Why Is There No #Include?

  • Article

/>A
common and entirely sensible programming practice is to put commonly used utility
functions in one file, and then somehow link that file in to many different programs.
In traditional compiled languages you can compile a bunch of utilities into a statically
linked .LIB file or a dynamically linked .DLL file. But what do you do in script,
where a program is just text?

People
often ask me why there is no #include statement
in VBScript or JScript. Wouldn't it be nice to be able to write up a page full
of helpful utility functions and then in half a dozen different scripts say  #include
"myUtilityFunctions.vbs" ?

But
if you actually think for a bit about how such a statement would be implemented, you
see why we didn't do it. The reason is pretty simple: the script engine does
not know where the current file came from, so it has no way of finding the referenced
file. Consider IE, for example:

<script
language="vbscript">

#include
"myUtilityFunctions.vbs"

' ...

How
does the script engine know that this page was downloaded from https://www.rezrov.com/gnusto/ ?
The script engine knows nothing about where the script came from -- IE just hands
the text to the engine. Even assuming that it could figure it out, the script
engine would then have to call IE's code to download files off the internet.
Worse, what if the path was fully qualified to a path in a different domain?
Now the script engine needs to implement IE's security rules.

But
do any of those factors apply when running in ASP? No! ASP would have
to have a completely different #include mechanism
where the relative path was based on facts about the current virtual root and the
security semantics checked for cross-root violations. What about WSH?
There we're worried about the current directory but have no security considerations.

What
about third party hosts? Every host could have a different rule for where
the script comes from and what is a legal path. We cannot possibly put all
those semantics into the script engine. To make an
#include feature
work, we'd have to define a callback function into the host so that the host could
fetch the included script for us. It's just too much work for too little gain,
so we simply declared a rule: the host is responsible for implementing any mechanism
whereby script files can be included into the script namespace. This is
why IE and WSH have the <script
src= attribute.