How to create objects in JavaScript_javascript skills

What is an object?
From the definition of JavaScript, an object is a collection of unordered properties, and its properties can contain basic values, objects or functions. That is to say, an object is a set of attributes in no particular order. Each attribute is mapped to a value, which is a set of key-value pairs. The value can be data or an object.

The simplest object
A pair of curly brackets {} in JavaScript can define an object. This writing method is actually the same as calling the constructor of Object

Copy code The code is as follows:

var obj={};
var obj2= new Object();

The object constructed in this way only contains a pointer to the prototype of the Object. You can use some valueOf, hasQwnProperty and other methods, which have little practical effect. Custom objects always have some custom properties and methods.

Copy code The code is as follows:

var obj={};
                                                                                                                          =0; a:0,
fn:function( ){
                    alert(this);

You can add properties and methods to it through "." after defining the object, or you can use the literal assignment method to add properties and methods to it when defining the object. The object created in this way has the same methods as Attributes can directly use object references, similar to static variables and static functions of classes. There is an obvious flaw in creating objects in this way - it is very laborious to define a large number of objects, and it is necessary to write almost repetitive code over and over again.


Abstract it

Since it is repeated code, it can be abstracted, use functions to do these repeated tasks, and call a method specifically created when creating an object. For different attribute values, only You just need to pass in different parameters.

Copy code

The code is as follows:

function createObj(a,fn){

            obj={};

obj.a=a; obj.fn=fn; return obj; }        var obj=createObj(2,function(){              alert(this.a);

In this way, when creating a large number of objects, you can do some repetitive work by calling this method. This method is not perfect, because in many cases it is necessary to determine the type of the object. The objects created by the above code are the most The original Object object instance only extends some properties and methods.

Style

It’s time for function to appear again. In JavaScript, function is an object. When creating an object, you can discard the above createObj method and directly use function as an object. How to achieve reuse? This lies in the special nature of function as an object. Sex.

1. Function can accept parameters and can create objects of the same type and different values ​​based on the parameters

2. When function is used as a constructor (called through the new operator), it will return an object. Some basic knowledge of constructors is mentioned in jQuery of the poor and lower-middle-class peasant version. Simply copy it

The return value of the constructor is divided into two situations. When the function has no return statement or returns a basic type (bool, int, string, undefined, null), it returns an anonymous object created by new, which is It is a function instance; if the function body returns a reference type object (Array, Function, Object, etc.), the object will overwrite the anonymous object created by new as the return value.

３. So how to use function to solve the type identification problem? Each function instance object will have a constructor attribute (not "has", but can correspond). This attribute can indicate who constructs it, or it can Use the instanceof operator to determine whether the object is an instance of XXX.

Don’t just talk without practicing, code

Copy code The code is as follows:

function Person(name){
this.name =name;
this.fn=function(){
alert(this.name);
}
}

      var person1=new Person('Byron');

console.log(person1.constructor==Person);//true
console.log(person1 instanceof Person); //true

This would be perfect, no! Although the constructor can be object-specific, the methods must be repeated in every instance of the object!

Copy code The code is as follows:

function Person(name){
this.name =name;
this.fn=function(){
alert(this.name);
}
}

      var person1=new Person('Byron');
var person2=new Person('Frank');

console.log(person1.fn==person2.fn);//false

Look, although the fn of the two instances are exactly the same, they are not the same thing. If a function object has a thousand methods, then each of its instances must contain copies of these methods, which is very frustrating. The memory is speechless.

No more nonsense

Is there a nearly perfect way to construct an object that does not require repeated work, is stylish, and does not require the repetition of common methods of objects? ? In fact, we can find that using function is already close to the requirements. There is only one difference - a container shared by all instances of function objects is needed. In this container, instances need to share attributes and methods. It just so happens that this container is ready - —prototype, students who don’t know prototype can take a look at JavaScript prototype

Copy code The code is as follows:

function Person(name){
this.name=name;
}

Person.prototype.share=[];

Person. prototype. printName=function(){
                                                                    ; ;

console.log(person1.printName==person2.printName);//true



In this way, each Person instance has its own attribute name, as well as the attribute share and method printName shared by all instances. The basic problems are solved. For general object processing, you can always create this stylish and loving way. Object mode.