If you are used to writing code in Flash ActionScript and you begin to use JScript you'll quickly realize that they are quite similar. The syntax is identical and most of the core objects exist for both. For example both have the Array object, and both have nearly the same methods on the Array object. However, there are items in Flash that are not present in JScript, such as MovieClip, _global, and others.
You cannot directly access something in Flash from JScript, just as you cannot access something directly in JScript from Flash. However, there are objects and methods built into every script module you create in SWF Studio that will make accessing your Flash movie from JScript possible.