JS-ES6 Study note-class (4)

1 Object.getPrototypeOf . The method can be used to get the parent class from the subclass. Therefore, you can use this method to determine whether a class inherits another class.

2, super this keyword, can be used as a function, can also be used as an object. In both cases, its usage is completely different.

The first case, super as a function call, represents the constructor of the parent class. ES6 requirements, the constructor of a subclass must perform a super function once .

Class A {}class B extends a {  constructor () {    super ();  }}

Note that although the constructor for the parent class is represented, an instance of the subclass is returned, that is super A , the B super internal this refers B to, so it is super() equivalent here A.prototype.constructor.call(this) .

The second case, super as an object, points to the prototype object of the parent class .

class A {  p () {    return 2;  }} Class B extends A {  constructor () {    super ();     // 2 New B ();    

In a subclass B super.p() , super It is used as an object. At this super point, pointing A.prototype , so the super.p() equivalent A.prototype.p() . It is important to note that because of the super prototype object pointing to the parent class, the method or property defined on the parent class instance cannot be super called .

ES6 Specifies that super when a method of the parent class is called, super the this subclass is bound .

3, because of the binding subclass this , if by super assigning a value to a property, then that super is, the this assignment of the property will become the property of the child class instance.

class A {  constructor () {    this. x =1;  }} Class B extends A {  constructor () {    super ();     this. x = 2;     = 3;     // undefined    Console.log (this//  3  new B ();

In the above code, the super.x assignment is 3 equal to the this.x value 3 assigned to. And when read super.x , read is A.prototype.x , so return undefined .

4, when using, super you must explicitly specify whether as a function, or as an object to use, or will error.

5, prototype of the subclass, is the prototype of the parent class. As a result, the behavior of __proto__.__proto__ the parent class instance can be modified through the properties of the subclass instance.

JS-ES6 Study note-class (4)