Enforcing Object Construction

The following code includes a mistake. Can you find it?

function Employee(a) {
  this.name = a;
}

function init(){
  John = Employee("Johnson");
  alert(John.name);
}

You guessed it right. The code is missing the keyword new before the constructor name Employee. The correct code for init() is:

function init(){
  John = new Employee("Johnson");
  alert(John.name);
}

Calling the first init() above will give an error. One defensive programming action would be to add a check inside the constructor. It will match the calling object (this) with the constructor class (Employee) by the following check:

(this instanceof Employee)

function Employee(a) {
  if (!(this instanceof Employee)) return new Employee(a);
  this.name = a;
}

function init(){
  John = Employee("Johnson");
  alert(John.name);
}

Try calling init(). You will not get an error, because we create the object for you.

Next: How to distinguish between private and public data elements