https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/super
The super keyword is used to access and call functions on an object's parent.
super关键字用于访问和调用一个对象的父对象上的函数。
The super.prop
and super[expr]
expressions are valid in any method definition in both classes and object literals.
super.prop
和super[expr]
表达式在类和对象字面量任何方法定义中都是有效的。
super([arguments]); // calls the parent constructor.调用 父对象/类 的构造函数
super.[functionOnParent]([arguments]); // calls the parent functions 调用 父对象/类 上的方法
Description描述
When used in a constructor, the super
keyword appears alone and must be used before the this
keyword is used. The super
keyword can also be used to call functions on a parent object.
在构造函数中使用时,super
关键字将单独出现,并且必须在使用this
关键字之前使用。super
关键字也可以用来调用父对象上的函数。
Example示例
Using super in classes
Here super()
is called to avoid duplicating the constructor parts' that are common between Rectangle
and Square
.
class Rectangle {
constructor(height, width) {
this.name = 'Rectangle';
this.height = height;
this.width = width;
}
sayName() {
console.log('Hi, I am a ', this.name + '.');
}
get area() {
return this.height * this.width;
}
set area(value) {
this.height = this.width = Math.sqrt(value);
}
}
class Square extends Rectangle {
constructor(length) {
this.height; // ReferenceError, super needs to be called first! super需要先被调用
// Here, it calls the parent class' constructor with lengths
// provided for the Rectangle'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';
}
}
Super-calling static methods
class Rectangle{
constructor() {}
static logNbSides() {
return 'I have 4 sides';
}
}
class Square extends Rectangle {
constructor() {}
static logDescription() {
return super.logNbSides() + ' which are all equal';
}
}
Square.logDescription(); // 'I have 4 sides which are all equal'
Deleting super properties will throw an error
You cannot use the 大专栏 JavaScript关键字之super()eb/JavaScript/Reference/Operators/delete">delete operator and super.prop
or super[expr]
to delete a parent class' property, it will throw a ReferenceError
.
你不能使用 delete 操作符 加 super.prop
或者 super[expr]
去删除父类的属性,这样做会抛出 ReferenceError
。
class Base {
constructor() {}
foo() {}
}
class Derived extends Base {
constructor() {}
delete() {
delete super.foo; // this is bad
}
}
new Derived().delete();
// ReferenceError: invalid delete involving 'super'.
super.prop
cannot overwrite non-writable properties
When defining non-writable properties with e.g. Object.defineProperty
, super
cannot overwrite the value of the property.
当使用 Object.defineProperty
定义一个属性为不可写时,super
将不能重写这个属性的值。
class X {
constructor() {
Object.defineProperty(this, 'prop', {
configurable: true,
writable: false,
value: 1
});
}
f() {
super.prop = 2;
}
}
var x = new X();
x.f(); // TypeError: "prop" is read-only
console.log(x.prop); // 1
Using super.prop
in object literals
Super can also be used in the object initializer / literal notation. In this example, two objects define a method. In the second object, super
calls the first object's method. This works with the help of Object.setPrototypeOf()
with which we are able to set the prototype of obj2
to obj1
, so that super
is able to find method1
on obj1
.
Super
也可以在object initializer / literal 符号中使用。在下面的例子中,两个对象各定义了一个方法。在第二个对象中, 我们使用super
调用了第一个对象中的方法。 当然,这需要我们先利用 Object.setPrototypeOf()
将obj2
的原型加到obj1
上,然后才能够使用super
调用 obj1
上的method1
。
var obj1 = {
method1() {
console.log("method 1");
}
}
var obj2 = {
method2() {
super.method1();
}
}
Object.setPrototypeOf(obj2, obj1);
obj2.method2();
// logs "method 1"
Specifications规范
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262)super | Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262)super | Living Standard |