Understanding prototypes in JavaScript

Phototype in JS is a difficult part to understand in JS. JavaScript methods can be divided into three categories: class methods, object methods, and prototype methods.

Example:

function People(name)
{
    this.name=name;
    //对象方法
    this.Introduce=function(){
        alert("My name is "+this.name);
    }
}
//类方法
People.Run=function(){
    alert("I can run");
}
//原型方法
People.prototype.IntroduceChinese=function(){
    alert("我的名字是"+this.name);
}
//测试
var p1=new People("Windking");
p1.Introduce();
People.Run();
p1.IntroduceChinese();

obj1.func.call(obj) method means treating obj as obj1 and calling the func method.

What does prototype mean?

Every object in JavaScript has a prototype attribute. The explanation of the prototype attribute of an object in Javascript is: return a reference to the prototype of the object type.

?

A.prototype = new B();

Understanding prototype should not be confused with inheritance. The prototype of A is an instance of B. It can be understood that A has cloned all the methods and properties in B. A can use B's methods and properties. The emphasis here is on cloning rather than inheritance. This situation can occur: A's prototype is an instance of B, and B's prototype is also an instance of A.

Let’s look at an experimental example first:

function baseClass()
{
    this.showMsg = function()
    {
        alert("baseClass::showMsg");   
    }
}
function extendClass()
{
     
}
extendClass.prototype = new baseClass();
instance = new extendClass();
instance.showMsg(); // 显示baseClass::showMsg

We first define the baseClass class, and then we define extentClass, but we plan to use an instance of baseClass as the prototype, and the extendedClass to be cloned also contains the showMsg object method. extendClass.prototype = new baseClass() can be read as: extendClass is created by cloning an instance of baseClass as the prototype.

Then there will be a question, what if extendClass itself contains a method with the same name as the method of baseClass?

The following is extended experiment 2:

function baseClass()
{
    this.showMsg = function()
    {
        alert("baseClass::showMsg");   
    }
}
function extendClass()
{
    this.showMsg =function ()
    {
        alert("extendClass::showMsg");
    }
}
extendClass.prototype = new baseClass();
instance = new extendClass();
instance.showMsg();//显示extendClass::showMsg

The experiment proves: when the function is running, it will first look for the function in the ontology. If it is found, it will be run. If it cannot be found, it will look for the function in the prototype. Or it can be understood that the prototype will not clone the function with the same name.

Then there will be a new question: What if I want to use an instance of extendClass to call the object method showMsg of baseClass?

The answer is that you can use call:

extendClass.prototype = new baseClass();
instance = new extendClass();
var baseinstance = new baseClass();
baseinstance.showMsg.call(instance);//显示baseClass::showMsg

baseinstance.showMsg.call(instance); here reads as "call instance as baseinstance and call its object method showMsg". Okay, someone may ask here, why not use baseClass.showMsg.call(instance); this is the difference between object methods and class methods. What we want to call is the object method of baseClass

Finally, if you understand the following code clearly, Then you can understand what this article says:

<script type="text/javascript">
function baseClass()
{
    this.showMsg = function()
    {
        alert("baseClass::showMsg");   
    }
    
    this.baseShowMsg = function()
    {
        alert("baseClass::baseShowMsg");
    }
}
baseClass.showMsg = function()
{
    alert("baseClass::showMsg static");
}
function extendClass()
{
    this.showMsg =function ()
    {
        alert("extendClass::showMsg");
    }
}
extendClass.showMsg = function()
{
    alert("extendClass::showMsg static")
}
extendClass.prototype = new baseClass();
instance = new extendClass();
instance.showMsg(); //显示extendClass::showMsg
instance.baseShowMsg(); //显示baseClass::baseShowMsg
instance.showMsg(); //显示extendClass::showMsg
baseClass.showMsg.call(instance);//显示baseClass::showMsg static
var baseinstance = new baseClass();
baseinstance.showMsg.call(instance);//显示baseClass::showMsg
</script>