ES6 之 Class 的基本语法和继承
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2020-12-31 19:48
Class 类基本用法
class Person {
constructor(name,age) {
this.name = name;
this.age = age;
}
getName(){
console.log(`My name is ${this.name}!`)
}
static sayHi() {
console.log('Hi');
}
}
let p = new Person();
typeof Person // "function"
p instanceof Person // true
constructor 方法
class Person {}
Person.prototype // {construtor:f}
class Foo {
constructor(){
return Object.create(null);
}
}
new Foo() instanceof Foo // false
class Foo {
constructor(){}
}
Foo() // TypeError: Class constructor Foo cannot be invoked without 'new'
Class 的自定义方法
class Person {
constructor() {}
aaa(){}
bbb(){}
}
Object.getOwnPropertyNames(Person.prototype) // ["constructor", "aaa", "bbb"]
Person.prototype.constructor === Person // true
p.constructor === Person // true
Object.assign
向 prototype
一次性添加多个。class Person {
constructor() {}
}
Object.assign(Person.prototype, {
aaa(){},
bbb(){}
})
function
关键字,也不需要逗号分隔。Class
内部所有定义的方法,都是不可枚举的。class Person {
constructor() {}
aaa(){}
}
Object.keys(Person.prototype) // []
Object.getOwnPropertyNames(Person.prototype) // ["constructor", "aaa"]
prototype
原型定义的方法是可枚举的。let Person = function(){};
Person.prototype.aaa = function(){};
Object.keys(Person.prototype) // ["aaa"]
Object.getOwnPropertyNames(Person.prototype) // ["constructor", "aaa"]
let methodName = 'getName';
class Person {
constructor() {}
[methodName](){}
}
Object.getOwnPropertyNames(Person.prototype) // ["constructor", "getName"]
取值函数 getter 和存值函数 setter
class Person {
constructor() {
this.name = 'dora';
}
get author() {
return this.name;
}
set author(value) {
this.name = this.name + value;
console.log(this.name);
}
}
let p = new Person();
p.author // dora
p.author = 666; // dora666
Object.getOwnPropertyNames(Person.prototype) // ["constructor", "author"]
Object.getOwnPropertyDescriptor(Person.prototype,'author')
// { get: ƒ author()(),
// set: ƒ author()(value),
// ...
// }
Class 的 static 静态方法
class Person {
static sayHi() {
console.log('Hi');
}
}
Person.sayHi() // "Hi"
let p = new Person();
p.sayHi() // TypeError: p.sayHi is not a function
this
关键字,这个 this
指的是类,而不是实例。静态方法可以与非静态方法重名。class Person {
static sayHi() {
this.hi();
}
static hi(){
console.log('hello')
}
hi(){
console.log('world')
}
}
Person.sayHi() // "hello"
实例属性的另一种写法
class Person {
name = 'dora';
getName() {
return this.name;
}
}
let p = new Person();
p.name // "dora"
Object.keys(p) // ["name"]
Class 的继承
class Person {
constructor() {}
sayHi() {
return 'Hi';
}
}
class Teacher extends Person {
constructor() {
super();
}
}
let t = new Teacher();
t.sayHi(); // "Hi"
子类的 constructor
class Teacher extends Person {
}
let t = new Teacher();
t.sayHi(); // "Hi"
class Teacher extends Person {
constructor(...args) {
super(...args);
}
}
super 关键字
super() 方法
class A {
constructor() {
console.log(this.constructor.name)
}
}
class B extends A {
constructor() {
super();
}
}
new A() // A
new B() // B
super 对象
class A {
constructor() {
this.a = 3;
}
p() {return 2;}
}
A.prototype.m = 6;
class B extends A {
constructor() {
super();
console.log(super.a); // undefined
console.log(super.p()); // 2
console.log(super.m); // 6
}
}
new B();
super
调用父类方法时,方法内部的 this
指向当前的子类实例。class A {
constructor() {
this.x = 'a';
}
aX() {
console.log(this.x);
}
}
class B extends A {
constructor() {
super();
this.x = 'b';
}
bX() {
super.aX();
}
}
(new B()).bX() // 'b'
class A {
static m(msg) {
console.log('static', msg);
}
m(msg) {
console.log('instance', msg);
}
}
class B extends A {
static m(msg) {
super.m(msg);
}
m(msg) {
super.m(msg);
}
}
B.m(1); // "static" 1
(new B()).m(2) // "instance" 2
任意对象的 super
let obj = {
m() {
return super.constructor.name;
}
};
obj.m(); // Object
class B extends A {
m() {
console.log(super);
}
}
// SyntaxError: 'super' keyword unexpected here
静态方法的继承
class Person {
static sayHi() {
return 'hello';
}
}
class Teacher extends Person {
}
Teacher.sayHi() // "hello"
class Teacher extends Person {
static sayHi() {
super.sayHi();
}
}
Teacher.sayHi() // "hello"
new.target 属性
class Shape {
constructor() {
if(new.target === Shape) {
throw new Error('本类不能实例化');
}
}
}
class Circle extends Shape {
constructor(radius) {
super();
console.log('ok');
}
}
let s = new Shape(); // 报错
let cir = new Circle(6); // 'ok'
本文完〜
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