Imagine some language with the following type and functions:

  type Person { name: String }
  function parsePerson(json: String): Person { ... }
  function usePerson(person: Person) { ... }

The compiler will guarantee that usePerson will always get the correct shape of data for it's person variable. The only function you need to worry about is parsePerson, which will generally throw a runtime error if the incoming JSON does not conform to the expected shape. And in most web frameworks, the JSON to type parsing is done for you so once the framework hands you data in the type you are asking for you can expect it to be correct.