeakSet</code> instances inherit from {{jsxref("WeakSet.prototype")}}.</p>

<h3 id="Properties_2">Properties</h3>

<p>{{page('en-US/Web/JavaScript/Reference/Global_Objects/WeakSet/prototype','Properties')}}</p>

<h3 id="Methods">Methods</h3>

<p>{{page('en-US/Web/JavaScript/Reference/Global_Objects/WeakSet/prototype','Methods')}}</p>

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

<h3 id="Example.3A_Using_the_WeakSet_object">Example: Using the <code>WeakSet</code> object</h3>

<pre class="brush: js">
var ws = new WeakSet();
var obj = {};
var foo = {};

ws.add(window);
ws.add(obj);

ws.has(window); // true
ws.has(foo);    // false, foo has not been added to the set

ws.delete(window); // removes window from the set
ws.has(window);    // false, window has been removed

ws.clear(); // empty the whole WeakSet
</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('ES6', '#sec-weakset-objects', 'WeakSet')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

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

<p>{{ CompatibilityTable() }}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{ CompatChrome(36) }}</td>
   <td>{{ CompatGeckoDesktop(34) }}</td>
   <td>{{ CompatNo() }}</td>
   <td>{{ CompatOpera(23) }}</td>
   <td>{{ CompatNo() }}</td>
  </tr>
  <tr>
   <td><code>new WeakSet(iterable)</code></td>
   <td>38</td>
   <td>{{ CompatGeckoDesktop(34) }}</td>
   <td>{{ CompatNo }}</td>
   <td>25</td>
   <td>{{ CompatNo }}</td>
  </tr>
  <tr>
   <td>Constructor argument: <code>new WeakSet(null)</code></td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{CompatGeckoDesktop("37")}}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
  </tr>
  <tr>
   <td>Monkey-patched <code>add()</code> in Constructor</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{CompatGeckoDesktop("37")}}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{ CompatNo() }}</td>
   <td>{{ CompatGeckoMobile(34) }}</td>
   <td>{{ CompatNo() }}</td>
   <td>{{ CompatNo() }}</td>
   <td>{{ CompatNo() }}</td>
  </tr>
  <tr>
   <td><code>new WeakMap(iterable)</code></td>
   <td>{{ CompatNo() }}</td>
   <td>{{ CompatGeckoMobile(34) }}</td>
   <td>{{ CompatNo }}</td>
   <td>{{ CompatNo }}</td>
   <td>{{ CompatNo }}</td>
  </tr>
  <tr>
   <td>Constructor argument: <code>new WeakSet(null)</code></td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{CompatGeckoMobile("37")}}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
  </tr>
  <tr>
   <td>Monkey-patched <code>add()</code> in Constructor</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{CompatGeckoMobile("37")}}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
   <td>{{ CompatUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

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

<ul>
 <li>{{jsxref("Map")}}</li>
 <li>{{jsxref("Set")}}</li>
 <li>{{jsxref("WeakMap")}}</li>
</ul>