Metaprogramming
JavaScript
Dan Webb (dan@danwebb.net)

What?
“ the writing of computer programs
that write or manipulate other
programs (or themselves) as their
data”

With JavaScript?!
★ Small and fiesty but widely misunderstood
★ Relies on few but powerful constructs
★ You can work with functions, arguments,
built-ins types and mechanisms like
inheritance at runtime
★ Techniques underpin many JS libraries

JavaScript Exposed:
Objects Out Of
Control!

Play along!

4 building blocks

(Nearly) everything
is a hash

number
string
boolean
undefined

everything else is an
Object

This works with
any object

Expandos: a special case

Inspecting objects

Inspecting types

Wha?!

First typeof
then instanceof

Functions are Objects

'Methods' are just
functions that are
assigned to a property
of an object

arguments

Functions can return
other functions

Closure

Back to wikipedia...
“
A closure occurs when a function
is defined within another function,
and the inner function refers to
local variables of the outer
function.

Why is that useful?

Functions can be used
to make objects

Constructor functions
are just functions

The magic is in
the new operator

inheritance

The point is:
its all just functions,
objects and properties

We can hack it all.

So what can you do
with this stuff?

Patch holes in
bad implementations

Self optimising code

Pretty APIs

Domain Specific
Languages

DOMBuilder
http://danwebb.net/lowpro

DIY language features

John Resig's method
overloading
http://ejohn.org/blog/javascript-method-overloading/

Prototype 1.6 Classes
http://prototypejs.org/learn/class-inheritance

and a lot more...

Looking back...
★ JavaScript is small but totally flexible
★ A handful of techniques provide everything
you need
★ It's all about objects, properties and
functions
★ Don't worry if its not all sunk in. Have a
play around. Slides will be available.

Coming Soon...