JavaScript Advanced Programming Reading Notes (14) Implementation of js inheritance mechanism_javascript skills

Inheritance

Inheritance is an essential feature of object-oriented languages, that is, one class can reuse the methods and properties of another class. There are five main ways to implement inheritance in JavaScript: object impersonation, call(), apply(), prototype chain, and hybrid methods.

They are introduced below.

Object impersonation

Principle: The constructor uses the this keyword to assign values ​​to all properties and methods. Because the constructor is just a function, you can make ClassA's constructor a method of ClassB and then call it. ClassB will receive the properties and methods defined in the constructor of ClassA.

Example:

Copy code The code is as follows:

function ClassA(sColor ){
this.color=sColor;
this.sayColor=function(){
alert(this.color);
}
}

function ClassB(sColor ,sName){
this.newMethod=ClassA;
this.newMethod(sColor);
delete this.newMethod;

this.name=sName;
this.sayName= function(){
alert(this.name);
}
}

Call:

Copy code The code is as follows:

var objb=new ClassB("blue","Test");
objb.sayColor();//
blueobjb.sayName(); // Test　

Note: The reference to ClassA must be deleted here, otherwise new methods and attributes defined later will overwrite the relevant attributes and methods of the super class. Multiple inheritance can be achieved in this way.

call() method

Due to the popularity of object impersonation methods, two new methods call() and apply( were added to the Function object in the third edition of ECMAScript ) method to achieve similar functions.

The first parameter of the call() method is used as the object of this, and the other parameters are passed directly to the function itself. Example:

Copy code The code is as follows:

function sayColor(sPrefix,sSuffix){
alert(sPrefix this.color sSuffix);
}

var obj=new Object();
obj.color="red";

//output The color is red, a very nice color indeed.
sayColor.call(obj,"The color is ",", a very nice color indeed.");

Use this method to implement inheritance , just replace the assignment, call, and deletion codes in the first three lines:

Copy the code The code is as follows:

function ClassB(sColor,sName){
//this.newMethod=ClassA;
//this.newMethod(sColor);
//delete this.newMethod;
ClassA.call(this,sColor);

this.name=sName;
this.sayName=function(){
alert(this.name);
}
}

apply() method

The apply() method is similar to the call() method, except that the second parameter is in apply( ) method is passed an array.

Copy code The code is as follows:

function sayColor(sPrefix,sSuffix){
alert (sPrefix this.color sSuffix);
}

var obj=new Object();
obj.color="red";

//output The color is red , a very nice color indeed.
sayColor.apply(obj,new Array("The color is ",", a very nice color indeed."));

Use this method to To implement inheritance, you only need to replace the assignment, call, and deletion codes in the first three lines:

Copy the code The code is as follows:

function ClassB(sColor,sName){
//this.newMethod=ClassA;
//this.newMethod(sColor);
//delete this.newMethod;
ClassA.apply(this,new Array(sColor));

this.name=sName;
this.sayName=function(){
alert(this.name);
}
}

One difference from call() is that if the order of parameters in the super class is exactly the same as the order of parameters in the subclass, arguments can be used as the second parameter.

Prototype chain

This form of inheritance was originally used for prototype chain in ECMAScript. Any properties and methods of the Prototype object are passed to all instances of that class. The prototype chain utilizes this functionality to implement the inheritance mechanism.

Example of using prototype chain to implement inheritance:

Copy code The code is as follows:

function ClassA(){
}

ClassA.prototype.color="red";
ClassA.prototype.sayColor=function(){
alert(this.color);
};

function ClassB(){
}

ClassB.prototype=new ClassA();

Note: Call the constructor of ClassA , no parameters are passed to it. It's standard practice in the prototype chain to ensure that the constructor doesn't have any parameters.

Hybrid method

This method mixes object impersonation and prototype chaining methods. Example:

Copy code The code is as follows:

function ClassA(sColor){
this .color=sColor;
}
ClassA.prototype.sayColor=function(){
alert(this.color);
}

function ClassB(sColor,sName){
ClassA.call(this,sColor);
this.name=sName;
}

ClassB.prototype=new ClassA();
ClassB.prototype.sayName=function (){
alert(this.name);
}

Call example:

Copy code The code is as follows:

var objb=new ClassB("red","test");
objb.sayColor();// output red
objb. sayName();// output test

Author: Artwl