Detailed explanation of JavaScript function objects_javascript skills

Function

A function is an event-driven or reusable block of code that executes when it is called.

function One(leve , leve){
  //code
  return leve+leve
 }

Note:

Formal parameters do not need to be typed;

The return statement is optional, and a function without a return statement returns undefined;

Local variables and global variables

Declaration within the function: local variable

Declaration outside the function: global variable

When assigning a value to a new variable name, var is not used: the variable will become a new global variable

Functions can be used as values

Form 1:

 function init(){
  alert("One")
 }
 window.onload = init;

Format:

window.onload = function(){
  alert("One");
 }

Note: The above two methods can make the browser prompt: One.

Object

Everything in JavaScript is an object: strings, numbers, arrays, functions, and in addition, JavaScript allows custom objects.

Object reference

When assigning an object to a variable, the variable will contain a reference to the object, not the object itself.

When calling a function and passing in an object, only the object reference is actually passed (copy a copy of the reference, pass it to the formal parameter, and point to the object, that is, the two references point to the same object)

Create object

 var dog = {
  name : "myDog",
  weight : ,
  bark :function(){
   alert("woof!");
  } 
 }
 dog.bark();

Note: There must be "," after each attribute (except the last one).

Constructor constructs object

function Dog(name,weight){
  this.name = name;
  this.weight = weight;
  this.bark = function(){
   if(this.weight > ){
    alert(this.name + "Woof!");
   }else{
    alert(this.name + "Yip!");
   }
  };//这里也不能忘了分号
 }
 var myDog = new Dog("hello","");
 myDog.bark();

PS:

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 I gave after consulting relevant information. The explanation is very book-like but the meaning is still very clear. Please see a small example below:

The code is as follows:

　

Copy code The code is as follows:

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 this " request" object and assign it an initial value. So we can know: "The 'function' used with the 'new' operator to create and initialize the object is the constructor".

For example, our common way to declare an array is the standard constructor: var array = new Array();

2. What is an instantiated object

The code is as follows:

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 a "class"? According to the literal understanding, we can understand it as a "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 language, "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 a series of it. 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.