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javascript 类
In 2015 the ECMAScript 6 (ES6) standard introduced classes.
2015年,ECMAScript 6(ES6)标准引入了类。
JavaScript has a quite uncommon way to implement inheritance: prototypical inheritance. , while in my opinion great, is unlike most other popular programming language’s implementation of inheritance, which is class-based.
JavaScript有一种非常不常见的实现继承的方法:原型继承。 在我看来, 虽然很棒,但与大多数其他流行的编程语言基于类的继承不同。
People coming from Java or Python or other languages had a hard time understanding the intricacies of prototypal inheritance, so the ECMAScript committee decided to sprinkle syntactic sugar on top of prototypical inheritance so that it resembles how class-based inheritance works in other popular implementations.
来自Java或Python或其他语言的人很难理解原型继承的复杂性,因此ECMAScript委员会决定在原型继承的基础上撒上语法糖,以使其类似于基于类的继承在其他流行实现中的工作方式。
This is important: JavaScript under the hood is still the same, and you can access an object prototype in the usual way.
这很重要:底层JavaScript仍然相同,并且您可以以通常的方式访问对象原型。
This is how a class looks.
这是一个类的外观。
class Person { constructor(name) { this.name = name } hello() { return 'Hello, I am ' + this.name + '.' }}
A class has an identifier, which we can use to create new objects using new ClassIdentifier()
.
一个类具有一个标识符,我们可以使用它使用new ClassIdentifier()
创建新对象。
When the object is initialized, the constructor
method is called, with any parameters passed.
初始化对象时,将调用传递任何参数的constructor
方法。
A class also has as many methods as it needs. In this case hello
is a method and can be called on all objects derived from this class:
一个类还具有所需的许多方法。 在这种情况下, hello
是一种方法,可以在派生自此类的所有对象上调用:
const flavio = new Person('Flavio')flavio.hello()
A class can extend another class, and objects initialized using that class inherit all the methods of both classes.
一个类可以扩展另一个类,并且使用该类初始化的对象将继承这两个类的所有方法。
If the inherited class has a method with the same name as one of the classes higher in the hierarchy, the closest method takes precedence:
如果继承的类具有与层次结构中较高级别的类之一名称相同的方法,则最接近的方法优先:
class Programmer extends Person { hello() { return super.hello() + ' I am a programmer.' }}const flavio = new Programmer('Flavio')flavio.hello()
(the above program prints “Hello, I am Flavio. I am a programmer.”)
(上面的程序打印“ 您好,我是Flavio。我是程序员。 ”)
Classes do not have explicit class variable declarations, but you must initialize any variable in the constructor.
类没有显式的类变量声明,但是必须在构造函数中初始化任何变量。
Inside a class, you can reference the parent class calling super()
.
在类内部,您可以引用父类调用super()
。
Normally methods are defined on the instance, not on the class.
通常,方法是在实例上定义的,而不是在类上定义的。
Static methods are executed on the class instead:
静态方法改为在类上执行:
class Person { static genericHello() { return 'Hello' }}Person.genericHello() //Hello
JavaScript does not have a built-in way to define private or protected methods.
JavaScript没有内置的方法来定义私有或受保护的方法。
There are workarounds, but I won’t describe them here.
有解决方法,但在此不再赘述。
You can add methods prefixed with get
or set
to create a getter and setter, which are two different pieces of code that are executed based on what you are doing: accessing the variable, or modifying its value.
您可以添加以get
或set
开头的方法来创建getter和setter,这是基于您正在执行的操作的两个不同的代码段:访问变量或修改其值。
class Person { constructor(name) { this._name = name } set name(value) { this._name = value } get name() { return this._name }}
If you only have a getter, the property cannot be set, and any attempt at doing so (outside of the constructor, which sets the value when you initialize a new object with this class) will be ignored:
如果只有getter,则无法设置该属性,并且这样做的任何尝试(在构造函数之外,当您使用此类初始化新对象时都会设置值)将被忽略:
class Person { constructor(name) { this._name = name } get name() { return this._name }}
If you only have a setter, you can change the value but not access it from the outside:
如果只有二传手,则可以更改值,但不能从外部访问它:
class Person { constructor(name) { this._name = name } set name(value) { this._name = value }}
Getters and setters are very useful when you want to execute some code upon changing the property value, or if you want to create a “computed” property. You can alter the values you return by using a getter.
当您想在更改属性值时执行一些代码,或者要创建“计算”属性时,getter和setter很有用。 您可以使用getter更改返回的值。
You can also run some code, like logging to the console or to a file when a value is changed.
您还可以运行一些代码,例如在更改值时登录到控制台或文件。
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javascript 类
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