Question

Possible Duplicate:
Why is it necessary to set the prototype constructor?

I’m struggling to understand the necessity of setting the ‘constructor’ property of a javascript object to the subclass when building a hierarchy. I find that the code below does what is expected without changing the constructor property, but in almost all references I find about the subject the constructor is set explicitly. Am I missing something ? (I don’t find any explicit use of it in the ECMAScript specs either).

A = function() {
    this.value = "a";

    this.A = function() {
        window.alert( this.value + " instanceof A : " + ( this instanceof A ) );
    }
}


B = function() {
    this.value = "b";

    this.B = function() {
        window.alert( this.value + " instanceof B : " + ( this instanceof B ) );
    }
}

B.prototype = new A();

test = function() {
    var b = new B();
    b.A();
    b.B();
}

Answer 1

First of all, proper JS inheritance means putting methods in the prototype:

var A = function() {
    this.value = "a";
};

A.prototype.A  = function() {
        window.alert( this.value + " instanceof A : " + ( this instanceof A ) );
};

var B = function() {
    this.value = "b";
};

Secondly, don’t run the constructor when establishing a prototype chain:

B.prototype = Object.create( A.prototype );

Whenever you reassign the entire .prototype, you are completely overwriting the object. So the constructor
property needs (If it’s going to be used) to be reassigned:

B.prototype.constructor = B;

B.prototype.B = function() {
    window.alert( this.value + " instanceof B : " + ( this instanceof B ) );
};

Object.create is not supported in older browsers but you can do something like:

Object.create = Object.create || function( proto ) {
     if( proto == null ) {
         return {};
     }
     function f(){}
     f.prototype = proto;
     return new f();
};