What does new mean in JavaScript?

New in JavaScript is an operator used to instantiate an object; for example, to create a new instance of Person, you must use the new operator.

The operating environment of this article: windows7 system, javascript version 1.8.5, Dell G3 computer.

What does new mean in JavaScript? What exactly does new() in js do?

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) Change the function of the constructor The domain is assigned 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 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 we see in the Javascript engine The model 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 change this empty object The __proto__ member of the 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 a value to the obj object An id member 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.

Recommended study: "javascript Advanced Tutorial"

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