CODEWITHSUNDEEP
javascriptprototypal-inheritance
tt0686
tt0686

Reputation: 1849

Javascript constructors inheritance

Good Morning in my timezone.

I am learning JavaScript inheritance and i start reading the MDN pages. I understand that we have to use the prototype property from the constructor to build a inheritance chain, for example :

function Employee(){
 this.name = "Dave";
 this.dept = "";
}

function Manager(){
 this.projects = [];
}
Manager.prototype = new Employee;

If we do this :

var jane = new Manager();

jane.name -> It will retrive "Dave" because it will find on the Employee object.

What i can not understand is if you do this way:

 function Employee(name,dept){
 this.name = name || "Dave";
 this.dept = dept || "General";
}

function Manager(){
 this.base = Employee;
 this.base("Jack","CustpmDept");
 this.projects = [];
}

Now if i do the same :

var jane = new Manager();

jane.name -> It will retrive "Jack" because it will find on the Employee object.

In this last example i did not use the line Manager.prototype = new Employee; And it stil works , the Manager object have as their prototype object the Employee object. How is this possible ? Can you clarify me

Thanks in advance

Best regards

Upvotes: 2

Views: 2165

Answers (5)

Krasimir
Krasimir

Reputation: 13549

Check out the following code:

function Employee(name,dept){
    this.name = name || "Dave";
    this.dept = dept || "General";
    console.log(this instanceof Manager);
}

function Manager(){
     this.base = Employee;
     this.base("Jack","CustpmDept");
     this.projects = [];
}

var jane = new Manager();
console.log(jane.name);

In the example

console.log(this instanceof Manager);

returns true because when you call

this.base = Employee;

you are basically sending Manager's this to the Employee. this.name and this.dept are actually attached to the Manager.

Upvotes: 3

Matt R. Wilson
Matt R. Wilson

Reputation: 7575

OK so js is fun with how it handles context.

When you call this.base() in the Managers 'constructor' you are calling the function base from the context of that manager. So by the time we get to the this.name = bit, the keyword this refers back to that manager, not the function that line was written in.

Upvotes: 0

Joe Simmons
Joe Simmons

Reputation: 1848

In the latter, new Manager() returns 'Jack' because you call this.Employee('Jack', 'CustpmDept') which sets jane.name to 'Jack', because 'this' is 'Manager', or in this case, 'jane'.

It's a bit confusing.

Upvotes: 0

Arun P Johny
Arun P Johny

Reputation: 388436

function Manager(){
    this.base = Employee;
    this.base("Jack","CustpmDept");
    this.projects = [];
}

when you used the this.base("Jack","CustpmDept"), in invokes the Employee with this inside the Employee pointing to the new manager instance

Upvotes: 2

grape_mao
grape_mao

Reputation: 1153

The second way is like constructor stealing. this.base("Jack","CustpmDept"); will call the Employee constructor, since this key word will point to the object to be creates when we new the instance, so jane can get these properties.

Upvotes: 1

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