JavaScript inheritance extend function

Question

I'm having some trouble understanding the IF clause at the end of this function from Pro JavaScript Design Patterns:

function extend(subClass, superClass) {
    var F = function() {};
    F.prototype = superClass.prototype;
    subClass.prototype = new F();
    subClass.prototype.constructor = subClass;

    subClass.superclass = superClass.prototype;
    if(superClass.prototype.constructor == Object.prototype.constructor) {
        superClass.prototype.constructor = superClass;
    }
}

The book explains that these lines ensure that the superclass's constructor attribute is correctly set, even if the superclass is the Object class itself. Yet, if I omit those three lines and do the following:

function SubClass() {};
extend(SubClass, Object);

alert(Object.prototype.constructor == Object);

The alert says 'true', which means the superclass's constructor is set correctly even without those last three lines. Under what conditions, then, does this IF statement do something useful?

Thanks.

I have a question: why the intermediary function F, instead of just subClass.prototype = new superClass();? — 755, Feb 19 '13 at 10:00

Christian C. Salvadó · Accepted Answer · 2010-04-21T20:40:28.810

The problem that those two lines try to avoid, is generally produced when you replace the prototype property of a Constructor Function, for example:

function Foo () {};
Foo.prototype = {
  bar: 'baz'
};

var foo = new Foo();
foo.constructor === Object; // true, but `constructor` should refer to Foo

When functions objects are created, the prototype property is initialized with a new object, which contains a constructor property that refers to the function itself, e.g.:

function Bar () {};
var bar = new Bar();
bar.constructor === Bar; // true

When you replace the prototype property with another object, this object has it's own constructor property, generally inherited from other constructor, or from Object.prototype.

var newObj = {};
newObj.constructor === Object;

JavaScript inheritance extend function

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