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TypeScript Deep Dive & Type Metaprogramming
TypeScript is a structural type system with Turing-complete compile-time type manipulation.
Key Mental Models & Invariants
- -Structural typing ('duck typing') compares shapes, not nominal declarations.
- -Discriminated unions enable exhaustive pattern matching with never checks.
- -Conditional types (`T extends U ? X : Y`) power advanced utility transformations.
- -Template literal types allow compile-time string validation (e.g. `api/${string}`).
Deep Dive Architecture
### Discriminated Unions & Exhaustiveness
A discriminated union has a literal property common to all variants. By checking `never`, the compiler guarantees you handled all cases:
```typescript
type NetworkState =
| { status: "idle" }
| { status: "loading" }
| { status: "success"; data: string[] }
| { status: "error"; error: Error };
function render(state: NetworkState) {
switch (state.status) {
case "idle": return "Waiting";
case "loading": return "Loading...";
case "success": return state.data.join(", ");
case "error": return state.error.message;
default: {
const _exhaustive: never = state;
throw new Error(`Unhandled state: ${_exhaustive}`);
}
}
}
```
Code Exampletypescript
// DeepReadonly type utility using recursive conditional types
type DeepReadonly<T> = T extends Function | boolean | number | string | null | undefined
? T
: T extends Array<infer U>
? ReadonlyArray<DeepReadonly<U>>
: { readonly [K in keyof T]: DeepReadonly<T[K]> };Recursively iterates nested object properties and marks all fields and arrays readonly at compile time.