Interface Programs

Interfaces describe structural contracts for values and classes. An object implements an interface when it has the required members; TypeScript does not create runtime interface objects. Interfaces help decouple a reporting function from the details of each data source.

Both the class instance and the object literal satisfy the Printable shape. print depends on the label method rather than a particular class.

An interface is erased after compilation; instanceof Printable is not a valid runtime check.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

interface Printable { label(): string }
class Book implements Printable {
  constructor(readonly title: string) {}
  label(): string { return `Book: ${this.title}`; }
}
function print(item: Printable): string { return item.label(); }
console.log(print(new Book("Learning TypeScript")));
console.log(print({ label: () => "Note: review" }));

Verified output

Book: Learning TypeScript
Note: review

Compile: npx tsc --strict --target es2022 --module commonjs interface-programs.ts; run: node interface-programs.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

Both the class instance and the object literal satisfy the Printable shape. print depends on the label method rather than a particular class.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

An interface is erased after compilation; instanceof Printable is not a valid runtime check.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

Define an interface Priceable with a price() method and implement it for two product types.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

Both the class instance and the object literal satisfy the Printable shape. print depends on the label method rather than a particular class.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.