Detailed explanation of JavaScript constructor and instantiated object instances

Constructors in JavaScript are different from constructors in other languages. Functions called with the new keyword are considered constructors. Any JavaScript function can be used as a constructor, and the constructor must Use the new operator as a prefix to create new instances.

Preface--Let’s talk about it earlier

I think there are many beginner friends who have rarely been exposed to backend programming languages ​​​​like me, and they are not familiar with a series of "" in JavaScript "Noun" is so confusing. It seems that I roughly know what it is about, but in fact I still don’t understand it clearly; I think that when learning any kind of knowledge, the first step should be to clarify the most basic terminology (knowing what it is about will help us better understand it) (Understand it.) That is, knowing what it is and why it is, will be very helpful for future advanced learning. Below I will briefly talk about my own understanding of some seemingly unimportant but necessary knowledge points. (If there are any discrepancies, please correct me)

1. What is a constructor?

Constructor is a special method. It is mainly used to initialize the object when creating the object, that is, to assign initial values ​​to the object member variables. It is always used together with the new operator in the statement to create the object.

This is the explanation from Baidu Encyclopedia. The explanation is very bookish but the meaning is still very clear. Please see a small example below:

　var request = new XMLHttpRequest();

This expression is often used when we create request objects when using AJAX technology. Then we can clearly see that "new XMLHttpRequest();" is a standard constructor! We declare a "request" object in "var" and use the constructor "new XMLHttpRequest();" to initialize the "request" object and assign it an initial value. So we can know: "The 'function' used with the 'new' operator to create and initialize an object is a constructor".

For example, our common declaration array is the standard constructor: var array = new Array();

2. What is an instantiated object

var request = new XMLHttpRequest();

In object-oriented programming, the process of creating objects using classes is usually called instantiation.

I highlighted the key points of the explanation in red and blue above. To put it bluntly, instantiating an object is the process of creating an object!

So what is "class"? Literally we can understand it as "type". For example, "cake" is a category of desserts, that is, a type; then cheesecake is a specific individual of the category of cake in desserts, that is, an object.

We know that in programming languages, "class" is abstract. We have no way to operate it or use its methods and properties. Only by instantiating this class into an object can we call it. A series of methods and properties. In fact, this is also easy to understand. In life, we have no way to see or capture abstract things, so naturally we have no way to use some of its functions. We can only concrete the abstract things into each individual, individual or actual situation. Only with the object can we clearly understand or recognize it; the same is true for programming. Therefore, instantiating an object is a process from abstract to concrete, and this process is called instantiation.

The above is the entire content of this article, I hope you all like it.

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