Revision 944527 of extends

<div>{{jsSidebar("Classes")}} {{Harmony}}</div>

<p>The <strong><code>extends</code></strong> keyword is used in a <a href="/en-US/docs/Web/JavaScript/Reference/Statements/class">class declarations</a> or <a href="/en-US/docs/Web/JavaScript/Reference/Operators/class">class expressions</a> to create a class with a child of another class.</p>

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

<pre class="syntaxbox">
class ChildClass extends ParentClass { ... }</pre>

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

<p>The <code>extends</code> keyword can be used to subclass custom classes as well as built-in objects.</p>

<p>The <code>.prototype</code> of the extension must be an {{jsxref("Object")}} or {{jsxref("null")}}.</p>

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

<h3 id="Using_extends">Using <code>extends</code></h3>

<p>The first example creates a class called <code>Square</code> from a class called <code>Polygon</code>. This example is extracted from this <a href="https://googlechrome.github.io/samples/classes-es6/index.html">live demo</a> <a href="https://github.com/GoogleChrome/samples/blob/gh-pages/classes-es6/index.html">(source)</a>.</p>

<pre class="brush: js">
class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = 'Square';
  }

  get area() {
    return this.height * this.width;
  }

  set area(value) {
    this.area = value;
  } 
}</pre>

<h3 id="Using_extends_with_built-in_objects">Using <code>extends</code> with built-in objects</h3>

<p>This example extends the built-in {{jsxref("Date")}} object. This example is extracted from this <a href="https://googlechrome.github.io/samples/classes-es6/index.html">live demo</a> <a href="https://github.com/GoogleChrome/samples/blob/gh-pages/classes-es6/index.html">(source)</a>.</p>

<pre class="brush: js">
class myDate extends Date {
  constructor() {
    super();
  }

  getFormattedDate() {
    var months = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'];
    return this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear();
  }
}</pre>

<h3 id="Extending_null">Extending <code>null</code></h3>

<p>Extending from {{jsxref("null")}} works like with a normal class, except that the prototype object does not inherit from {{jsxref("Object.prototype")}}.</p>

<pre class="brush: js">
class nullExtends extends null {
  constructor() {}
}

Object.getPrototypeOf(nullExtends); // Function.prototype
Object.getPrototypeOf(nullExtends.prototype) // null</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-class-definitions', 'extends')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-class-definitions', 'extends')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td>&nbsp;</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(42.0)}}</td>
   <td>{{CompatNightly("firefox")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
  <tr>
   <td>Array subclassing</td>
   <td>{{CompatChrome(43.0)}}</td>
   <td>{{CompatNo}}</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>
   <th>Chrome for Android</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNightly("firefox")}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatChrome(42.0)}}</td>
  </tr>
  <tr>
   <td>Array subclassing</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatChrome(43.0)}}</td>
  </tr>
 </tbody>
</table>
</div>

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

<ul>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Classes">Classes</a></li>
 <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/super">super</a></li>
</ul>