extends
语法
class ChildClass extends ParentClass { ... }
描述
示例
使用 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。
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;
}
}
使用 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。
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()
);
}
}
扩展 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
(en-US)。
js
class nullExtends extends null {
constructor() {}
}
Object.getPrototypeOf(nullExtends); // Function.prototype
Object.getPrototypeOf(nullExtends.prototype); // null
new nullExtends(); //ReferenceError: this is not defined
规范
Specification |
---|
ECMAScript Language Specification # sec-class-definitions |
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