The Language of Expressions

The Language of Expressions

The After Effects expression language is based on a subset
of JavaScript. JavaScript is a scripting language used largely
for web page design and includes many features specifi -
cally aimed at that task. The JavaScript implementation for
expressions includes the core features only. That means
there’s a lot about JavaScript that you won’t need to know,
but it also means that any JavaScript reference you pick up
(and you’re going to need one if you really want to master
expressions) is going to have a lot of content that will be of
little or no use to you.
The rest of the expression language consists of extensions
that Adobe has added specifi cally for After Effects. This
means that in addition to a good JavaScript reference,
you’ll also be frequenting Adobe’s After Effects Expression Element
Reference. The most up-to-date version of this reference
can be found at Adobe’s Help on the Web. For machines
with an Internet connection, the After Effects Help menu
will take you there: Help > After Effects Help. Alternatively,
you can go to www.adobe.com/go/learn_ae_cs4helphome.
This chapter focuses on working examples rather than the
details of JavaScript. The book’s disc, however, contains an
abbreviated JavaScript guide, and I recommend that you
glance through it before you really dive into the sample
expressions discussed here. In addition, I’ll point you to
the appropriate sections of that guide as you encounter
new JavaScript elements for the fi rst time.