174 lines
4.9 KiB
Markdown
174 lines
4.9 KiB
Markdown
---
|
||
title: Classes
|
||
localeTitle: 类
|
||
---
|
||
## 类
|
||
|
||
JavaScript本身没有类的概念。
|
||
|
||
但是我们可以通过利用JavaScript的原型性质来模拟类的功能。
|
||
|
||
本文假设您对[原型](/src/pages/javascript/prototypes/index.md)有基本的了解。
|
||
|
||
为了清楚起见,我们假设我们想要创建一个可以执行以下操作的类
|
||
|
||
```javascript
|
||
var p = new Person('James','Bond'); // create a new instance of Person class
|
||
p.log() // Output: 'I am James Bond' // Accessing a function in the class
|
||
// Using setters and getters
|
||
p.profession = 'spy'
|
||
p.profession // output: James bond is a spy
|
||
```
|
||
|
||
### 使用class关键字
|
||
|
||
与任何其他编程语言一样,您现在可以使用`class`关键字来创建类。
|
||
|
||
旧版浏览器不支持此功能,并且已在ECMAScript 2015中引入。
|
||
|
||
```javascript
|
||
class Person {
|
||
constructor(firstName, lastName) {
|
||
this._firstName = firstName;
|
||
this._lastName = lastName;
|
||
}
|
||
|
||
log() {
|
||
console.log('I am', this._firstName, this._lastName);
|
||
}
|
||
|
||
// setters
|
||
set profession(val) {
|
||
this._profession = val;
|
||
}
|
||
// getters
|
||
get profession() {
|
||
console.log(this._firstName, this._lastName, 'is a', this._profession);
|
||
}
|
||
|
||
}
|
||
```
|
||
|
||
|
||
|
||
|
||
`class`只是JavaScript现有的基于原型的继承模型的语法糖。
|
||
|
||
通常,程序员使用以下方法在JavaScript中创建类。
|
||
|
||
### 使用添加到原型的方法:
|
||
|
||
在这里,所有方法都被添加到原型中
|
||
|
||
```javascript
|
||
function Person(firstName, lastName) {
|
||
this._firstName = firstName;
|
||
this._lastName = lastName;
|
||
}
|
||
|
||
Person.prototype.log = function() {
|
||
console.log('I am', this._firstName, this._lastName);
|
||
}
|
||
|
||
// This line adds getters and setters for the profession object. Note that in general you could just write your own get and set functions like the 'log' method above.
|
||
// Since in this example we are trying the mimic the class above, we try to use the getters and setters property provided by JavaScript
|
||
Object.defineProperty(Person.prototype, 'profession', {
|
||
set: function(val) {
|
||
this._profession = val;
|
||
},
|
||
get: function() {
|
||
console.log(this._firstName, this._lastName, 'is a', this._profession);
|
||
}
|
||
})
|
||
```
|
||
|
||
您还可以在函数`Person`编写原型方法,如下所示
|
||
|
||
```javascript
|
||
Person.prototype = {
|
||
log: function() {
|
||
console.log('I am ', this._firstName, this._lastName);
|
||
}
|
||
set profession(val) {
|
||
this._profession = val;
|
||
}
|
||
|
||
get profession() {
|
||
console.log(this._firstName, this._lastName, 'is a', this._profession);
|
||
}
|
||
|
||
}
|
||
```
|
||
|
||
### 使用内部添加的方法
|
||
|
||
这里的方法是内部添加而不是原型
|
||
|
||
```javascript
|
||
function Person(firstName, lastName) {
|
||
this._firstName = firstName;
|
||
this._lastName = lastName;
|
||
|
||
this.log = function() {
|
||
console.log('I am ', this._firstName, this._lastName);
|
||
}
|
||
|
||
Object.defineProperty(this, 'profession', {
|
||
set: function(val) {
|
||
this._profession = val;
|
||
},
|
||
get: function() {
|
||
console.log(this._firstName, this._lastName, 'is a', this._profession);
|
||
}
|
||
})
|
||
}
|
||
```
|
||
|
||
### 用符号隐藏类中的细节
|
||
|
||
大多数情况下,必须隐藏一些属性和方法以防止从函数外部进行访问。对于类,要获得此功能,一种方法是使用符号。 Symbol是一种新的内置JavaScript类型,可以调用它来提供新的符号值。每个符号都是唯一的,可以用作对象的键。因此,符号的一个用例是,您可以向您可能不拥有的对象添加内容,并且您可能不希望与对象的任何其他键冲突,因此创建新的并使用符号将该属性添加到该对象是最安全的。此外,当符号值添加到对象时;没有人会知道如何得到它。
|
||
|
||
```javascript
|
||
class Person {
|
||
constructor(firstName, lastName) {
|
||
this._firstName = firstName;
|
||
this._lastName = lastName;
|
||
}
|
||
|
||
log() {
|
||
console.log('I am', this._firstName, this._lastName);
|
||
}
|
||
|
||
// setters
|
||
set profession(val) {
|
||
this._profession = val;
|
||
}
|
||
// getters
|
||
get profession() {
|
||
console.log(this._firstName, this._lastName, 'is a', this._profession);
|
||
}
|
||
// With the above code, even though we can access the properties outside the function to change their content what if we don't want that.
|
||
// Symbols come to rescue.
|
||
let s_firstname = new Symbol();
|
||
|
||
class Person {
|
||
constructor(firstName, lastName) {
|
||
this[s_firstName] = firstName;
|
||
this._lastName = lastName;
|
||
}
|
||
|
||
log() {
|
||
console.log('I am', this._firstName, this._lastName);
|
||
}
|
||
|
||
// setters
|
||
set profession(val) {
|
||
this._profession = val;
|
||
}
|
||
// getters
|
||
get profession() {
|
||
console.log(this[s_firstName], this._lastName, 'is a', this._profession);
|
||
}
|
||
```
|
||
|
||
#### 更多信息: |