Are you asking what the properties are for or simply how is it possible that properties are being created ? The phrasing of your question seems to imply you're asking the second question.
If you want to know what the properties mean, you need to supply a link to the original source code so we can see everything in context. But from what I can tell, they're probably part of some code that displays a visual representation of an audio source's levels.
If you just want to know why this is possible... Well, it's possible because a ScriptProcessorNode
is just an object, and you can always add properties to an object in JavaScript (unless it's been frozen).