I have a class with a property that can be an Image (i.e. an IMG element) or a Canvas. When I serialize it to JSON, I need to convert this to a text string. If it is a Canvas, then I can call Canvas#toDataURL. But if it is an Image, I first need to draw it to a Canvas with Canvas#drawImage, then serialize that canvas with toDataURL.

So how do I determine if the object is a Canvas or an Image? (Since Canvas#drawImage is capable of accepting either Image or Canvas objects as an argument, there must be a way.)

I have seen that some programmers test for the existence of certain properties or functions to determine class, but I was wondering if there is a smarter way that won't break if the API presented by these objects changes.