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TypeScript Type System — It All Disappears After Compilation

What structural typing is and why it differs from Java's nominal typing

Java: Cat and Dog with different names are incompatible. TypeScript: if property and method structure matches, they're compatible.

What Remains After Compilation

: string is gone. TypeScript compiler performs type checking then erases all type info (type erasure). Can't check TypeScript types at runtime with typeof.

When This Is a Problem

TypeScript types don't exist at runtime — can't validate API responses. Need runtime validation libraries (Zod, etc.).

Type Narrowing

To check types at runtime, use JavaScript methods: typeof, in, instanceof. TypeScript compiler recognizes these patterns and narrows types inside branches.

Key Points

TypeScript types exist only at compile time — no trace in runtime JS (type erasure)

Structural typing — compatibility by "structure" not "name"

Runtime validation (API responses) needs separate libraries like Zod/Valibot

Type Narrowing possible via typeof/in/instanceof patterns

Use Cases

API response validation — Zod + TypeScript for both compile and runtime coverage Library type definitions — understanding why .d.ts files are needed separately

References

🔗 TypeScript Handbook — Type Compatibility

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TypeScript Generics — Understanding T Once and for All

Using "types decided later" in functions, classes, and interfaces

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← ⛓️ Promise Internals — How then() Enters the Microtask Queue 🔤 TypeScript Generics — Understanding T Once and for All →