Detailed explanation of new() in Javascript

Like other high-level languages, JavaScript also has the new operator. We know that the new operator is used to instantiate a class and allocate 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 (for this new object) Object adds attributes);

(4) Returns a new object.

new operator

After the introduction of the above basic concepts, and adding the new operator, we can complete the traditional oriented The object's class + new method creates objects. 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 we see in the Javascript engine The model is:

What exactly does the new operator do? In fact, it is very simple, it only 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 set the __proto__ member of this empty object Points 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 to the obj object Variable, the value of this member variable is "base", regarding the usage of the call function.

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 __proto_ _ characteristics, the toString method can also be accessed as a method of a new object. 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.

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Detailed explanation of the new operator in JavaScript

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