I started with Sololearn. It's free (at time of writing) and it help me get good enough to get going here.

freecodecamp has exercises similar to these katas and they're simple so that you can pickup the syntax at your own pace.

also, if simple kata is what you look for. give yourself simple exercises like the one you created in your question. write down five simple problems that you THINK you can do, and figure out how to do it in your browser's console.