TS Lazy Conditional Type Evaluation.md

TS Lazy Conditional Type Evaluation

Below is a five-line “playground-size” demo that lets you watch the conditional type stay in its unevaluated form inside a generic function and collapse as soon as you call the function with a concrete argument.

// 1 ─ a tiny conditional type
type Kind<T> = T extends string ? "str" : "other";

// 2 ─ generic function: the test type receives a **bare** type parameter
function lazy<T>(value: T) {
  type Inside = Kind<T>;          // 3: Inside is still  Kind<T>
  return null as unknown as Inside;
}

// 4 ─ call sites supply concrete types, so the compiler finally reduces it
const a = lazy("hello");   // a: "str"
const b = lazy(123);       // b: "other"

• Line 3 (inside lazy) T is a bare type parameter, so the compiler postpones the test T extends string ? … : …. Hover in an editor and you’ll see Inside reported literally as Kind<T>—the conditional hasn’t run yet.

• Lines 4 – 5 (outside lazy) Each call substitutes a concrete type (string or number) for T. Now the condition can be decided, so Kind<string> reduces to "str" and Kind<number> to "other".

That postponement is exactly what TypeScript’s documentation refers to as “deferred” or “lazy” evaluation of conditional types involving bare type parameters.