Javascript Inheritance call behaviour

Question

Was going through Mozilla's Reference and came across this simple relationship.

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

function Manager() {
  Employee.call(this);
  this.reports = [];
}
Manager.prototype = Object.create(Employee.prototype);

Apparantly, this forms a inheritance-like relationship from the Employee to the Manager.

Two questions here, Not sure what and why Employee.call(this); is there and instead of assigning the object to Employee.prototype it is assigned to Manager.prototype. My thoughts were that Manager is inheriting from Employee, not vice versa. Perhaps this is the concept of the prototype chain where it really means both objects can grab properties from each other?

Would like some clarification.

Manager will inherit Employee class's properties and will also have its own properties/methods. However Employee class won't inherit Manager's properties though. — David R
– David R, Commented Aug 23, 2016 at 18:14
"instead of assigning the object to Employee.prototype" - which object? — Bergi
– Bergi, Commented Aug 23, 2016 at 18:16
@Bergi Employee.prototype = Object.create(Manager.prototype); — Aaron
– Aaron, Commented Aug 23, 2016 at 18:16
@Aaron yes, that sets the employees' prototype to an object inheriting from the managers' prototype. — Bergi
– Bergi, Commented Aug 23, 2016 at 18:17
@Aaron: Oops, I missed that you swapped Employee with Manager there. Right. But no, employee's prototype should not "have a manager object". The Employee class doesn't care which subclasses are extending it, and it won't hold references to them. If managers are inheriting from employees, then the prototype chain link goes from Manager.prototype to Employee.prototype, not the other way round. — Bergi
– Bergi, Commented Aug 23, 2016 at 18:23

Jed Fox · Accepted Answer · 2016-08-23 18:25:58Z

2

Employee.call(this) is like calling Employee(), but instead of using it to create a new object, it instead modifies the current Manager. In this example, it sets name and dept in the Manager.
This statement:
```
Manager.prototype = Object.create(Employee.prototype);
```
means that any attributes set like Employee.prototype.x = val will also be available in Manager.prototype; however if you override them in Manager.prototype, they will be overriden.
You should also run Manager.prototype.constructor = Manager to make that property correct.

edited Aug 23, 2016 at 18:25

answered Aug 23, 2016 at 18:16

Jed Fox

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12 Comments

Aaron Over a year ago

Is it okay to think of Employee.call(this) as super() in Java?

Jed Fox Over a year ago

I think it’s similar. (I don’t know Java though).

Bergi Over a year ago

@Aaron: Yes, that's exactly what it does. It calls the parent constructor on the child instance to create and initialise the parent properties.

Bergi Over a year ago

@JF: Object.create does not copy anything.

Aaron Over a year ago

Is JF's answer wrong? His explanation was extremely clear to me. Not understanding the downvotes.

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