BB Code Parser Understandings

[General Ray]

<font face="courier new">There are a few things regarding the bb code parser I wish to clarify.
I understand the the parser replaces HTML code, however, does it insert the HTML Code directly into the post itself, than parse it? Say, for instance, I insert a js script that requires a function. Will that function be inserted into every post that uses the tag creating multiple instances of the function on the page or post? Is all the data Processed by that code, than the end result is inserted into the post?
How exactly does this work?</font>