How does new() in js change the constructor return value and this pointer?

This time I will show you how new() in js changes the return value of the constructor and the point of this. What are the precautions for changing the return value of the constructor and the point of this in new() in js? The following is a practical case, let's take a look. take a look.

Preface

Like other high-level languages, JavaScript also has the new operator. We know that the new operator is used to instantiate a class. Thus allocating an instance object in memory. But in JavaScript, everything is an object. Why do we need to use new to generate objects? This article will take you to explore the mystery of new in javascript...

To create a new instance of Person, you must use the new operator.

Calling the constructor in this way will actually go through the following 4 steps:

(1) Create a new object;

(2 ) Assign the scope of the constructor to the new object (so this points to the new object);

(3) Execute the code in the constructor (add attributes to the new object);

(4) Return the new object.

new operator

After the introduction of the above basic concepts, and adding the new operator, we can complete the traditional oriented Objects are created using class new. In JavaScript, we call this method Pseudoclassical.

Based on the above example, we execute the following code

var obj = new Base();

What is the result of this code? The object model we see in the Javascript engine is:

What exactly does the new operator do? It’s actually very simple, it does three things.

var obj = {};
obj.__proto__ = Base.prototype;
Base.call(obj);

In the first line, we create an empty object obj

In the second line, we point the __proto__ member of this empty object to the Base function object prototype member object

In the third line, we replace the this pointer of the Base function object with obj, and then call the Base function. So we assign an id member variable to the obj object. The value of this member variable is "base". About the call function usage.

What will be the effect if we add some functions to the Base.prototype object?

For example, the code is as follows:

Base.prototype.toString = function() {
 return this.id;
}

Then when we use new to create a new object, according to the characteristics of __proto__, the toString method can also create a new object method is accessed. So we see:

In the constructor, we set the member variables of the 'class' (for example: id in the example), and in the constructor object prototype we set the public methods of the 'class'. Therefore, the effects of classes and class instantiation are simulated through function objects, Javascript's unique __proto__ and prototype members, and the new operator.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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