You'll need to know the minimal concepts of JavaScript, e.g. variable types (strings, integers, boolean, objects, arrays, etc), operations (especially joining strings).
Then, you should know about functions related to the DOM, e.g. document.querySelector(), maybe DOM tree traversal.
You can move up from there to advanced functions, especially those for arrays, e.g. forEach(), map().
Also, don't rely on/learn about JavaScript frameworks, jQuery. As much as possible, learn and use native/raw JavaScript. This is because Launch itself doesn't provide any framework, so you'll have the added complication of needing to bundle that framework in your code if you choose to go down that route.
Specifically for Launch, you should also be aware of its event-driven system. That means, when an Event in a Rule gets triggered, then that is when data elements get evaluated (especially those with "None" storage duration) and actions get run. That can affect the returned values from data elements or some JavaScript code.
