Javascript Constructor Detailed _ Basics

First, what is the constructor

Constructors are common in some object-oriented languages, such as Java, C + +, and PHP. In JavaScript, a constructor is first a normal function that can be invoked using the new operator and generate a special type of object.

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"Benjamin" is a constructor
var benjamin = new Benjamin ("Zuojj", "male");

In the above example, Benjamin is a Benjamin object, so how is it instantiated?

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function Benjamin (username, sex) {
This.username = Username;
This.sex = sex;
}
var benjamin = new Benjamin ("Zuojj", "male");
Outputs:benjamin{sex: "Male", Username: "ZUOJJ"}
Console.log (Benjamin);

As we can see, the "Benjamin" constructor simply receives the passed arguments and assigns them to the this object. This is because when the constructor is called by the new operator, the this object of the constructor is assigned the object returned by the new operation.
This means that the code above is equivalent to:

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Benjamin = {
"username": "ZUOJJ",
"Sex": "Male"
}

Second, why use constructors

There are several reasons why you can use constructors:
1. Using constructors means that all of these objects can be created using the same basic structure
2. Using constructors means that the "Benjamin" object is explicitly labeled as an instance of the "Benjamin" function

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function Benjamin (username, sex) {
This.username = Username;
This.sex = sex;
}
var benjamin = new Benjamin ("Zuojj", "male");
var ben = {
"username": "ZUOJJ",
"Sex": "Male"
}
Outputs:true
Console.log (Benjamin Instanceof Benjamin);
Outputs:false
Console.log (Ben instanceof Benjamin);

3. Using constructors means that we can define common methods on the prototype for multiple instances to share

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function Benjamin (username, sex) {
This.username = Username;
This.sex = sex;
}
Benjamin.prototype.getName = function () {
return this.username;
}
var benjamin = new Benjamin ("Zuojj", "male");
var ben = new Benjamin ("Lemon", "female");
Outputs:zuojj
Console.log (Benjamin.getname ());
Outputs:lemon
Console.log (Ben.getname ());

III. Matters of note

1.new keywords
When instantiating a constructor, be sure to use the New keyword and use the new keyword, which has a large impact on the This object, which points to the Global Object (window in browser and global in node) without the new keyword. So when you define a constructor, we recommend that you capitalize the first letter of the function name.
2. If the called function does not have an explicit return expression, it implicitly returns the This object-that is, the newly created object, or it will affect the result returned, but only to return an object

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function Bar () {
return 2;
}
var bar = new Bar ();
Returns the newly created object
Outputs:bar {}
Console.log (bar);
function Test () {
This.value = 2;
return {
Foo:1
};
}
var test = new Test ();
The returned object
Outputs:object {foo:1}
Console.log (test);

What we need to be aware of is:
A The new Bar () returns the newly created object, not the literal value of 2. So the new bar (). constructor = = Bar, but if the return is a numeric object, the result is different;
b Here's the new Test () is the object returned by the function, not the newly created object by the newer keyword, as follows:

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function Bar () {
return 2;
}
var bar = new Bar ();
function BarN () {
return new number (2);
}
var barn = new Barn ();
Outputs:true
Console.log (Bar.constructor = = bar);
Outputs:number {}
Console.log (barn);
Ouputs:false
Console.log (Barn.constructor = = = Barn);
Outputs:true
Console.log (Barn.constructor = = number);
/* -------------------------------------- */
function Test () {
This.value = 2;
return {
Foo:1
};
}
var test = new Test ();
outputs:undefined
Console.log (Test.value);
Ouputs:1
Console.log (Test.foo);

The above is the summary of the constructor function, I hope to help beginners, the article in the wrong place, hope Criticism, treatise.