Thanks for your comments. I started the blog just as a way to keep a programming reference for myself and the things I've learned, and also as a way to share what I've learned with others.

Granted, the concept has its proper place, and the scenario you point to makes sense. My point is just that if you don't know what properties/methods exist on an object, and you constantly find yourself "guessing" or "testing the waters" by using typeof to determine if a method or property exists, you might want to rethink the design a tad. In my experience, when programming like that is overused, it leads to unwieldy and difficult to maintain code.

One remark to your statements "[..] frequently using the typeof operator exclusively to check for the existence of a variable [..] is probably a sign of bad program design" and "[indicates] that your objects [..] are too tightly coupled":

I don't agree here, because I suggest using this check as good practice especially in loosley coupled objects to verify pre-conditions. For instance in setter methods with variant parameters this makes very much sense.