TypeScript Practice 8/12

Narrowing with in and instanceof

Lesson 4 narrowed a string | number union with typeof. But typeof has a blind spot: for a Date, a plain object, an array - it answers "object" for all of them. A union of object types needs sharper questions:

instanceof asks "was this built by that class?" - the right check for class instances like Date.
in asks "does this object have that property?" - the right check for plain object shapes: "url" in value is true only for the member of the union that carries a url.

Inside each branch, TypeScript narrows the type - and at runtime, the same checks route each value to code that can actually handle it. One function, three shapes, no crashes:

function describe(value: string | Date | { url: string }): string {
  // typeof for the primitive, instanceof for the class, in for the shape
}

Your task: complete describe so it returns "text: " plus the string for strings, "year: " plus the full year for Dates, and "link: " plus the url for link objects.

You'll practice:

instanceof as a runtime check and a type narrower
The in operator for telling object shapes apart

function describe(value: string | Date | { url: string }): string {
  if (typeof value === "string") return "text: " + value;
  // Add a branch for Date (instanceof) returning "year: " + the full year,
  // and a branch for the link object (in) returning "link: " + the url.
  return "unknown";
}

const a = describe("hello");
const b = describe(new Date(2030, 0, 1));
const c = describe({ url: "https://example.com" });