CODEWITHSUNDEEP
javascript
JC JC
JC JC

Reputation: 12132

Why does 'call()' add properties to the Function's instance?? (see small code)

Why is it that just by using call you're adding properties to the Employee instance named jc?

I'm wondering why jc.hasOwnProperty('firstName'); results to true.

I didn't even do prototype inheritance yet. This is all the code, no more no less:

I understand that call just changes the this while passing in arguments, but to add properties? I don't know what's going on...

function Person(firstName, lastName){
  this.firstName = firstName;
  this.lastName = lastName;
}
function Employee(firstName, lastName, position){
  Person.call(this,firstName,lastName);
  this.position = position;
}
var jc = new Employee('JC','Viray','Developer');
jc.hasOwnProperty('firstName'); //true

UPDATE

I get it now.. the solution to my problem is: stop thinking in C#/Java mentality.. I lost track that Person is still a FUNCTION despite it being a "constructor type function"... -_- once a function, still a function..

Upvotes: 1

Views: 65

Answers (2)

Adam Rackis
Adam Rackis

Reputation: 83366

I'm wondering why jc.hasOwnProperty('firstName'); results to true.

Because of this line in the Person function:

this.firstName = firstName;

When you say

Person.call(this, firstName, lastName);

You're actually calling the Person function, and telling Person to set its this value to what's provided in the first argument—in your case, the Employee object you're in the process of building. The subsequent arguments to call then get passed along as regular parameters.

As a result, Person gets called with this set to be your current Employee object, and picks up a firstName and lastName property.

Or consider an alternate example.

var obj = { };
Person.call(obj, "Adam", "Rackis");
obj.hasOwnProperty('firstName'); //still true

Upvotes: 4

Felix Kling
Felix Kling

Reputation: 816770

You are calling 

Person.call(this,firstName,lastName);

which means that the function Person, this refers to whatever this refers to in Employee (a new object). Inside Person, you are assigning values to this.firstName and this.lastName.

You could have written the same as:

function Employee(firstName, lastName, position){
  this.firstName = firstName;
  this.lastName = lastName;
  this.position = position;
}

I understand that call just changes the this while passing in arguments, but to add properties?

call does not add properties to the object, but your function does. And that's exactly why it is used in this case. You want to apply everything inside Person to the new object as if you had called new Person(a, b).

Upvotes: 4

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