I use ajv's schema-based validation at all entry points to my application (user input from forms, urls, whatever comes back from an API, etc) and find I can very safely assume all data after that point is totally safe, so I'm using TypeScript to ensure _I'm_ not doing anything stupid. Hopefully that makes sense. Runtime type checking would be neat for sure, but I'm mostly worried about the dumb things I'll do. It's not too hard to strictly and cleanly validate foreign data at runtime, but it's historically very easy to shoot yourself in the foot with JavaScript. So I'm very happy.