extends
关键字用于类声明或者类表达式中,以创建一个类,该类是另一个类的子类。
class ChildClass extends ParentClass { ... }
extends
关键字用来创建一个普通类或者内建对象的子类。
继承的.prototype
必须是一个Object
或者 null
。
extends
第一个例子是根据名为 Polygon
类创建一个名为Square
的类。这个例子是从这个在线演示中提取出来的。
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;
}
}
extends
与内置对象这个示例继承了内置的Date
对象。这个例子是从这个在线演示中提取出来的。
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();
}
}
null
可以像扩展普通类一样扩展null
,但是新对象的原型将不会继承 Object.prototype
。
class nullExtends extends null {
constructor() {}
}
Object.getPrototypeOf(nullExtends); // Function.prototype
Object.getPrototypeOf(nullExtends.prototype) // null
new nullExtends(); //ReferenceError: this is not defined
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) extends |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) extends |
Draft |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
extends | 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
|