Beside SQL which is strongly recommended, it's good for you to know static typings, java and c# comes to mind (haven't tried go, anyone can recommend it or other suitable)
TypeScript is the obvious choice for an experienced webdev. It's worth spending extensive time on to get good. Learn generics and advanced types to the point of not needing to look things up.
I need to mention that though TS is good, it's static typing is optional so op may find it different with java or csharp
Setting noImplicitAny should be adequate to address that. Java is getting type inference soon too.