be inheritable to be visible—and with that simple observation, the entire justification
for inheritance hierarchies collapses like a house of cards.
The most basic approach to function reuse is manual delegation—any public function
can be invoked directly via call or apply. It’s a powerful and easily overlooked feature.
However, aside from the verbosity of serial call or apply directives, such delegation is
so convenient that, paradoxically, it sometimes actually works against structural disci-
pline in your code—the invocation process is sufficiently ad hoc that in theory there is
no need for developers to organize their code at all.
Mixins are a good compromise: by encouraging the organization of functionality along
thematic lines they offer something of the descriptive prowess of the class hierarchy,
yet they are light and
flexible enough to avoid the premature organization traps (and
head-spinning dizziness) associated with deeply chained, single-ancestry models. Bet-
ter still, mixins require minimal syntax and play very well with unchained JavaScript
prototypes.
The Basics
Traditionally, a mixin is a class that defines a set of functions that would otherwise be
defined by a concrete entity (a person, a circle, an observer). However, mixin classes
are considered abstract in that they will not themselves be instantiated—instead, their
functions are copied (or borrowed) by concrete classes as a means of inheriting behav-
ior without entering into a formal relationship with the behavior provider.
Okay, but this is JavaScript, and we have no classes per se. This is actually a good thing
because it means we can use objects (instances) instead, which offer clarity and
flexi-
bility: our mixin can be a regular object, a prototype, a function, whatever, and the
mixin process becomes transparent and obvious.
The Use Case
I’m going to discuss a number of mixin techniques, but all the coding examples are
directed toward one use case: creating circular, oval, or rectangular buttons (some-
thing that would not be readily possible using conventional classical inheritance tech-
niques). Here’s a schematic representation: square boxes represent mixin objects, and
rounded boxes represent the actual buttons.
CHAPTER ONE: BEAUTIFUL MIXINS
4
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