JavaScript is just a language standard, used by many different softwares. There are a few open source impllementations out there that can be used to embed it in any software, e.g. SpiderMonkey, KJS, Rhino.
JavaScript is a scripting language, and does not share much more than the name and parts of the syntax with Java, which is a full blown high-level language. In the case of Rhino which is used by Scriptographer, JavaScript is implemented in Java and can access the Java class library that is exposed to the scripting environment through JavaScript wrapper objects.
Processing does not use JS. It is based on Java and uses the Java compiler together with some preprocessing of the code. This produces much higher performance than interpretated JS code, and is needed for getting decent performance in animations and such.
I am not aware in what way ARTCOM is using JS for their installations. If you can point to some examples that would be nice, but I think it is Java what they use.
JS can be used for installations though, for example Hektor is driven with JavaScript and Scriptographer.