class 声明创建一个基于原型继承的具有给定名称的新类。
你也可以使用类表达式定义类。但是不同于类表达式,类声明不允许再次声明已经存在的类,否则将会抛出一个类型错误。
class name [extends] {
// class body
}
和类表达式一样,类声明体在严格模式下运行。构造函数是可选的。
类声明不可以提升(这与函数声明不同)。
在下面的例子中,我们首先定义一个名为Polygon的类,然后继承它来创建一个名为Square的类。注意,构造函数中使用的 super() 只能在构造函数中使用,并且必须在使用 this 关键字前调用。
class Polygon {
constructor(height, width) {
this.name = 'Polygon';
this.height = height;
this.width = width;
}
}
class Square extends Polygon {
constructor(length) {
super(length, length);
this.name = 'Square';
}
}
重复声明一个类会引起类型错误。
class Foo {};
class Foo {};
// Uncaught TypeError: Identifier 'Foo' has already been declared
若之前使用类表达式定义了一个类,则再次声明这个类同样会引起类型错误。
let Foo = class {};
class Foo {};
// Uncaught TypeError: Identifier 'Foo' has already been declared
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) Class definitions |
Standard | Initial definition. |
ECMAScript 2016 (ECMA-262) Class definitions |
Standard | |
ECMAScript 2017 (ECMA-262) Class definitions |
Standard | |
ECMAScript Latest Draft (ECMA-262) Class definitions |
Draft |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
class | Chrome Full support 42 | Edge Full support Yes | Firefox Full support 45 | IE No support No | Opera Full support 36 | Safari Full support 10.1 | WebView Android Full support 42 | Chrome Android Full support 42 | Edge Mobile Full support Yes | Firefox Android Full support 45 | Opera Android ? | Safari iOS Full support 10.1 | Samsung Internet Android Full support 4.0 | nodejs Full support 6.0.0 |
Array subclassing | Chrome Full support 43 | Edge ? | Firefox ? | IE No support No | Opera Full support 36 | Safari ? | WebView Android Full support 43 | Chrome Android Full support 43 | Edge Mobile ? | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support 4.0 | nodejs ? |
Allowed in sloppy mode | Chrome Full support 49 | Edge ? | Firefox ? | IE No support No | Opera Full support 36 | Safari ? | WebView Android Full support 49 | Chrome Android Full support 49 | Edge Mobile ? | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support 5.0 | nodejs ? |