How to Hide Objects/Text When Video Starts

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Or is there possibly even a JS-encyclopedia or command documentation for Storyline anywhere?

Storyline doesn't run JS, so there is little need for documentation. It stores the script and passes it to the browser. It is the browser that executes the script.

That said, there are some places where JS and SL interact, mainly passing variables back and forth. Also, the GSAP library can control some elements on the page, giving the appearance of SL doing it. So your main ability to use JS for this would be to pass SL a new value for a variable, and trigger your actions on when the variable changes.

W3schools has a pretty comprehensive reference of  JS commands.

Personally, I use JS only as a very last resort, because when it comes time to do maintenance, there may not be an ID with programming experience available.