Kev Kev - 5 months ago 27
Javascript Question

Why does document.querySelectorAll return a StaticNodeList rather than a real Array?

It bugs me that I can't just do

document.querySelectorAll(...).map(...)
even in Firefox 3.6, and I still can't find an answer, so I thought I'd cross-post on SO the question from this blog:

http://blowery.org/2008/08/29/yay-for-queryselectorall-boo-for-staticnodelist/

Does anyone know of a technical reason why you don't get an Array? Or why a StaticNodeList doesn't inherit from an Array in such a way that you could use
map
,
concat
, etc?

(BTW if it's just one function you want, you can do something like
NodeList.prototype.map = Array.prototype.map;
...but again, why is this functionality (intentionally?) blocked in the first place?)

Answer

I believe it to be a philosophical decision of the W3C. The design of the W3C DOM [spec] is quite orthogonal to the design of JavaScript, as the DOM is meant to be platform and language neutral.

Decisions like "getElementsByFoo() returns an ordered NodeList" or "querySelectorAll() returns a StaticNodeList" are very much intentional, so that implementations don't have to worry about aligning their returned data structure based on language-dependent implementations (like .map being available on Arrays in JavaScript and Ruby, but not on Lists in C#).

The W3C aim low: they'll say a NodeList should contain a readonly .length property of type unsigned long because they believe every implementation can at least support that, but they won't say explicitly that the [] index operator should be overloaded to support getting positional elements, because they don't want to stymie some poor little language that comes along that wants to implement getElementsByFoo() but cannot support operator overloading. It's a prevalent philosophy present throughout much of the spec.

John Resig has voiced a similar option as yours, to which he adds:

My argument isn't so much that NodeIterator isn't very DOM-like it's that it isn't very JavaScript-like. It doesn't take advantage of the features present in the JavaScript language and use them to the best of its ability...

I do somewhat empathize. If the DOM was written specifically with JavaScript features in mind it would be a lot less awkward and more intuitive to use. At the same time I do understand the W3C's design decisions.

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