typeof, instanceof, in, discriminated unions, custom type predicates.

Type Guards — Narrowing Union Types at Runtime

Type guards narrow the type of a value in a conditional branch. Built-in: typeof (primitive types), instanceof (class instances), in (property existence). Discriminated unions use a literal type "tag" field for exhaustive narrowing. Custom type predicates (value is Type) create reusable narrowing functions. The never type and exhaustive checks catch missing union cases at compile time.

Interactive Pipeline Flow

UnderstandImplementOptimise

🔀 Union — A | B | C

Type narrowing is the process of refining a broad type to a more specific type based on runtime checks. TypeScript tracks narrowing through control flow.

Architecture Diagrams

Exhaustive type checking with discriminated unions.

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Key Design Decisions

🏷️Discriminated unions are the cleanest narrowing pattern

Add a kind or type literal field to every variant in a union. TypeScript narrows to the exact variant in a switch/if block — no casting, no unsafe access, catches missing cases.

🛑Add an exhaustive never check to every discriminated union switch

default: const _: never = value — this line produces a compile error if a new union member is added but not handled. It's your compiler-enforced completeness check.

✅Type predicates replace as casts

value as User is an unsafe assertion. isUser(value): value is User is a runtime-verified narrowing. Use type predicates for unknown data, catch(e: unknown), and JSON responses.

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