How much JS is needed before starting Node?

JS the most carefree ( read careless) language there is. Understand some of its concepts - Promises, call stack, closures, etc and remember its shortcomings, limitations and bad implementation (loosely typed, mutability, bad typecasting, undefined) That being said for nodejs you need to know more backend concepts than the JS. What's a server, what's rest, how to connect to a database, writing, async fetching. You can start with a bare bones expressjs app. Once you learn to connect to a database, create and test APIs with postman, consume these apis in your frontend... You can then move to Typescript and to more beefier and capable Node Frameworks.