The initial value of Error.prototype.constructor is the built-in Error() constructor.
You can use this as one way of creating Error objects, although it is more appropriate to use the new Error() technique.
This property is useful if you have an object that you want to clone, but you don't know what sort of object it is. Simply access the constructor belonging to the object you have a reference to.
Netscape provides constructors for many objects, virtually all of them in fact, even when it is highly inappropriate to do so. MSIE is far more selective and there are some occasions when you might wish for a constructor that MSIE does not make available.