Detailed explanation of six ways to define classes by JS

In front-end development, it is often necessary to define JS classes. So in JavaScript, there are several ways to define classes, and what are the differences? This article explains the following six ways of defining JS classes (case description):

1. Factory method

 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 is created and all properties and methods are assigned. Use this function to create 2 objects with exactly the same properties.

Of course, you can modify this method by passing parameters to it.

 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, each time the function Car() is called, showcolor() is created, which means that each object has its own showcolor() method.

But in fact, each object shares the same function. Although a method can be defined outside the function, then by pointing the function's properties to the method.

 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:

 function Car(color,door){this.color = color;this.doors = door;this.showColor = function(){alert(this.color)};}var car1 = new Car("red",4);var car2 = new Car("blue",4);

You can see that the constructor method does not create objects inside the function, and the keyword this is used. Because an object has been created when calling the constructor, and this can only be used to access the object properties within the function.

Now use new to create objects, it looks like that! But it's the same as the factory. Each call creates its own method for the object.

3. Prototype method

This method takes advantage of the prototype attribute of the object. First, create the class name with an empty function, and then all attributes and methods are assigned to the prototype attribute.

 function Car(){}Car.prototype.color = "red";Car.prototype.doors = 4;Car.prototype.showColor = function(){alert(this.color);}var car1 = new Car();var car2 = new 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 assigned to the object to be created immediately. All objects in this function store pointers to showColor(), which syntactically seems to belong to the same object.

However, this function has no parameters, and the attribute cannot be initialized by passing parameters. The default value of the attribute must be changed after the object is created.

A very serious problem with the prototype method is that when the attribute points to an object, such as an array.

 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,cclert(car2.arr); //output:aa,bb,cc

Here, because of the reference value of the array, both Car objects point to the same array, so when the value is added in car1, you can also see it in car2.

Union is a method that can create objects like other programming languages using constructor/prototype. It is a method that uses constructors to define non-functional properties of objects and uses prototypes to define objects.

 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,ccalert(car2.arr); //output:aa,bb

5. Dynamic prototype method

The dynamic prototype method is similar to the mixed constructor/prototype method. The only difference is the position given to the object's method.

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

6. Mixed factory method

Its purpose teacher creates a fake constructor that returns only new instances of another object.

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

Unlike the factory method, this method uses the new operator.

The above are all the methods of creating objects. The most widely used method is the hybrid constructor/prototype method, and dynamic prototype method is also very popular. Functionally equivalent to the constructor/prototype method.

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