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The extends
keyword is used in class declarations or class expressions to create a class which is a child of another class.
The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
Syntax
class ChildClass extends ParentClass { ... }
Description
The extends
keyword can be used to subclass custom classes as well as built-in objects.
The .prototype
of the extension must be an Object
or null
.
Examples
Using extends
The first example creates a class called Square
from a class called Polygon
. This example is extracted from this live demo (source).
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; } }
Using extends
with built-in objects
This example extends the built-in Date
object. This example is extracted from this live demo (source).
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(); } }
Extending null
Extending from null
works like with a normal class, except that the prototype object does not inherit from Object.prototype
.
class nullExtends extends null { constructor() {} } Object.getPrototypeOf(nullExtends); // Function.prototype Object.getPrototypeOf(nullExtends.prototype) // null new nullExtends(); //ReferenceError: this is not defined
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'extends' in that specification. |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) The definition of 'extends' in that specification. |
Draft |
Browser compatibility
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome
Full support
49
| Edge Full support 13 | Firefox Full support 45 | IE No support No | Opera Full support 36 | Safari Full support 9 | WebView Android ? | Chrome Android Full support Yes | Edge Mobile Full support 13 | Firefox Android Full support 45 | Opera Android ? | Safari iOS Full support 9 | Samsung Internet Android Full support Yes | nodejs
Full support
6.0.0
|
Legend
- Full support
- Full support
- No support
- No support
- Compatibility unknown
- Compatibility unknown
- See implementation notes.
- See implementation notes.
- User must explicitly enable this feature.
- User must explicitly enable this feature.