Bob Ippolito (@etrepum) on Python, Erlang, JavaScript, etc.

JavaScript frameworks


JavaScript, as a language, gives you just about nothing useful. All you have are objects (little more than key-value bags, or null), extremely lame exception handling, and a few primitives (number, string, undefined, function). That means no classes, macros, continuations, coroutines, metaclasses, etc. Forget even thinking about overloading operators or inventing new control structures.

To make JavaScript a half-assed environment for writing code that does more than twiddle the visibility of DOM elements, you have to really add a lot of junk to it. There are two or three out there worth their salt, but all of them have their issues. Here's my beef with Prototype, for example:

  • Object.prototype hacking breaks for (key in obj) syntax in just about every imaginable way
  • No documentation
  • No tests
  • No good method for doing deferred operations
  • Crufty code evolved from JavaScript frameworks past that attempts to work around bugs in browsers that wouldn't even consider Prototype valid code in the first place!

There really isn't enough useful code in Prototype to make it worth my while to embrace and extend it, so I went and wrote my own (MochiKit). It's not released yet, but we're planning to put it out there this month. Currently it has some really interesting stuff, though. Here's just a small taste.


  • A complete set of operator-wrappers for use with functional programming (e.g. operator.add, operator.truth, etc.)
  • Wrapper functions for common operations (itemgetter(nameOfKey), forward(forwardMethodName), typeMatcher(tA, tB, ...), isUndefinedOrNull, etc.
  • Partial function application partial(func, arg0, arg1, ...) and function-binding bind(func, obj).
  • Functional programming constructs for Array-like-objects and argument lists: map(fn, seq0, seq1, ...), xmap(fn, p0, p1, p2, ...), filter(pred, seq), xfilter(pred, p0, p1, p2, ...), concat(seq0, seq1, ...)
  • An adapter registry concept that makes certain bits of the framework more like generic functions.
  • COMPARISON THAT WORKS. compare(a, b) for any reasonable a and b will do the right thing. This includes arrays, dates, numbers, and strings. If you want to teach it how to compare something else, there is registerComparator(...) which is a wrapper around an adapter registry! Also, keyComparator(...) and reverseKeyComparator(...) that conveniently wrap up compare(a.key, b.key).
  • repr(obj) gives you Python-like repr for objects, if obj.repr() is implemented OR if there is an adapter for it via registerRepr(...). Built-in repr for array, string, undefined, and null. The default is obj.toString(), of course.
  • min/max functions that use compare(...): listMax(lst), listMin(lst), objMax(p0, p1, ...), objMin(p0, p1, ...)
  • nodeWalk(node, visitor), a non-recursive object walker (great for finding things in DOM trees).

Every feature has tests. Tests are run on Safari, Firefox, and IE. The tests all pass.

Every function has documentation. The documentation is in reStructuredText. A documentation tool extractor and generator will ship with MochiKit.

I'll save talking about Async, DOM, Iter, and DateTime for a later entry. Needless to say, they're much cooler than Base.

You can check out MochiKit at! We ended up splitting the docs and source, though, so there is no doc extraction tool. It's still reStructuredText, though!
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