Summary of methods for defining classes in JavaScript_Basic knowledge

There are many ways to define classes in JS:

1. Factory method

Copy code The code is as follows:

function Car(){
var ocar = new Object;
ocar.color = "blue";
ocar.doors = 4;
ocar.showColor = function(){
Document.write(this.color)
};
Return ocar;
}
var car1 = Car();
var car2 = Car();

When this function is called, a new object will be created and all properties and methods will be assigned to it. Use this function to create 2 objects with exactly the same properties. Of course, my sister can modify this method by passing parameters to it.

Copy code The code is as follows:

function Car(color,door){
var ocar = new Object;
ocar.color = color;
ocar.doors = door;
ocar.showColor = function(){
Document.write(this.color)
};
Return ocar;
}
var car1 = Car("red",4);
var car2 = Car("blue",4);
car1.showColor() //output:"red"
car2.showColor() //output:"blue"

Now you can get objects with different values ​​by passing different parameters to the function.
In the previous example, showcolor() is created every time the function Car() is called, which means that each object has its own showcolor() method.

But in fact, each object shares the same function.
Although it is possible to define a method outside a function and then point the function's properties to the method.

Copy code The code is as follows:

function showColor(){
alert(this.color);
}
function Car(){
var ocar = new Object();
ocar.color = color;
ocar.doors = door;
ocar.showColor = showColor;
Return ocar;
}

But this doesn’t look like a function method.
2. Constructor method
The constructor method is as simple as the factory method, as shown below:

Copy code The code is as follows:

function Car(color,door)

You can see that the constructor method does not create an object inside the function, but uses the this keyword. Because the object has been created when the constructor is called, and only this can be used to access the object properties inside the function.
Now use new to create objects, it looks like this! But it's the same as the factory approach. Each call creates its own method for the object.

3. Prototype method

This method uses the prototype attribute of the object. First the class name is created with an empty function, then all properties and methods are assigned the prototype attribute.

Copy code The code is as follows:

function Car()

In this code, an empty function is first defined, and then the properties of the object are defined through the prototype attribute. When this function is called, all properties of the prototype will be immediately assigned to the object to be created. All objects of this function store pointers to showColor(), and syntactically they all appear to belong to the same object.
However, this function has no parameters, and the properties cannot be initialized by passing parameters. The default value of the properties must be changed after the object is created.
A serious problem with the prototype method is when the property points to an object, such as an array.

Copy code The code is as follows:

function Car(){
}
Car.prototype.color = "red";
Car.prototype.doors = 4;
Car.prototype.arr = new Array("a","b");
Car.prototype.showColor = function(){
alert(this.color);
}
var car1 = new Car();
var car2 = new Car();
car1.arr.push("cc");
alert(car1.arr); //output:aa,bb,cc
alert(car2.arr); //output:aa,bb,cc

Due to the reference value of the array, the two objects of Car point to the same array, so when the value is added to car1, it can also be seen in car2.
Union is a method that uses the constructor/prototype method to create objects like other programming languages. It uses the constructor to define the non-functional attributes of the object and the prototype method to define the object.

Copy code The code is as follows:

function Car(color,door){
This.color = color;
This.doors = door;
This.arr = new Array("aa","bb");
}
Car.prototype.showColor(){
alert(this.color);
}
var car1 = new Car("red",4);
var car2 = new Car("blue",4);
car1.arr.push("cc");
alert(car1.arr); //output:aa,bb,cc
alert(car2.arr); //output:aa,bb

4. Dynamic prototype method

The principle of dynamic prototype method is similar to that of mixed constructor/prototype method. The only difference is where the object methods are assigned.

Copy code The code is as follows:

function Car(color,door){
This.color = color;
This.doors = door;
This.arr = new Array("aa","bb");
If(typeof Car._initialized == "undefined"){
Car.prototype.showColor = function(){
alert(this.color);
};
Car._initialized = true;
}
}

The dynamic prototype method uses a flag to determine whether a method has been assigned to the prototype. This ensures that the method is only created once

6. Mixing factory method

Its purpose is to create a fake constructor that just returns a new instance of another object.

Copy code The code is as follows:

function Car(){
var ocar = new Object();
ocar.color = "red";
ocar.doors = 4;
ocar.showColor = function(){
alert(this.color)
};
Return ocar;
}

The difference from the factory method is that this method uses the new operator.

PS (personal understanding):

1) The members (methods or attributes) defined by a class through prototype are common to every class object and are generally not used to define member attributes. If one object modifies the attribute value, all objects will be modified;

2) Classes have prototype attributes, but class objects do not;

3) Every time a new class object or a class is called directly (in the form of a factory method below), the statement defining the class (function) will be executed once (the singleton mode can avoid this situation);

4) Classes are of function type, class objects are of object type, and only function types have prototype attributes;

5) The method defined by prototype cannot access the private variables of the class (local variables defined by the class), but can access the member properties and member methods of the class (variables and methods defined by this) through this;

6) How to define a class:

a. Factory method (Object)

b. Inheritance method (prototype)

c. Constructor method (this)

　d. Mixing method

7) [Question] Why can properties defined through prototype be changed by any object? And the properties defined in the constructor method only belong to the object and will not affect the property values ​​of other objects?

The above are all the methods to create objects. The most widely used method at present is the mixed constructor/prototype method. In addition, the dynamic prototype method is also very popular. Functionally equivalent to the constructor/prototype approach.