Module boundaries and type-only dependencies

The idea

A module can export both runtime values and compile-time types. import type explicitly marks a dependency needed only for checking and removed from emitted JavaScript. This helps readers distinguish a shape contract from a value whose module must exist at runtime. Actual module resolution and supported file formats still depend on the compiler configuration and execution host.

A type-only import cannot be used to construct a runtime value. Conversely, a class may be needed both as a type and as a constructor, so import it normally when new is required. Keep modules’ initialization effects predictable, and do not assume importing a type loads a validator. The example shows separate files to explain the boundary; it requires an appropriate module toolchain rather than a single-file browser execution claim.

// model.ts
export type Ticket = {id: string; done: boolean};
// report.ts
import type {Ticket} from "./model";
export function countDone(items: readonly Ticket[]): number {
  return items.filter(item => item.done).length;
}
// Example call after importing countDone:
// countDone([{id: "a", done: true}, {id: "b", done: false}])

The example call returns 1.

Ticket guides checking without being a runtime object imported by the generated program.