JS Pro-Detailed explanation of inheritance in object-oriented programming_javascript skills

Prototype chaining:

Use prototypes to inherit properties and methods. Review the relationship between constructor, prototype and instance. Each constructor has a prototype attribute, which points to a prototype object; the prototype object also has a constructor attribute, which points to the function; and the instance also has an internal pointer (__proto__) pointing to the prototype object. What would happen if this prototype object was an instance of another object? In this way, the prototype object contains a pointer to another type. Correspondingly, the other prototype also contains a pointer to another constructor.

JS inheritance is very simple, that is, set the prototype of the subclass to an (instantiated) object of the parent class

Copy code The code is as follows:

function SuperType(){
this.property = true;
}

SuperType.prototype.getSuperValue = function(){
Return this.property;
};

function SubType(){
this.subproperty = false;
}

//inherit from SuperType
SubType .prototype = new SuperType();

SubType.prototype.getSubValue = function (){
return this.subproperty;
};

var instance = new SubType() ;
alert(instance.getSuperValue()); //true

Final result: instance's __proto__ points to the SubType.prototype object, and the __proto__ attribute of the SubType.prototype object And points to the SuperType.prototype object. getSuperValue() is a method, so it still exists in the prototype, and property is an instance property, so it now exists in the instance of SubType.prototype. instance.constructor now points to SuperType. This is because SubType.prototype points to SuperType.prototype, and the constructor property of SuperType.prototype points to the SuperType function, so instance.constructor points to SuperType.

By default, all reference types inherit from Object. This is because the prototype object of all functions is an instance of Object by default, so the internal prototype(__proto__) points to Object.Prototype.

Relationship between prototype and instance: You can use 2 methods to determine the relationship between prototype and instance.
- instancef operator: Use this operator to test the constructor that appears in the instance and prototype chain, and it will return true

Copy codeThe code is as follows:

alert(instance instanceof Object); //true
alert(instance instanceof SuperType); //true
alert(instance instanceof SubType); / /true

- isPrototypeOf() method: As long as it is a prototype that appears in the prototype chain, it can be said to be the prototype of the instance derived from the prototype chain.

Copy code The code is as follows:

alert(Object.prototype.isPrototypeOf(instance)); //true
alert(SuperType.prototype.isPrototypeOf(instance)); //true
alert(SubType.prototype.isPrototypeOf(instance)); //true

given Note on adding methods to a class: Sometimes we add methods to subclasses, or override some methods of the parent class. At this time, it should be noted that these methods must be defined after inheritance. In the following example, after SubType inherits SuperType, we add a new method getSubValue() to it and rewrite the getSuperValue() method. For the latter, only SubType instances will use the overridden method, and SuperType instances will still use the original getSuperValue() method.

Copy code The code is as follows:

function SuperType(){
　this.property = true;
}
SuperType.prototype.getSuperValue = function(){
return this.property;
};
function SubType(){
this.subproperty = false;
}
//inherit from SuperType
SubType.prototype = new SuperType();
//new method
SubType.prototype.getSubValue = function (){
　return this. subproperty;
};
//override existing method
SubType.prototype.getSuperValue = function (){
　return false;
};
var instance = new SubType();
alert(instance.getSuperValue()); //false

Another thing to note is that when implementing inheritance through the prototype chain, you cannot use object literals to create prototype methods, because this will overwrite the prototype chain. As shown in the code below, after SubType inherits SuperType, it uses object literals to add methods to the prototype. However, doing so will rewrite the SubType prototype. The rewritten SubType.prototype contains an instance of Object, thus cutting off the connection with SuperType relationship.

Copy code The code is as follows:

function SuperType(){
　this.property = true;
}
SuperType.prototype.getSuperValue = function(){
return this.property;
};
function SubType(){
this.subproperty = false;
}
//inherit from SuperType
SubType.prototype = new SuperType();
//try to add new methods - this nullifies the previous line
SubType.prototype = {
getSubValue: function (){
　 return this.subproperty;
　},
someOtherMethod: function (){
return false;
}
};
var instance = new SubType();
alert(instance.getSuperValue()); //error!

Prototype chain problem: Same as prototype, when using reference type value Sometimes, there will be problems with the prototype chain. Recall from the previous content that a prototype property containing a reference type value will be shared by all instances, which is why we define the reference type value in the constructor rather than in the prototype. When inheritance is implemented through the prototype chain, the prototype will actually become an instance of another type, so the original instance properties will smoothly become the current prototype properties.

Copy code The code is as follows:

function SuperType(){
　this.colors = ["red", "blue", "green"];
}
function SubType(){
}
//inherit from SuperType
SubType.prototype = new SuperType();
var instance1 = new SubType();
instance1.colors.push(“black”);
alert(instance1.colors); //”red,blue,green,black”
var instance2 = new SubType();
alert(instance2.colors); //"red,blue,green,black"

In the SuperType constructor, we define a colors array, Each SuperType instance will have its own colors array. But when SubType uses the prototype chain to inherit SuperType, SubType.prototype becomes an instance of SuperType, so it has its own colors attribute, that is, SubType.prototype.colors attribute. Therefore, when creating a SubType instance, all instances share this property. As shown in the code above.

The second problem is: when creating an instance of a subclass, parameters cannot be passed to the constructor of the superclass. In fact, it should be said that there is no way to pass parameters to the constructor of a superclass without affecting all object instances. Because of these issues, we will not use the prototype chain alone.

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Constructor stealing:
To solve the above problem, developers invented a technique called constructor stealing. The idea behind this technique is to call the supertype constructor inside the subtype constructor. (A function is nothing but an object that executes code in a specific environment?) We can execute a constructor on a newly created object by using the apply() or call() method.

Copy code The code is as follows:

function SuperType(){
　this.colors = ["red", "blue", "green"];
}
function SubType(){
　//inherit from SuperType
　SuperType.call(this);
}
var instance1 = new SubType();
instance1.colors.push(“black”);
alert(instance1.colors); //”red,blue,green,black”
var instance2 = new SubType();
alert(instance2.colors); //"red, blue, green"

We use the call() method in SubType to call the SuperType constructor. In fact It is to execute all the object initialization code defined in the SuperType() function on the new SubType object. The result is that each SubType instance has its own copy of the colors property.

Passing parameters: A big benefit of using the borrowed constructor method is that we can pass parameters from the constructor of the subclass to the constructor of the parent class.

Copy code The code is as follows:

function SuperType(name){
　this.name = name;
}
function SubType(){
　//inherit from SuperType passing in an argument
　SuperType. call(this, “Nicholas”);
　//instance property
　this.age = 29;
}
var instance = new SubType();
alert(instance.name); //"Nicholas";
alert(instance.age); //29

The new SuperType constructor adds a new parameter name. We pass it to SuperType while calling SuperType Parameter "Nicholas". In order to prevent the supertype constructor from overwriting the properties of the subtype, you can define the properties of the subclass after calling the supertype constructor.

Problems with borrowing constructors: Methods are all defined in the constructor and cannot be reused. Moreover, methods defined in the supertype's prototype are not visible to subtypes. As a result all types can only use the constructor pattern.

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Combined inheritance:
An inheritance pattern that combines the advantages of prototype chains and borrowed constructors. Use the prototype chain to inherit prototype properties and methods, and use borrowed constructors to inherit instance properties. As in the following example, we use the call() method to call the constructor of SuperType (each SubType instance has its own name and colors attributes, as well as the age attribute of SubType); then assign the SuperType instance to the SubType prototype to make it inherit. SuperType's sayName() method (this method is shared by each instance).

Copy code The code is as follows:

function SuperType(name){
this.name = name;
this.colors = ["red", "blue", "green"];
}

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

function SubType(name, age){
//inherit properties
SuperType.call(this, name);
this.age = age;
}
//inherit methods
SubType.prototype = new SuperType();
SubType.prototype.sayAge = function(){
alert(this.age);
};

var instance1 = new SubType("Nicholas", 29);
instance1.colors.push("black");
alert(instance1.colors); //"red,blue,green,black "
instance1.sayName(); //"Nicholas";
instance1.sayAge(); //29
var instance2 = new SubType("Greg", 27);
alert(instance2 .colors); //"red,blue,green"
instance2.sayName(); //"Greg";
instance2.sayAge(); //27

Prototypal Inheritance:

Copy code The code is as follows:

function object(o){
　function F(){}
　F.prototype = o;
　return new F();
}
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Parasitic Inheritance:

Same disadvantages as constructor