Question

Possible Duplicate:
Why are my JS object properties being overwritten by other instances

Why isn’t the attribute “t” changed after setT was called? I would expect “4” as output, but it prints “default”.

function Car(i) {
  var id = i;
  var t = "default";

  this.getT = function() { return t; }
  this.setT = function(p) {
    t = p;  // attribute t isn't changed ..
  }
}

function ECar(id) {  
  Car.call(this, id);  // super constructor call

  this.setT = function(p) {  // override
    ECar.prototype.setT.call(this, p); // super call
  }
}

ECar.prototype = new Car();

ecar = new ECar(3);
ecar.setT(4);
alert(ecar.getT()); // prints default, not 4

Answer 1

ECar.prototype = new Car();

At this line ECar‘s prototype get a context, in which all ECar‘s instance will be shared.

ECar.prototype.setT.call(this, p);

This line will call at that context, not what has been created while calling super at Car.call(this, id);.

You can fix your code with

function ECar(id) {  
  Car.call(this, id);  // super constructor call
  var carSetT = this.setT;
  this.setT = function(p) {
    carSetT.call(this, p);
  }
}

but it would be better (and more readable) to use real prototypes, such as

function Car() {}

Car.prototype.getT = function () { /* ... */ };
Car.prototype.setT = function () { /* ... */ };

function ECar() {}

ECar.prototype = new Car();
ECar.prototype.setT = function () { /* ... */ };

Edit: note (as @Bergi suggested)

You should only use Child.prototype = new Parent() as inheritance if you must support legacy browsers & then you should only use empty constructors.

The most (other-language) compatible way in JavaScript for inheritance is

Child.prototype = Object.create(Parent.prototype)

(MDN says it is supprted from IE 9)