在MDN文章《面向对象Javascript介绍》中关于继承的部分,我注意到他们设置了prototype.constructor:

// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;  

这有什么重要的目的吗?可以省略吗?


当前回答

下面是MDN的一个例子,我发现它对理解它的用法很有帮助。

在JavaScript中,我们有返回AsyncFunction对象的async函数。AsyncFunction不是一个全局对象,但可以通过使用构造函数属性检索它并利用它。

function resolveAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

// AsyncFunction constructor
var AsyncFunction = Object.getPrototypeOf(async function(){}).constructor

var a = new AsyncFunction('a', 
                          'b', 
                          'return await resolveAfter2Seconds(a) + await resolveAfter2Seconds(b);');

a(10, 20).then(v => {
  console.log(v); // prints 30 after 4 seconds
});

其他回答

得到了一个很好的代码示例,为什么真的有必要设置原型构造函数..

function CarFactory(name){ 
   this.name=name;  
} 
CarFactory.prototype.CreateNewCar = function(){ 
    return new this.constructor("New Car "+ this.name); 
} 
CarFactory.prototype.toString=function(){ 
    return 'Car Factory ' + this.name;
} 

AudiFactory.prototype = new CarFactory();      // Here's where the inheritance occurs 
AudiFactory.prototype.constructor=AudiFactory;       // Otherwise instances of Audi would have a constructor of Car 

function AudiFactory(name){ 
    this.name=name;
} 

AudiFactory.prototype.toString=function(){ 
    return 'Audi Factory ' + this.name;
} 

var myAudiFactory = new AudiFactory('');
  alert('Hay your new ' + myAudiFactory + ' is ready.. Start Producing new audi cars !!! ');            

var newCar =  myAudiFactory.CreateNewCar(); // calls a method inherited from CarFactory 
alert(newCar); 

/*
Without resetting prototype constructor back to instance, new cars will not come from New Audi factory, Instead it will come from car factory ( base class )..   Dont we want our new car from Audi factory ???? 
*/

它并不总是必要的,但它确实有它的用途。假设我们想在基类Person上创建一个复制方法。是这样的:

// define the Person Class  
function Person(name) {
    this.name = name;
}  

Person.prototype.copy = function() {  
    // return new Person(this.name); // just as bad
    return new this.constructor(this.name);
};  

// define the Student class  
function Student(name) {  
    Person.call(this, name);
}  

// inherit Person  
Student.prototype = Object.create(Person.prototype);

现在,当我们创建一个新的Student并复制它时会发生什么?

var student1 = new Student("trinth");  
console.log(student1.copy() instanceof Student); // => false

副本不是Student的实例。这是因为(如果没有显式检查),我们将无法从“基”类返回Student副本。我们只能返回Person。然而,如果我们重置了构造函数:

// correct the constructor pointer because it points to Person  
Student.prototype.constructor = Student;

...然后一切都按照预期进行:

var student1 = new Student("trinth");  
console.log(student1.copy() instanceof Student); // => true

现在不需要加糖的函数'类'或使用'New'。使用对象字面量。

Object原型已经是一个“类”。当你定义一个对象文字时,它已经是原型对象的一个实例。这些也可以作为另一个对象的原型,等等。

const Person = {
  name: '[Person.name]',
  greeting: function() {
    console.log( `My name is ${ this.name || '[Name not assigned]' }` );
  }
};
// Person.greeting = function() {...} // or define outside the obj if you must

// Object.create version
const john = Object.create( Person );
john.name = 'John';
console.log( john.name ); // John
john.greeting(); // My name is John 
// Define new greeting method
john.greeting = function() {
    console.log( `Hi, my name is ${ this.name }` )
};
john.greeting(); // Hi, my name is John

// Object.assign version
const jane = Object.assign( Person, { name: 'Jane' } );
console.log( jane.name ); // Jane
// Original greeting
jane.greeting(); // My name is Jane 

// Original Person obj is unaffected
console.log( Person.name ); // [Person.name]
console.log( Person.greeting() ); // My name is [Person.name]

这里值得一读:

Class-based object-oriented languages, such as Java and C++, are founded on the concept of two distinct entities: classes and instances. ... A prototype-based language, such as JavaScript, does not make this distinction: it simply has objects. A prototype-based language has the notion of a prototypical object, an object used as a template from which to get the initial properties for a new object. Any object can specify its own properties, either when you create it or at run time. In addition, any object can be associated as the prototype for another object, allowing the second object to share the first object's properties

给定简单构造函数:

function Person(){
    this.name = 'test';
}


console.log(Person.prototype.constructor) // function Person(){...}

Person.prototype = { //constructor in this case is Object
    sayName: function(){
        return this.name;
    }
}

var person = new Person();
console.log(person instanceof Person); //true
console.log(person.sayName()); //test
console.log(Person.prototype.constructor) // function Object(){...}

默认情况下(来自规范https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor),所有原型都会自动获得一个称为constructor的属性,该属性指向属性所在的函数。 根据构造函数的不同,可能会将其他属性和方法添加到原型中,这不是很常见的做法,但仍然允许进行扩展。

所以简单地回答:我们需要确保原型中的值。构造函数被正确地设置为规范所假定的样子。

我们必须总是正确地设置这个值吗?它有助于调试,并使内部结构与规范保持一致。当我们的API被第三方使用时,我们肯定应该这样做,但当代码最终在运行时执行时,我们不应该这样做。

这是必要的。类继承中的任何类都必须有自己的构造函数,在原型继承中也是如此。它也方便对象的构建。但这个问题是不必要的,真正需要理解的是在JavaScript世界中调用函数作为构造函数的效果和解析对象属性的规则。

使用表达式new <函数名>([parameters])将函数作为构造函数执行的效果

创建类型名称为函数名的对象 函数中的内部属性附加到创建的对象 函数的属性原型作为原型自动附加到创建的对象上

对象属性解析规则

不仅要在对象上查找属性,还要在对象的原型、原型的原型上查找属性,以此类推,直到找到具有匹配名称的属性或到达原型链的末尾。

基于这些底层机制,语句<构造函数名称>.prototype. conf。构造函数= <构造函数名称>等于在构造函数体中附加表达式this的效果。构造函数= <构造函数名称>。如果第二次发声,构造函数将在对象上解析;如果第一次发声,构造函数将在对象的原型上解析。