class继承
// 继承Person类中的sayHi方法
class Person {
sayHi(){
console.log("hello");
}
}
class Chinese extends Person {}
// 继承Person类中的属性和方法
class Person {
constructor(name, age, gender){
this.name = name;
this.age = age;
this.gender = gender;
}
sayHi(){
console.log("hello");
}
}
// 子类
class Chinese extends Person {
constructor(name, age, gender, skin){
// 在子类中,必须在constructor函数中,首先调用super()
super(name, age, gender); // 相当于Person.call(this,name,age,gender)
// 调用super之后才可以去写其他代码
this.skin = skin;
}
}
let xm = new Chinese("xm", 20, "male", "黄");
console.log(xm);
xm.sayHi();
class Point {
}
class Student extends Point {
}
上面示例中,Point
是父类,Student
是子类,它通过extends
关键字,继承了Point
类的所有属性和方法。但是由于没有部署任何代码,所以这两个类完全一样,等于复制了一个Point
类。
下面,我们在Student
内部加上代码。
class Point { /* ... */ }
class Student extends Point {
constructor(x, y, color) {
super(x, y); // 调用父类的constructor(x, y)
this.color = color;
}
toString() {
return this.color + ' ' + super.toString(); // 调用父类的toString()
}
}
上面示例中,constructor()
方法和toString()
方法内部,都出现了super
关键字。super
在这里表示父类的构造函数,用来新建一个父类的实例对象。
ES6 规定,子类必须在constructor()
方法中调用super()
,否则就会报错。这是因为子类自己的this
对象,必须先通过父类的构造函数完成塑造,得到与父类同样的实例属性和方法,然后再对其进行加工,添加子类自己的实例属性和方法。如果不调用super()
方法,子类就得不到自己的this
对象。
class Point { /* ... */ }
class Student extends Point {
constructor() {
}
}
let cp = new Student(); // ReferenceError
class Foo {
constructor() {
console.log(1);
}
}
class Bar extends Foo {
constructor() {
super();
console.log(2);
}
}
const bar = new Bar();
// 1
// 2
class Point {
constructor(x, y) {
this.x = x;
this.y = y;
}
}
class ColorPoint extends Point {
constructor(x, y, color) {
this.color = color; // ReferenceError
super(x, y);
this.color = color; // 正确
}
}
class ColorPoint extends Point {
}
// 等同于
class ColorPoint extends Point {
constructor(...args) {
super(...args);
}
}
有了子类的定义,就可以生成子类的实例了。
let cp = new ColorPoint(25, 8, 'green');
cp instanceof ColorPoint // true
cp instanceof Point // true
class A {
static hello() {
console.log('hello world');
}
}
class B extends A {
}
B.hello() // hello world
class Foo {
#p = 1;
#m() {
console.log('hello');
}
}
class Bar extends Foo {
constructor() {
super();
console.log(this.#p); // 报错
this.#m(); // 报错
}
}
class Foo {
#p = 1;
getP() {
return this.#p;
}
}
class Bar extends Foo {
constructor() {
super();
console.log(this.getP()); // 1
}
}
super关键字
class A {}
class B extends A {
constructor() {
super();
}
}
class A {
constructor() {
console.log(new.target.name);
}
}
class B extends A {
constructor() {
super();
}
}
new A() // A
new B() // B
class A {}
class B extends A {
m() {
super(); // 报错
}
}
class A {
p() {
return 2;
}
}
class B extends A {
constructor() {
super();
console.log(super.p()); // 2
}
}
let b = new B();
class A {
constructor() {
this.p = 2;
}
}
class B extends A {
get m() {
return super.p;
}
}
let b = new B();
b.m // undefined
class A {}
A.prototype.x = 2;
class B extends A {
constructor() {
super();
console.log(super.x) // 2
}
}
let b = new B();
class A {
constructor() {
this.x = 1;
}
print() {
console.log(this.x);
}
}
class B extends A {
constructor() {
super();
this.x = 2;
}
m() {
super.print();
}
}
let b = new B();
b.m() // 2
class A {
constructor() {
this.x = 1;
}
}
class B extends A {
constructor() {
super();
this.x = 2;
super.x = 3;
console.log(super.x); // undefined
console.log(this.x); // 3
}
}
let b = new B();
class Parent {
static myMethod(msg) {
console.log('static', msg);
}
myMethod(msg) {
console.log('instance', msg);
}
}
class Child extends Parent {
static myMethod(msg) {
super.myMethod(msg);
}
myMethod(msg) {
super.myMethod(msg);
}
}
Child.myMethod(1); // static 1
var child = new Child();
child.myMethod(2); // instance 2
class A {
constructor() {
this.x = 1;
}
static print() {
console.log(this.x);
}
}
class B extends A {
constructor() {
super();
this.x = 2;
}
static m() {
super.print();
}
}
B.x = 3;
B.m() // 3
class A {}
class B extends A {
constructor() {
super();
console.log(super); // 报错
}
}
class A {}
class B extends A {
constructor() {
super();
console.log(super.valueOf() instanceof B); // true
}
}
let b = new B();
var obj = {
toString() {
return "MyObject: " + super.toString();
}
};
obj.toString(); // MyObject: [object Object]