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.

Accepted Answer

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

Solution

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