Assuming I understand you question: you can define an interface to describe what an object parsed from JSON contains, and then cast your raw JSON-parsed object into that type.

The casting doesn't guarantee the JSON from your server fits the interface, but it WILL ensure any downstream use is correct, and also enable auto complete for its fields (assuming your IDE supports it).

I've been using JavaScript on and off for nearly 20 years, and TypeScript is a massive improvement.