Typescript 类的属性/方法的默认可见性是什么

在Typescript中，默认情况下，Typescript类中所有属性或方法的可见性都是“public”。一个被标记为public的方法可以从任何地方访问，它没有限制。可见性有三种类型： public 、 private 和 protected 。

示例1：具有public成员可见性的类

公共函数和属性可以从任何地方访问。默认情况下，所有方法和属性都是“public”。在这个例子中，我们创建了一个名为student的类，它有两个属性和一个方法。我们使用public关键字来表示函数具有公共可见性，但是默认情况下所有属性和方法都是公共的，我们不需要指定。

Javascript

class Student { 
  name: string; 
  id: number; 
  constructor(name: string, id: number) { 
    this.name = name; 
    this.id = id; 
  } 
  public student_details() { 
    console.log( 
      "my name is : " + 
        this.name + 
        " my roll no is : " + 
        `${this.id.toString()}` 
    ); 
  } 
} 
  
let obj = new Student("rahul", 367236); 
obj.student_details();

输出: 已创建一个对象，并调用了student_details()方法。

my name is : rahul my roll no is : 367236

示例2：具有受保护成员可见性的类

具有“受保护”可见性的成员或属性只能在其类或其子类中访问。在这个例子中，在声明函数和属性’name’之前使用了 protected 关键字。类classRepresentative是从类Student派生而来的。当我们试图从外部访问“name”时，会返回一个错误，因为我们只能在该类或其子类中使用和访问它。名字也在student_name()函数中使用，但不会引发错误，因为它在类内部。由于student_details()是受保护的，所以不能从外部访问。

JavaScript

class Student { 
  protected name: string; 
  id: number; 
  constructor(name: string, id: number) { 
    this.name = name; 
    this.id = id; 
  } 
  protected student_details() { 
    console.log( 
      "my name is : " + 
        this.name + 
        " my roll no is : " + 
        `${this.id.toString()}` 
    ); 
  } 
  student_name() { 
    console.log("My name is : " + self.name); 
  } 
} 
  
class classRepresentative extends Student { 
  detail() { 
    console.log("I am the class representative, my name is :" 
      + this.name); 
  } 
} 
  
let obj = new Student("rahul", 367236); 
let obj1 = new classRepresentative("samuel", 287636); 
  
obj.name; // Error as name is protected 
obj.student_name(); // No error 
obj.student_details(); // Error 
obj1.detail(); // No error 
obj1.student_details(); //Error

输出：

error TS2445: Property 'name' is protected and only accessible within 
class 'Student' and its subclasses.
    obj.name; // error as name is protected
        ~~~~

two.ts:131:5 - error TS2445: Property 'student_details' is protected and only accessible 
within class 'Student' and its subclasses.
    obj.student_details(); //error
        ~~~~~~~~~~~~~~~

two.ts:133:6 - error TS2445: Property 'student_details' is protected and only accessible 
within class 'Student' and its subclasses.
    obj1.student_details(); //error
~~~~~~~~~~~~~~~

示例3：具有私有成员可见性的类

Private 与protected类似，但它还阻止子类访问成员。派生类无法改善其可见性，因为私有成员对派生类不可见。在前面的示例中，派生类在detail()函数中使用this.name属性，并且没有引发错误，因为该属性可以在子类中使用，但是当其可见性设置为private时，无法在子类或派生类中访问。

JavaScript

class Student { 
  private name: string; 
  id: number; 
  constructor(name: string, id: number) { 
    this.name = name; 
    this.id = id; 
  } 
  private student_details() { 
    console.log( 
      "my name is : " + 
        this.name + 
        " my roll no is : " + 
        `${this.id.toString()}` 
    ); 
  } 
  student_name() { 
    console.log("My name is : " + self.name); 
  } 
} 
class classRepresentative extends Student { 
  detail() { 
    console.log("I am the class representative, my name is :"
      + this.name); 
  } 
    
} 
  
let obj = new Student("rahul", 367236); 
let obj1 = new classRepresentative("samuel", 287636); 
  
obj.name; // Error as name is private 
obj.student_name(); // No error 
obj.student_details(); // Error 
obj1.detail(); // No error 
obj1.student_details(); // Error

输出:

error TS2341: Property 'name' is private and only accessible within class 'Student'.
        console.log("I am the class representative, my name is :" + this.name);
                                                                         ~~~~

two.ts:161:5 - error TS2341: Property 'name' is private and only accessible
 within class 'Student'.
    obj.name; // error as name is protected
        ~~~~

two.ts:163:5 - error TS2341: Property 'student_details' is private and only accessible 
within class 'Student'.
    obj.student_details(); //error
        ~~~~~~~~~~~~~~~

two.ts:165:6 - error TS2341: Property 'student_details' is private and only accessible 
within class 'Student'.
    obj1.student_details(); //error
~~~~~~~~~~~~~~~

参考： https://www.typescriptlang.org/docs/handbook/classes.html#public-private-and-protected-modifiers