JavaScript inheritance extend function
- by Zach
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.
