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DOM's Gotchas

Bill Venners: What's are some of DOM's gotchas?

Elliotte Rusty Harold: Take namespaces, for example. There are two basic models for handling namespaces in an XML API. In one model, you assign each element and attribute a certain namespace, and you figure out where the namespace declarations need to go when you serialize the document. In the other model, you don't provide any special support for namespaces—you just treat the namespaces as attributes. That also works, although it's harder on the end user. DOM is the only API I know of that does both, simultaneously. DOM requires client programmers to understand and use both models. Otherwise they'll produce namespace-malformed documents, which is truly evil. DOM has all the complexity of both approaches and the simplicity of neither.

There are a lot of other issues with DOM that stem from its cross-language nature. For example, DOM defines exactly one exception, DOMException, which has short type codes to indicate which kind of exception it is. To a Java programmer, this is just plain weird. Java programmers use many different exception classes, and never use shorts for anything. When was the last time you used a short in code? Have you ever? I don't think I've ever used short, except when I was trying to demonstrate all the data types. But using a short makes sense from a C or C++ programming perspective, where shorts are more common, and having many exception types is not.

Some of the languages DOM needs to support, especially JavaScript, did not support method overloading at the time DOM was invented. Therefore, DOM could not have two methods such as createElement, one that takes an element name and a namespace, and another that takes only a local name. Instead, DOM has createElement, which takes just the name, and createElementNamespace, which takes both a name and a namespace. There are many non-overloaded methods in the DOM API that, to any Java or C++ programmer, should clearly be overloaded.

There are several other DOM design decisions that confuse people. For example, DOM trees do not allow nodes to be detached from their parent document. Only the document that created the node is allowed to hold the node. Also, DOM's DocType object is read-only. Why? I can't explain these design decisions. I just know that they are painful when you're actually trying to get work done with DOM.