Discriminated unions model finite states with a shared literal tag, enabling exhaustive narrowing in switch statements, Result-style error handling, reducer actions, and pattern-matched API responses.
TypeScript narrowing refines broad types to specific ones within code branches. Covers typeof, instanceof, in, equality, assignment narrowing, discriminated unions, control flow analysis, and the never type.
User-defined type guards narrow types at runtime using the `is` predicate, assertion functions, generic guards, and class-based patterns. Covers API validation, DOM guards, and when to use Zod.
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