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Structural typing, generics, narrowing, and modelling domain constraints in the type system.
3 questions in Node.js & TypeScript.
You shrink the set of values the compiler accepts: discriminated unions instead of optional-field soup, branded types so one id cannot stand in for another, and a never-typed fallback that fails the build when a variant is added. Types vanish at runtime, so input is parsed by a schema the type is inferred from.
TypeScript erases at compile time, so a type annotation on request data is an assertion about the future rather than a check. Anything crossing a boundary the compiler cannot see - a request body, a database row, a JSON file, an API response - has to be validated at run time, with the type derived from the validator.
TypeScript allows extra properties under structural typing, but excess property checking rejects extra fields on fresh object literals. Assign the same object to a variable first and the extra fields usually pass, which can leak undeclared fields into API responses.